Comprehensive Biochemistry for Dentistry Textbook for Dental Students

2.10  Golgi Body 21

2.10 G olgi Body

Definition
Golgi body is the highly organized complex of interconnected flattened sacs
and vesicles that function as secretory and intracellular transport
organelle.

History

In 1898, Camillo Golgi (Italian scientist) described a complicated internal network
inside nerve cells of barn owl and called as internal reticular apparatus. Later on,

it was named as Golgi bodies.
It is also called as Golgi complex or Golgi apparatus.

Occurrence

• Golgi body is present in all eukaryotic cells except mature RBC.
• It is absent in prokaryotic cells.

Size and Number
Size and number of Golgi complex are variable and depend on metabolic activity of
cells. In secretory cells, their size is large in comparison with nonsecretory cells.
Liver cell may contain 30–50 Golgi bodies near the nucleus.

Position
Golgi complex is single and its location is permanent. It is located near the nucleus
and above centrioles. In liver cells and nerve cells, multiple units of Golgi bodies are
found scattered in cytoplasm.

Structure
Depending on electron microscope study, Golgi body is composed of three
structural elements as:
Cisternae

• Cisterna is a flattened double-walled saclike structure. Cisternae are compactly
arranged parallel to each other to form a stack. About 4–6 cisternae are placed in
a stack. Adjacent cisternae are separated by a distance of 100–300 Å. It is called
as intercisternal space which contains fluid and protein fibrils. Ends of each cis-
tern are swollen and called as Golgian vacuoles.

• Cisterna is surrounded by double-layered membrane. Internally, a cistern has
lumen of 90 Å. It contains a fluid.

22 2  Cell and Organelles

• Cisternae curved and exhibit polarity. Each cistern has two distinct sides as:
–– Forming Face: It is convex in shape and is positioned toward the nucleus.
It is also called as F-face or cis-face. Thickness of F-face membrane is
40  Å. The cis-face of cisternae together constitutes cis-Golgi network
(CGN).
–– Maturing Face: It is concave in shape and positioned toward plasma mem-
brane. It is also called as M-face or trans-face. Thickness of M-face mem-
brane is 80  Å. The trans-face of cisternae represents trans-Golgi network
(TGN) as in Fig. 2.11.

• Transitional vesicles contain secretory proteins (enzymes, hormones, antibodies
synthesized by ribosomes and delivered to lumen of RER and packaged into tran-
sitional vesicles). Transitional vesicles are pinched off from RER. They are coated
with COAP-I (coat protein complex). They transport secretory proteins from RER
to cis-Golgi network (anterograde transport). Transitional vesicles fuse with
membrane of cis-face (F-face) of cisternae. Secretory proteins are processed within
lumen of cisternae and packaged into secretory vesicles. They are transported to
different locations including fusion with plasma membrane as in Fig. 2.11.

Tubules

• A network of thin tubules is located at periphery of cisternae. Tubules have diam-
eter of 300 Å. They help to interlink adjacent cisternae.

Maturing face Vacuoles
Forming face Inter cisternal space
Cisterna

Transition vesicles

Fig. 2.11  Golgi complex

2.10  Golgi Body 23

Vesicles

• Vesicles are membrane-bound saclike structures. They have diameter between
200 and 800  Å. Vesicles are oriented toward tubular Golgi network (TGN).

They contain processed substances and are pinched off from M-face of Golgi

body. These vesicles may also be called as secretory vesicles. They are two types

as:
–– Smooth Vesicles

These vesicles have smooth surface. They contain secretory proteins.
–– Coated Vesicles

These vesicles have rough surface which is coated with coat protein (e.g.,
clathrin protein). Clathrin coat vesicles fulfill intracellular vesicular traffick-

ing function.

Golgian Vacuoles

• They are dilated ends of cisternae. They are modified and contain granular sub-
stances. They are released from trans-face of Golgi network. They may function
as lysosomes.

Formation of Vesicles
Ribosomes synthesize proteins and deliver them into lumen of RER. These large
molecules are unable to pass through plasma membrane of cytoplasmic organelles.

Within RER, large molecules are packaged into tiny, membrane-bound, saclike
structures called as vesicles. Plasma membrane of ER bulges out and pinches off
transport vesicles. They are transported to cis-face cisternae and fuse with its mem-
brane. In this way, transport vesicles deliver proteins from ER to Golgi body with-
out moving through plasma membrane. These proteins are called as cargo
proteins.

Within lumen of cisterna, proteins undergo processing. Carbohydrate and lipid
moieties are added to proteins to form glycoproteins, lipoproteins, and proteogly-
cans. It is called as post-transcriptional modifications.

Proteins are transported from cis-face to trans-face of Golgi body through shuttle
vesicles.

At trans-Golgi network, TGN (M-face), proteins are packaged into vesicles
that are transported to different intracellular locations and even extracellular sites.

Types of Secretions in Vesicles

1 . Vesicles may contain antibodies secreted by plasma cells. These vesicles are
pinched off from M-face and transported toward plasma membrane. They fuse
with membrane to release their contents to extracellular space.

2. Vesicles may contain peptide hormones and stored in cell. After a signal trans-
duction, vesicles release hormones in extracellular space.

3 . Vesicles may contain hydrolytic enzymes. These are transferred to lysosomes.

24 2  Cell and Organelles

Model of Intracellular Vesicular Trafficking

• Anterograde Vesicular Transport
Cargo proteins move in COP-II-coated vesicles (coat protein complex-II)
from RER to cis-face of cisternae.

• Retrograde Vesicular Transport
Proteins also move in COP-I-coated vesicles from older cisternae (M-face) to
young cisternae (F-face).

2.10.1 Functions of Golgi Body

Synthesis of Secretory Proteins

• Secretory proteins (hormones, enzymes, antibodies) are synthesized in RER and
transported to inside of endoplasmic reticulum. Proteins are translocated in coat
vesicles (coated with COPII) to cisternae. Proteins undergo modification and
packaged into secretory vesicles. Membrane of vesicle fuses with plasma mem-
brane and imports secretory proteins in extracellular space (exocytosis).

Example: Golgi body synthesizes enzymes in the pancreas, thyroxine in the thy-
roid gland, and antibodies in plasma cells.

Synthesis of Glycoproteins and Glycolipids
Glycolipids

• Gangliosides (glycolipids in the brain) are synthesized in Golgi body. Its lipid
component (ceramide) is synthesized in smooth endoplasmic reticulum.
Ceramide is glycosylated with moieties glucosyl UDP and galactosyl
UDP.  Reactions are catalyzed by enzymes glucosyltransferase and galactosyl-
transferase along with other transferases located in Golgi body.

Glycoproteins

• Cell membrane proteins and secretory proteins are glycosylated with carbohy-
drate moieties. It can occur by two types of glycosylations as:
–– N-linked Glycosylation
Oligosaccharide moiety is attached to N-atom of asparagine residue in protein
molecule.
–– O-linked Glycosylation
Oligosaccharide moiety is attached to O-atom of either serine or threonine
residue in protein molecule (e.g., addition of glycosaminoglycans to proteins
forms proteoglycans).

2.11  Endoplasmic Reticulum 25

GLYCOGENIN is GLYCOSYLATED TYROSINE.
It is a primer to initiate synthesis of glycogen. It is a glycoprotein and acts a
glycosyltransferase enzyme.

Synthesis of Proteoglycans

• Proteins are glycosylated with mucopolysaccharides like hyaluronic acid, chon-

droitin sulfate, and keratin sulfate in Golgi body and form proteoglycans. They

serve as important structural constituents of ground substance of extracellular

matrix.
• The term ground substance refers to amorphous gelatinous substance which

is transparent, viscous, and colorless.
• It fills interstitial spaces between fibers and cells in connective tissues.

Synthesis of Acrosome

• Acrosome contains hydrolases to digest layers of egg. It is synthesized from
Golgi body.

Synthesis of Lysosomes

• Proteins are glycosylated at asparagine residues by addition of mannose-6-­
phosphate moiety within Golgi body. Mannose-6-phosphate is a signaling mol-
ecule for protein to serve as acid hydrolase. These proteins are packaged into
clathrin-c­oated vesicles and move to acidic vesicles called as endosomes
(membrane-­bound intermediate cellular organelle that helps in sorting of materi-
als). Mature endosomes fuse with lysosomes.

2.11 E ndoplasmic Reticulum

Definition
Endoplasmic reticulum is a network of interconnected structures that serve as
cytoskeletal of cell.

History

• In 1897, Garnier discovered endoplasmic reticulum.
• In 1945, structure of ER was described by Claude, Porter, and Fullam.
• The term endoplasmic reticulum was coined by Porter.

26 2  Cell and Organelles

Occurrence

• Endoplasmic reticulum is found in the cytoplasm of eukaryotic cells. Exception:
mature RBC, germinal cells.

• In prokaryotes, ER is absent.

Types

• Smooth endoplasmic reticulum (SER)
It does not contain ribosomes. It is also called as agranular endoplasmic
reticulum.

• Rough endoplasmic reticulum (RER)
It contains ribosomes. It is also called as granular endoplasmic reticulum.

Structure
Endoplasmic reticulum is a cytoskeleton with well-defined structures namely
cisternae, tubules, and vesicles which are described as follows:
Cisternae

• They are cylindrical double-walled structures of ER. Cisternae are placed one
above the other to form an interlinked compact structure.

• Diameter of a cisterna is about 40 μm.
• In cells with high protein synthesis, cisternae contain ribosomes.
• Cisternae are located nearer to the nucleus as in Fig. 2.12.

Ribosomes

Rough
endoplasmic
reticulum pore

Smooth
endroplasmic
reticulum

Vesicles

Tubules

Fig. 2.12  Components of endoplasmic reticulum

2.11  Endoplasmic Reticulum 27

Tubules

• Tubules are wide-branched double-walled structures. Each tubule has a diameter
100 μm.

• Tubules are devoid of ribosomes.
• Tubules are located close to plasma membrane.

Vesicles

• Vesicles are double-walled oval structures. Each vesicle has a diameter 500 μm.
• Vesicles contain ribosomes.

Characteristics of SER

• SER is located near plasma membrane.
• It lacks ribosomes.
• It is mainly found in tissues concerned with synthesis of lipids.

Example: Adipose cells, liver, muscle fibers, adrenal cortical cells.

Characteristics of RER

• RER is located close to the nucleus.
• It contains ribosomes.
• It is found in protein-synthesizing cells.

Examples: Islets of Langerhans, pancreatic acini, Nissl’s granules, plasma
cells.

Functions

• Endoplasmic reticulum acts as cytoskeleton. It gives mechanical support to cell.
It helps to maintain shape of cell.

• It helps in import and export of substances.
• Endoplasmic reticulum is involved in formation of lysosomes.
• Rough ER is essential for synthesis of nuclear envelope.
• RER is essential for synthesis of regulatory proteins and membrane

proteins.
• RER is converted into SER through loss of ribosomes.
• ER is helpful in glycogen storage.
• SER is essential for synthesis of triglycerides in adipose cells.
• SER is essential in synthesis of steroidal hormones.
• SER is essential in glycogenolysis in the liver.
• SER in skeletal muscles is helpful in action potential across the muscle fiber.
• SER in muscle fiber (sarcoplasmic reticulum) is essential in cytosolic release

of calcium ions which in turn promotes muscle contraction.

28 2  Cell and Organelles

2.12 Nucleus

Definition
Nucleus is defined as a double membrane-bound cytoplasmic organelle in
eukaryotic cells containing genetic material for inheritance of information and
regulation of cellular functions.

Nucleus is derived from the Latin word nucleus which means kernel or seed.

History

• During 1632–1723, Antonie van Leeuwenhoek (microscopist) was the first who
described presence of a lumen (nucleus) in the RBC of salmon.

• In 1931, Robert Brown (botanist) described nucleus in root cells of orchid plant.
• In 1953, J Hammerling described nucleus a storehouse of genetic material in his

work on Acetabularia (unicellular alga).

Facts

• Nucleus is a characteristic feature of eukaryotic cells.
• Nucleus is absent in prokaryotic cells.
• In humans, nucleus is absent in mature erythrocytes. It is present in erythrocytes

in maturation stage.

Number

• Anucleate
Mature RBCs are without nucleus.

• Uninucelate
A cell containing single nucleus is termed as uninucleate cell.
Example: adipose cells, fibroblasts, osteoblasts, plasma cells, monocytes,
lymphocytes.

• Multinucleate
A cell containing multiple nuclei is termed as multinucleate cell.
Example: Osteoclasts (four to five nuclei), skeletal muscle fibers,
proerythroblasts.

• Giant cell
Giant cell is multinucleated cell which is formed by fusion of macrophages or
monocytes in pathological conditions.
Example: Langerhans giant cell (granulomatous diseases), foreign body
giant cell.

• Syncytium
It is fusion of uninucleate cells by plasma membrane to appear as a multinucleate
single mass.

2.12 Nucleus 29

Example: Cardia muscle fibers are uninucleate cells. They are intercon-
nected by intercalated discs and function as a single mass of multinucleate
cells called as syncytium.

Shape

• Shape of nucleus is highly variable depending on metabolic activity of cells.
• Generally, nuclei have round or oval shapes.
• Neutrophils have trilobed nucleus which becomes hypersegmented (>3 lobes) in

megaloblastic anemia.

Size

• Size of nucleus is highly variable and depends on metabolic activity of cell.
• Nuclear size is directly proportional to volume of cytoplasm and it is termed as

nucleoplasmic ratio.
• Nucleoplasmic ratio is altered in megaloblastic anemia and malignancy.

Position

• Generally, nucleus is positioned in the center of cell which is based on metabolic
activity of cell.

• Nucleus occupies area of maximum activity inside cytoplasm.
• However, in plasma cells, nucleus is eccentric in positon:

In adipose cells, nucleus is confined to periphery of cytoplasm due to accumu-
lation of fats.
In glandular cells, nucleus assumes basal position.

Ultrastructure
Nucleus during interphase has a diameter between 5 and 20 μm. It is composed of
five parts as follows:

• Nuclear membrane
–– Nuclear membrane is also called as karyotheca. It separates nucleus from
cytoplasm.
–– It is a double-layered structure. Its outer layer is called outer nuclear mem-
brane, and inner layer is called inner nuclear membrane. These layers are
separated by a distance of 100–500  Å. The space between outer and inner
nuclear membranes is called perinuclear space.
–– Each membrane has unit membrane structure. Each membrane is composed
of phospholipid bilayer surrounded by two protein layers as in Fig. 2.13. Each
membrane has a width of 70–90 Å. The outer membrane is rough and bears
ribosomes. It is connected with endoplasmic reticulum. The inner membrane
is smooth and is in contact with nucleoplasm.

30 2  Cell and Organelles

Heterochromatin Chromatin
Karyo lymph threads

Peri nuclear space
Nuclear pore
Outer nuclear membrane
Inner nuclear membrane

Nucleolus

Fig. 2.13  Structure of Nucleus

–– Nuclear membrane is perforated by presence of nuclear pores. Both layers of
nuclear membrane are in contact with each other at nuclear pore.

–– Nuclear pores
Nuclear pores are channel exchange of water-soluble substances between
cytoplasm and nucleus. Each nuclear pore has a diameter of 1200 Å.
Nuclear pores belong to family of transmembrane protein complexes which
are located in nuclear membrane as in Fig. 2.13.
They are called as nuclear pore complex or nucleoporins. There are about
30 proteins that form nucleoporins.

Nucleoplasm

• It is a viscous, transparent, and granular fluid that fills up space inside the nucleus.
It is a type of protoplasm that is bound by nuclear membrane. It is also called as
nuclear sap or karyoplasm

• Nucleoplasm contains chromatin and nucleolus.
• Nucleoplasm contains organic and inorganic compounds like nucleoside phos-

phates, proteins, and enzymes and minerals.

Chromatin

• Chromatin is a DNA-protein complex (nucleoprotein) that has affinity to basic
dyes.

• Chromatin is in the form of highly fine, coiled filament-like structure present
in nucleoplasm. It is seen during interphase.

2.12 Nucleus 31

Fibrils Intra nuclear
Matrix chromatin

Granules

Perinuclear
chromatin

Fig. 2.14  Structure of Nucleolus

• During cell division, chromatin appears thick and ribbon shaped and called as
chromosome (color-stained bodies). Chromosomes are observed during cell
division (M phase).

• A chromosome is composed of helically coiled DNA which is coated with his-
tone protein (nucleoprotein).

• Chromatin is of two types as:
–– Euchromatin
Euchromatin is a light-stained diffusely condensed chromatin network. It has
a width of about 100  Å. It forms 95% part of chromatin. Euchromatin is
actively involved in transcription.
–– Heterochromatin
Heterochromatin is a dark-stained highly condensed chromatin. It has width
of 1000  Å. The DNA in heterochromatin is inert. Heterochromatin is not
involved in transcription of mRNA.
Heterochromatin is of two types as:
Constitutive heterochromatin
Constitutive heterochromatin is highly condensed chromatin. It is made up of
multiple tandem repeats (when nucleotides are repeated as ATTCG
ATTCG ATTCG). DNA with tandem repeats is called satellite or repetitive
DNA. It is not transcribed. Constitutive heterochromatin is present in centro-
meres and telomers. It is necessary for pairing of homologous chromosomes
in meiosis.
Facultative heterochromatin
Facultative heterochromatin can be condensed or uncondensed and can act as
euchromatin or heterochromatin. It has the ability of gene expression.
Example: Bar body

32 2  Cell and Organelles

2.13 N ucleolus

• It is a round or irregular structure which is attached to nucleolar organizer region
of chromosome.

• Nucleolus is not covered by membrane as in Fig. 2.14. It is composed of four
components as:
–– Nucleolar matrix
It is amorphous portion of nucleolus. It is in the form of semisolid viscous
fluid. It contains proteins.
–– Granular part
Granular part contains ribosomal subunits which are in formative stages.
–– Fibrillar part
Fibrillar part contains small fibrils of 50 Å length. Fibrils are made up of pro-
teins and rRNA molecules. These fibrils are precursors to ribosomes.
–– Chromatin
Chromatin is associated with peripheral portions of nucleolus and is called as
perinucleolar chromatin. It grows inwardly into nucleolus as ingrowths and
called as intranucleolar chromatin.
Nucleolus is the main site for synthesis of rRNA.

Functions of Nucleus

• Nucleus is an essential part of eukaryotic cell that contains genetic material in
the form of DNA.

• DNA in nucleus controls growth, proliferation, differentiation, metabolism, and
apoptosis of cells.

• Nucleus regulates cell metabolism by transcription of mRNA.
• DNA in nucleus is the site of crossing over and variations.
• DNA in nucleus is the site for mutation.
• Nucleus is responsible for speciation.
• Nucleus regulates cell division.
• Nucleolus in cell is the site for synthesis of rRNA.

Suggested Readings

Berg JM (2007) Biochemistry. Freeman WH, New York
Campbell NA, Reece JB (2005) Biology, 7th edn. Pearson Benjamin Cummings, San Francisco,

CA
Karp G (2007) Cell and molecular biology, 5th edn. Wiley, Hoboken

Part II
Structural Biochemistry

Protein and Amino Acids 3

3.1 Historical Facts

• In 1835, Dutch chemist, Mulder GJ, started chemical analysis of albuminous
substances from egg white, milk, gluten, and serum. Mulder concluded that a
“common fundamental radical” was present in all those substances. This funda-
mental radical is combined with sulfur and phosphorous in varying proportions
to yield albuminous nature of substances.

• In 1838, Swedish scientist, Berzelius JJ, proposed the word “protein” for the
common fundamental radical, suggested by Mulder.

• The word protein has its root in the Greek word “proteios” which stands for
“primary.” It signifies prime importance of proteins in the anatomy and physiol-
ogy of acellular to multicellular organisms.

3.2 D efinition

Protein is defined as “polymer of amino acids that constitutes as basic struc-
tural and functional units of body tissues of living organisms.”

Proteins are made up of carbon, hydrogen, and oxygen as the constituent ele-
ments. Nitrogen is an essential element in proteins, and its proportion is nearly 16%
of total molecular weight of protein. Other elements like sulfur and phosphorus are
also present in small proportion in some proteins. Proteins may have molecular
weight ranging from 5000 to millions.

3.3 C lassification of Proteins 35

Proteins can be classified by four criteria as follows:

1. Classification based upon size and shape

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A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_3

36 3  Protein and Amino Acids

2 . Classification based upon functions
3. Classification based upon chemical composition and physical properties
4. Classification based upon quality and nutritional value

3.3.1 Classification Based upon Size and Shape

Proteins can be classified into two groups based on size and shape as:

• Fibrous Proteins
–– These proteins have axial ratio more than 10.
–– They have an elongated shape which may be either threadlike or resemble a
sheet.
–– Proteins are water insoluble.
–– They provide mechanical strength and protection to body tissues.
Example: Collagen fibers, elastin fibers, alpha-keratin in hairs and nails,
beta-k­ eratin in silk

• Globular Proteins
–– They have an axial ratio less than 10.
–– They are also called corpuscular proteins.
–– These proteins have spherical or ovoid shape.
–– They form colloidal solution with water.
–– They may have a role as enzyme, hormone, or antibody.
Example: Hemoglobin, myoglobin, immunoglobulins, ribonuclease

3.3.2 C lassification Based upon Biological Functions

Proteins perform multiple functions. They can be categorized into several groups.

• Structural Proteins
–– They are the essential structural components of various tissues of the body.
Example: Collagen protein in bone, elastin protein in cartilage, alpha-­
keratin in skin and nails

• Enzymes
–– They catalyze metabolic reactions in body tissues.
Example: Amylase, lipase, protease

• Hormones
–– They are the secretions from endocrine glands. They regulate metabolic reac-
tions in body tissues.
Example: Insulin, thyroxin

• Transport Proteins
–– They help in the gaseous transportation from one type of tissues to another tis-
sues. Hemoglobin helps in transport of oxygen from pulmonary alveoli to body
tissues and carbon diaoxide from tissues to pulmonary alveoli. Myoglobin helps
in transport of oxygen from erythrocytes to mitochondria within muscles.
Example: Hemoglobin, myoglobin

3.3  Classification of Proteins 37

• Protective Proteins

–– They provide immunity against the pathogens.
Example: IgA, IgG, IgM, IgD, IgE

• Contractile Proteins

–– They are the functional and contractile units of skeletal muscles and are

responsible for muscle contraction.
Example: Actin, myosin

3.3.3 C lassification Based upon Chemical Composition
and Physical Properties

The classification is widely accepted by academicians and scientists. It was pro-
posed by the British Physiological Society and American Physiological Society in
1907–1908. This system classifies proteins on the basis of complexity in chemical
composition of protein molecules.

Proteins are classified into three groups as:

(a) Simple proteins
(b) Conjugated proteins
(c) Derived proteins

(a) Simple Proteins
Simple proteins are the linear polypeptides of amino acids which upon

hydrolysis yield constituents amino acids.
They are further subclassified on the basis of coagulability and solubility

into seven types of proteins.
1. Albumins

• These proteins are heat coagulable.

• They are soluble in water and dilute salt solutions.

• They have low isoelectric pH (4.7).

• They behave as acidic at pH 7.4.

• Albumins are precipitated by saturated ammonium sulfate solution.

• They are deficient in glycine amino acid.
Example: Ovalbumin of egg, lactalbumin of milk, albumin in plasma,
legumelin in legumes, leucosine of cereals

2. Globulins

• They are heat coagulable.

• They are water insoluble but soluble in dilute salt solutions.

• They are precipitated by half saturated ammonium sulfate solution or full

saturated sodium chloride solution.
Example: Plasma globulin, ovoglobulin, glycinin in soybeans, edestin
in hemp seeds, legumin in peas

38 3  Protein and Amino Acids

3. Gliadins
• They are plant proteins and also called prolamines.
• They are insoluble in water and absolute alcohol.
• They are soluble in 60–80% ethyl alcohol.
• Gliadins are rich in proline amino acid but poor in lysine.
Example: Hordein in barley, gliadin in wheat, zein in maize

4. Glutelins
• They are plant proteins.
• They are insoluble in water, alcohol, and salt solution.
• They are soluble in dilute solutions of acids and alkalies.
• They are coagulated by heat.
• Glutelins are rich in glutamic amino acid.
Example: Glutenin in wheat, glutelin in maize, oryzenin in rice

5 . Protamines
• They are basic proteins and mostly found in animals.
• They are soluble in water and dilute solutions of acids and alkalies.
• They are heat non-coagulable.
• They are rich in arginine amino acid.
• They are deficient in cysteine, tyrosine, and tryptophan amino acids.
• They have high isoelectric pH (7.4).
• They link to nucleic acids and form nucleoproteins.
Example: Salmine in sperm of salmon, cyprinine in carp, clupeine in
sperm of herring

6 . Histones
• They are basic proteins and mostly found in animals.
• They are soluble in water and dilute solutions of acids and alkalies.
• They are heat non-coagulable.
• They are rich in arginine and histidine amino acids.
• They have high isoelectric pH (7.4).
• They link to nucleic acids and form nucleoproteins.
• They are deficient in cysteine, tyrosine, and tryptophan amino acids.
Example: Globin in hemoglobin, nucleoproteins

7. Scleroproteins
• They are also called as albuminoids and occur as supporting tissues in animals.
• They are insoluble in water, solutions of acid and alkalies, and 60–80% alcohol.
• They are soluble in concentrated solution of acids.

Scleroproteins are characterized into three types of proteins based upon loca-
tion and function.
• Keratins

Alpha-keratin
–– They are insoluble in water, alcohol, acids, or alkali.
–– Alpha-keratin is rich in sulfur-containing amino acid, cysteine.
–– Alpha-keratin contains a right-handed, α helix.
–– Hard alpha-keratin contains higher number of disulfide bonds in compari-

son with soft or pseudo keratin.

3.3  Classification of Proteins 39

Example: Proteins in human hairs, nails, outermost layer of skin, and
mammalian tissues like wool, horns, claws, and hooves
Beta-keratin

–– It is rich in amino acids glycine and alanine and poor in cysteine.

–– Beta-keratin has β-pleated sheet structure.
–– It is comparatively harder than α keratin.

Example: Proteins in silk fibroin, scales of reptiles, feathers, beaks,
and claws of birds
• Elastins

–– They are rich in amino acids like valine, leucine, and proline.

–– They are deficient in cysteine, methionine, and histidine.

–– They provide elasticity to tissues.
Example: Proteins in elastic fibers in ligaments, cartilages, and loose
areolar connective tissues

• Collagen

–– They are long fibrous proteins.

–– They provide mechanical strength to tissues.
Example: Proteins in bones, muscles, skin, tendons

( b) Conjugated Proteins

These proteins are linked to non-protein components, called as prosthetic

groups which can be either a metal or an organic compound.

Simple protein + Prosthetic group ------ Conjugated protein

(Apo-protein) (Holoprotein)

Type of prosthetic group determines subclass of conjugated protein as follows:
• Mucoproteins
–– M ucoproteins are composed of simple proteins linked to mucopolysac-
charides as prosthetic group.

–– In mucoproteins, mucopolysaccharide concentration is >4%.
Example: Mucoproteins in blood group antigens, umbilical cord, beta
ovomucoid in egg white, vitreous humor

–– Mucins are mucoproteins containing nearly 70–80% of carbohydrates con-

sisting of N-acetylglucosamines, N-acetylgalactosamines, sialic acid, and

fucose.
Glycophorin is a mucoprotein in the membrane of erythrocytes. It
contains high content of carbohydrates nearly 60%.
• Glycoproteins
–– G lycoproteins are composed of simple proteins attached to mucopolysac-
charides as prosthetic group.

–– In glycoproteins, mucopolysaccharide concentration is <4%.
Example: Serum albumin, serum globulin, immunoglobulins

40 3  Protein and Amino Acids

• Nucleoproteins
–– N ucleoproteins are composed of simple proteins of protamine and histones
conjugated with nucleic acids as prosthetic group.
Example: Deoxyribonucleoproteins are present in nuclei of cells, chlo-
roplasts, and mitochondria.
Ribonucleoproteins are present in nucleoli within eukaryotic nuclei.

• Chromoproteins
–– Chromoproteins are composed of simple proteins with pigmented com-
pounds as prosthetic group.
Example:
1. H emoglobin, myoglobin, cytochromes, and enzyme catalase are
hemoproteins containing heme as prosthetic group.
2. Iodopsin, cyanopsin, porphyropsin, and rhodopsin are photosensi-
tive pigments in cones and rods in the retina of eyes.
3. Flavin adenine dinucleotide (FAD) and flavin mononucleotide
(FMN) are coenzymes containing riboflavin as prosthetic group.

• Lipoprotein
–– Lipoproteins are composed of simple proteins attached to lipids as pros-
thetic group.
Example: Serum lipoproteins

• Phosphoproteins
–– P hosphoproteins are composed of proteins linked to phosphoric acid as
prosthetic group.
Example: Casein protein in milk, vitellin protein in egg yolk

• Metalloproteins
–– M etalloproteins are composed of simple proteins and metallic ions as
prosthetic group.
Example:
Carbonic anhydrase contains zinc (Zn++).
Hemoglobin contains iron (Fe++).
Ceruloplasmin contains copper (Cu++).

(c) Derived Proteins
Derived proteins are obtained from the simple and conjugated proteins.
Derived proteins are produced due to activity of physical factors like X-rays, UV
rays, and heat and chemical factors like acids, alkali, and enzymes upon simple
and conjugated proteins.
Derived proteins can be subclassified into two groups of proteins as follows:
• Primary Derived Proteins
–– Primary derived proteins are denatured native proteins.
–– They have molecular weight similar to native proteins.
–– They differ from native proteins in terms of solubility and crystallization
properties.
–– In primary derived proteins, all secondary forces like ionic bonds, hydro-
phobic bonds, van der Waals forces, and hydrogen bonds are disrupted.
–– Peptide bonds remain intact.

3.3  Classification of Proteins 41

Types
1.  Coagulated proteins: They are insoluble in water.

Example: Coagulated egg and cooked meat
2.  Proteans: They are insoluble in water.

Example: Myosan derived from myosin, fibrin derived from fibrinogen
3.  Metaproteins: They are insoluble in water but soluble in acid or alkali.

Example: Acid metaproteins, alkali metaproteins
• Secondary Derived Proteins

–– Secondary derived proteins are obtained by hydrolysis of native
proteins.

–– They have molecular weight lesser than native protein.

–– In secondary derived proteins, hydrolysis occurs at peptide bonds.
Types

–– According to the level of hydrolysis, they are of three types as proteoses,

peptones, and peptides:
1.  Proteoses: They are produced by hydrolysis of proteins by acid or

enzyme. They are water soluble. They are heat coagulable.
2.  Peptones: They are produced by hydrolysis of proteoses by acid or

enzyme. They are soluble in water. They are non-coagulable by heat.
3.  Peptides: They are made up of few amino acids linked by peptide

bonds. They can be dipeptides or tripeptides. They are water soluble

and not coagulated by heat (Fig. 3.1).

3.3.4 Classification Based upon Quality and Nutritional Value

Quality of proteins is determined by the amount of essential amino acids in proteins.
The higher the amount of essential amino acids, the greater will be its nutritional
value. Proteins can be subclassified into three groups on the basis of quality and
nutritional value.

• Complete Proteins
–– These proteins contain all the essential amino acids in proportional
composition.
–– They are also called as first-class proteins.
–– These proteins can promote growth of children.
–– These proteins are generally obtained from animals.
Example: Egg, meat, fish, milk

• Incomplete Proteins
–– These proteins are deficient of one essential amino acid.
–– These proteins are generally obtained from plants.
–– These proteins are inadequate for normal growth of children.
–– These proteins can maintain normal metabolism in adults.
Example: Pulses are deficient of amino acid methionine. Cereals are defi-
cient of amino acid lysine

42 3  Protein and Amino Acids
Proteins
Fig. 3.1 Complete
hydrolysis of protein
molecule

Proteans

Metaproteins

Proteoses Peptones Peptides

Amino acid + Amino acid+ - - - - - - - - -+ Amino acid + Amino acids

• Poor Proteins
–– These proteins are deficient of more than one amino acid.
Example:
Legumes are deficient of amino acids tryptophan and methionine.
Maize is deficient of amino acids lysine and tryptophan.

3.4 Structural Organization of Proteins

Proteins are biopolymers of α-amino acids. These are linked together through pep-
tide bonds. A condensation of two amino acids forms a dipeptide.

Example: Carnosine, anserine, pseudoproline
Similarly, condensation of more amino acids results into formation of tripep-
tides, tetrapeptides, and oligopeptides and proteins.
A polypeptide is formed by linking of 10–50 amino acids, and a protein molecule
is composed of more than 50 amino acids. Natural proteins are composed of 20
α-amino acids.

3.4  Structural Organization of Proteins 43

For example, a tetrapeptide contains only four amino acids. These can be any 4
out of total of 20 amino acids. These four amino acids can have (204 = 160,000)
types of arrangements. Therefore, proteins are synthesized in the body by altering
the sequence of amino acids.

Except glycine, all amino acids contain chiral carbon and are L stereoisomers.
Proteins exhibit wide structural complexity. Proteins have four levels of struc-
tural organization as:

Primary structure
Secondary structure
Tertiary structure
Quaternary structure

3.4.1 P rimary Structure

D efinition
Primary structure of protein is defined as “linear arrangement of amino acids
in a definite sequence in a polypeptide chain.”

Characteristics of Primary Structure
• Primary structure of protein is formed by condensation of amino acids in a linear

arrangement in a polypeptide chain.
• Each polypeptide chain has definite sequence of amino acid residues.
• Example: In a tetrapeptide, four amino acids are condensed to form differ-

ent types of tetrapeptides as in Fig. 3.2.
• Polypeptide chain shows polarity.

–– Amino terminal (N-terminal)
On the left hand side of a chain, the first amino acid is present. It has a free
amino group. This end of a polypeptide chain is called “amino terminal” or
(N-terminal).

–– Carboxy terminal (C-terminal)
Opposite end of polypeptide chain contains last amino acid. This amino acid
has a free carboxylic group. This end of chain is called carboxy terminal or
C-terminal.
By convention, numbering and sequence of amino acids are determined
from N-terminal as in Fig. 3.3.

Fig. 3.2  Linear structure Ala – Thr – Val - Leu
of tripeptide 1st tetrapeptide

Ala – Leu – Thr – Val
2nd tetrapeptide

Ala – Val – Thr – leu
3rd tetrapeptide

44 3  Protein and Amino Acids

H HH H

Amino (N) H2N –C – CONH – C – CONH – C – CONH – C – COOH
Terminal
R1 R2 R3 R4 Carboxlic

Peptide Bonds C-Terminal

Fig. 3.3  Polarity in peptide chain

Fig. 3.4  Formation of peptide H2N H COOH + H2N H COOH
bond C C
R1 R2

Condensation

H2N H CONH H
C C COOH + H2O
R1 R2

• In a polypeptide chain, amino acids are linked together by formation of covalent
bonds called as peptide bonds (–C–N–) as in Fig. 3.4. It has the following
features:
–– Peptide bond is formed by condensation of carboxylic group (COOH) of one
amino acid with the amino group (NH2) of successive amino acid with the loss
water molecule. The (CO–NH) groups in a peptide are called peptide or
amide group.
–– The peptide bond is a covalent bond. The four atoms, namely, carbonyl car-
bon, carbonyl oxygen, amide nitrogen, and hydrogen in peptide group (CO–
NH), are located in the same plane. Peptide bond has a planar geometry.
–– Peptide bond is a partial double bond. The length of a single covalent C–N
bond in amines is around 1.49 Å, while the length of double (C〓N) bond in
imines is around 1.27 Å. However, the length of C–N bond in peptide bond is
1.32 Å. This indicates that the peptide bond has a length lesser than single
bond but greater than a double bond. Partial double bond nature of peptide
bond is due to resonance in peptide bond. The nitrogen atom donates its
valency electrons to carbonyl carbon. The valency electrons shift to oxygen
and make it negatively charged ions, and nitrogen assumes positive charge.
This delocalization of electrons is called resonance as in Fig. 3.5.

3.4  Structural Organization of Proteins 45
Fig. 3.5  Resonance in peptide bond
O
C

N
H

O– O

C C
N+ N
H H

–– Peptide bond is a rigid bond. There is absence of free rotation around the
C–N bond in peptide group. Therefore, peptide group can assume two
i­someric conformations, namely, cis and trans. In cis form, two α-carbons
lie on the same side of peptide bond, while in trans form, two α-carbons lie on
the opposite side of peptide bond.
However, in nature, most of peptide bonds in proteins belong to
trans-conformation.

–– Two single bonds are present on either side of peptide bond. One single bond is

formed by alpha carbon and carbonyl carbon (Cα–CI) of first amino acid, while

the other bond is between amide nitrogen and alpha carbon (NI–Cα) of second

amino acid. These single bonds can rotate freely around peptide bond and can

assume a number of positions. This free rotation is responsible for coiling and

recoiling of polypeptide chains. It determines shape of protein molecule.

• The amount of rotation of single bonds is determined by torsion angle or dihedral

angle. The angle formed by (Cα–CI) bond with the peptide bond (C–N) is called

Psi (Ψ) torsion angle, whereas the angle between (NI–Cα) and peptide bond
(C–N) is called Phi (Φ) torsion angle. Pioneering research over the (Ψ, Φ) tor-
sion angles of amino acid resides in small polypeptide chain was performed
by Dr. G N Ramachandran during 1960-1963.

A diagrammatic representation of torsion angles is called Ramachandran plot.
Example:
Dystrophin is a large-sized primary protein containing 3685 amino acid resi-
dues weighing about 427,000 dalton. It is present in skeletal, cardiac, and
smooth muscle fibers.

• In a primary structure of protein, a linear polypeptide may be linked to another

polypeptide with the help of disulfide bonds. These bonds can be within the same

polypeptide chain (intra-chain) or between two adjacent polypeptide chains

(inter-chain). For example, insulin represents a primary structure of protein.

46 3  Protein and Amino Acids

3.4.2 Insulin Structure

It was described by Sanger in 1955.

• Insulin is composed of two polypeptide chains as:
–– Chain A (glycine chain): It contains 21 amino acid residues.
–– Chain B (phenylalanine chain): It has 30 amino acid residues.
–– Chain A contains one intra-chain disulfide bridge between 6th cysteine resi-
due and 11th cysteine residue.

• Insulin molecule contains two inter-chain disulfide bridges:
–– First inter-chain disulfide bridge is formed between seventh cysteine resi-
dues of chain A and chain B.
–– Second inter-chain disulfide bridge is formed between 20th cysteine residue
of chain A and 19th cysteine residue of chain B as in Fig. 3.6.

• In chain A, amino acid sequence at eighth, ninth, and tenth positions under-
goes variation among different species.
–– In humans, sequence (Thr-Ser-Ile) is present.
–– In bovine, sequence (Ala-Ser-Val) is present.
–– In pig, sequence (Thr-Ser-Ile) is present.
–– In sheep, sequence (Ala-Gly-val) is present.

• Structure of insulin among human, pig, and bovine is almost identical except the
difference of amino acid at 30th position in chain B. It is amino acid threonine in
humans, whereas it is alanine in pig and bovine.

3.4.3 Secondary Structure

Secondary structure is defined as “steric relationship between the closely
placed amino acid resides in a polypeptide chain.”

A linear polypeptide chain undergoes coiling or folding to attain a three-­
dimensional shape of a helix or a sheet or an intermediate shape. Non-covalent force
like hydrogen bond is responsible for folding of polypeptide chain in secondary
structure.

Intra chain disulfide bridge

SS

A 67 11 20

Gly Ile Val Glu Gln Cys Cys Thr Ser Ile Cys Ser Leu Tyr Gln Leu Glu Asn Tyr Cys Asn

S S
Inter chain disulfide bridge
SS

Phe Val Asn Glu Ile Leu Cys Gly Ser His Leu Val Glu Ala Lys Tyr Leu Val Cys Gly Glu Aeg Gly Phe Phe Tyr Thr Ala Pro Lys Thr

7 19

Fig. 3.6  Structure of Insulin

3.4  Structural Organization of Proteins 47

• Hydrogen bonding
–– It is a non-covalent and weak electrostatic force of attraction. Hydrogen bond
is formed between a hydrogen atom covalently linked to an electronegative
atom like fluorine, oxygen, or nitrogen in one molecule and an electronegative
atom in another molecule.
–– In secondary protein structure, hydrogen bond is formed between the oxygen
atom of carbonyl (–C〓O) group in one amino acid and the hydrogen atom of
amide (–NH) group in another amino acid. Hydrogen bonding stabilizes the
secondary structure of proteins.

Secondary Structures in Proteins
1. Alpha helix
2. Beta-pleated sheet
3 . Beta-bends (reverse turns)
4. Triple helix

1. Alpha Helix (Historical Facts)
• In 1930, William Astbury was a molecular biologist who did pioneering

work in X-ray diffraction studies on hair and wool. On the basis of diffraction

patterns, he concluded that fibrous proteins have folded structure and unfolded
or stretched structure. He used nomenclature as alpha for folded form and
beta for stretched form of secondary structures.
• In 1951, it was Pauling, Corey, and Branson who described alpha helical
model of secondary structure of protein. The research was published in the

proceedings of the National Academy of Sciences.

• Possibly, they proposed the structure of helix earlier than sheet structure. So

nomenclature of alpha and beta are used.

• Alpha helix is the most common type of secondary structure of proteins.
Alpha helix Characteristics

• It is a helically coiled rodlike structure. It is formed by tight folding of poly-

peptide chain around an imaginary long axis. The carbonyl group (–C〓O)

and amide group (–NH) form the backbone of helix. Peptides and hydrogen

bonds are placed parallel to long axis of helix. The side chains of amino acid

residues are directed outwardly and are placed perpendicular to helical axis.
• The alpha helix is stabilized by hydrogen bonds.
• One complete turn of helix contains 3.6 amino acid residues. The distance

between two amino acid residues is 1.5 Å (translation) along the helical axis,
and each one has 100° of rotation. The vertical distance between two suc-
cessive turns is called pitch, and it is the product of amino acid residues (3.6)
and translation (1.5 Å) and is equal to 5.4 Å in alpha helix.

• The folding in alpha helix can be either right handed or left handed. Right-­

handed alpha helix offers more stable conformation than left-handed helix.

48 3  Protein and Amino Acids

• In left-handed folding, side chains exhibit steric interferences on the (–C–N–)
backbone. Therefore, left-handed alpha helix is unstable and rarely exists. In
nature, all alpha helices found in proteins are right-handed structures.

• Proline contains a secondary amino group (–NH), and it cannot furnish hydro-
gen atom for hydrogen bonding. So it does not favor formation of alpha helix.
Glycine has the shortest side chain and it has highly flexible steric conforma-
tion. Proline and glycine are present in beta-bends in proteins.

• Pauling and Corey α-helix is called as (3.613–helix). It indicates 3.6 amino
acid residues in 1 complete turn of helix which is formed by 13 atoms includ-
ing hydrogen bonds as in Fig. 3.7.
Example: Alpha-keratin, ferritin

2. Beta-Pleated Sheet Structure
• It was William Astbury, a molecular biologist who initially proposed beta
sheet structure in 1930. Later on, Pauling and Corey elucidated the beta sheet
structure and called it as beta sheet. The beta sheet structure of protein is dis-
tinctly different from alpha helical rod structure.
• A beta sheet is formed by polypeptide chains which are called beta
strands. There can be two or more beta strands in a beta sheet as in
Figs. 3.8 and 3.9.
• The beta strands have almost completely extended (unfolded) form. It con-
tains nearly four to ten amino acid residues. Adjacent beta strands are held
together by two or more hydrogen bonds which gives the sheet a pleated

Fig. 3.7  Alpha Helix

CR H
OC Intra chain
hydrogen bond
CR H
O 5.4A˚ pitch
R C C N C
3.6 Amino acids coil
H N C OH H O
R C R
H
C RH O CH
HN
HC
N
HH

CN

O

3.4  Structural Organization of Proteins 49
Amino Terminal
Fig. 3.8  Beta-pleated parallel RR Amino Terminal
sheets

CN C NH2
O H RO

NH R O
C NH R

NH2
R

Fig. 3.9  Beta-pleated anti- R
parallel sheets
H
CN C NH Amino Terminal
O H RO

HN O R HN
C

COOH Carboxy Terminal

R

appearance. The distance between two amino acid residues is 3.5 Å in the

beta strand. The peptide bonds in adjacent beta strands are placed adjacent to

each other. The side chains in adjacent strands are outwardly directed.
• A beta sheet can have adjacent beta strands either placed in the same

direction (parallel) or in the opposite direction (antiparallel) or follow
mixed pattern. Generally, beta sheet contains four to ten beta strands.
• In antiparallel arrangement of beta sheet, the N-terminus of one strand
faces C-terminus of adjacent strand as in Fig. 3.9. The hydrogen bonds are

placed perpendicular to strand and are planar. The space between a pair of

hydrogen bonds is less. Antiparallel beta sheet structure is highly stable.
Example: Superoxide dismutase enzyme, fibroin in silk
• In parallel arrangement of beta sheet, all the N-terminals of all strands
are placed in the same direction as in Fig. 3.8. The hydrogen bonds are non-­

planar and the hydrogen bond pairs are greatly spaced. This arrangement is
comparatively less stable. Example: Flavodoxin
• In mixed pattern of arrangement in sheet, a few strands run in parallel
direction, while others are directed in antiparallel direction. Example:
Carbonic anhydrase enzyme
3. Beta-Bends (Reverse Turns)
Beta-turns are the frequent structural motifs in protein molecule. It was
described by Venkatachalam in 1968.
• A beta-turn is made up of four amino acid residues. These are designated
as (i, i+1, i+2, i+3).These residues do not form alpha helix. The alpha carbon

of residue (i) and alpha carbon of residue (i+3) are located at a distance less

than 7 Å. The amino acid residues are held by intra-chain hydrogen bonding.

50 3  Protein and Amino Acids
Fig. 3.10  Beta bends
Beta bend

-C-Terminal

-N-Terminal

Polypeptide chain
Polypeptide chain

-N-Terminal

-C-Terminal

Beta-turn is the region in polypeptide chain where it reverses its direction by
180° and coils upon itself. These conformations are also called as reverse
turns as in Fig. 3.10.
• Beta-turns are of two types as type I and type II. These types differ from
each other by psi and phi angles at amino acid residues i+1 and i+2.
• Beta-turn determines the shape of protein molecule. It helps the polypeptide
chain to assume a globular shape. Generally, the turns are present on the outer
surfaces of proteins. So it may help in the recognition of an antigen. Also, it
may provide surface for the attachment of an antigen.
4 . Triple Helix
• Collagen protein forms a triple helix structure. It is called tropocollagen.
• It is made up of three polypeptide chains which run in parallel direction.
Each chain is composed of 1000 amino acid residues. The glycine, proline,
and hydroxyproline are the constituent amino acids as in Fig. 3.11.
• Each polypeptide chain has a repeating proline residue, and the chain is called
as poly-proline polypeptide chain. Within the chain, alpha carbon atoms on
(–CO) and (–NH) groups are located on the opposite side of peptide bond.
Thus chain is called as left-handed poly-proline II helix. Each chain under-
goes right-handed coiling around each other to form a triple helix.
• The polypeptide chains are linked by hydrogen bonds. The hydrogen is
donated by (–NH) group of glycine and oxygen of carbonyl group (–CO) on
the proline. Three poly-proline helices are tightly packed in triple helix. There
are 3.3 amino acid residues in triple helix. Every third residue in triple helix
is glycine. There is a repeating sequence of glycine-AA2-AA3, where AA2
and AA3 can be proline and hydroxyproline.

3.4  Structural Organization of Proteins 51

Collagen polypeptide chains [precursors]

Triple helix procollagen

Fig. 3.11  Triple helix collagen

3.4.4 T ertiary Structure

Tertiary structure can be defined as “steric relationship of distantly placed
amino acid residues in a polypeptide chain.”

Characteristics
• Tertiary structure is the three-dimensional conformation of the entire polypep-

tide chain. It is composed of a single polypeptide chain. The peptide groups form

the backbone of tertiary structure.

• The side chains of distant amino acid residues undergo various bond formations

between each other. These interactions produce several folds and super folds in

polypeptide chain. The distant amino acid residues come closer to each other in

the chain as in Fig. 3.12.
• The three-dimensional conformation is a physiologically active protein and

is called native protein.
• Tertiary structure is stabilized by five types of non-covalent bonds as:

–– Hydrogen bonds: These are non-covalent and weak electrostatic force of

attraction. Hydrogen bonds are formed between the polar side chains in amino

acid residues.
Example: Arginine, histidine, glutamate, aspartate, lysine, serine, tyrosine
–– Ionic bonds (electrostatic bonds): These are non-covalent bonds which are

formed between oppositely charged side chains in amino acid residues.
Example: Arginine, histidine, glutamate, aspartate, lysine

52 3  Protein and Amino Acids
Fig. 3.12  Tertiary Protein

SS H
H
H
H
SS
NH2
S
S

H SS

COOH Amino terminal

Carboxy terminal

–– Hydrophobic interactions: These are force of attraction between non-polar
side chains of amino acid residues. These are main forces that stabilize ter-
tiary structure of proteins.
Example: Alanine, leucine, isoleucine, phenylalanine

–– Van der Waals forces: These are very weak forces that occur between non-­
polar side chains.

–– Disulfide bonds: These are formed by oxidation of sulfhydryl groups in cys-
teine amino acid residues in side chains.
Example of tertiary structure: Myoglobin

3.4.5 Q uaternary Structure of Protein

Quaternary structure is the three-dimensional arrangement of two or more
folded polypeptide chains.

The folded protein subunits are positioned relative to each other in the qua-
ternary structure. These subunits are held by non-covalent forces.

• The complex aggregate of protein subunits is called as oligomer. It is biologi-
cally functional.

• Constituent polypeptide chains are called as monomers as in Figs. 3.13 and
3.14.

3.4  Structural Organization of Proteins 53
Monomer Monomer

NH2 NH2

COOH COOH

NH2 NH2

COOH COOH

Fig. 3.13  Quaternary Protein

Based on the number of monomeric units, quaternary structure is called as:

• Dimer (two monomers), for example, creatine phosphokinase is dimer
• Trimer (three monomers), for example, collagen, porins
• Tetramer (four monomers), for example, hemoglobin, immunoglobulin
• Pentamer (five monomers), for example, GABAA receptor
• Hexamer (six monomers), for example, viral capsides

Similarity or Dissimilarity of Monomers

• In a quaternary protein, monomers can be similar or dissimilar.
• Proteins with similar monomers are called as homodimer or homoteramer.

Example: Apoferritin is a protein. It is made up of 24 homo-monomers.

• Proteins with dissimilar monomers are called as heterodimer or heterotetramer.

Example: Hemoglobin is a heterotetramer. It is made up of two alpha and
two beta polypeptide chains.

Immunoglobulin is a heterotetramer. It is made up of two heavy chains and
two light chains.

54 3  Protein and Amino Acids
Primary structure
Fig. 3.14 Proteins Secondary structure
Organization
Tertiary structure

Quaternary structure

3.5 A mino Acids Definition

Amino acids are the organic compounds containing amino and carboxylic
groups.

These are fundamental structural components of proteins. Amino acids have
essential nutritive value for living organisms.

3.8  Important Characteristics of Amino Acids 55

3.6 N umber of Amino Acids

More than 300 amino acids have been discovered and isolated. There are only 20
amino acids in nature which are necessary for biosynthesis of all the proteins. It is
the definite and unique sequence of amino acids in a polypeptide chain that enable
the synthesis of large number of proteins.

3.7 Structure of Amino Acid

It is an amino-carboxylic acid that is made of five components.

• Asymmetric carbon atom (C)
–– It is the central carbon in amino acid. It linked to four different groups. So it
is a chiral carbon. All amino acids except glycine contain chiral carbon. This
carbon atom is placed immediately next to carbon of carboxylic group and is
also called as α-carbon, and amino acid is called as α-amino acid. All 20
amino acids are alpha in nature.
–– The amino acids have (d) and (l) configurations. In (d) amino acid, the amino
group is positioned on the right-hand side of alpha carbon. In (l) amino acid, it
is positioned on the left-hand side of alpha carbon in the molecule. In nature, all
alpha amino acids have (l) configuration except glycine. Amino acid serine has
both (d and l)) configurations. d-serine is found in the cell wall of bacteria.

• Amino group (–NH2)
• Carboxylic group (–COOH)
• Hydrogen atom (H)
• Hydrocarbon side chain (R)

–– Hydrocarbon side chains provide identity to amino acids. Side chain may
contain acidic or basic groups. It may contain hydrophilic or hydrophobic
groups. These are necessary for the steric conformation of proteins. They
determine the structure and properties of proteins as in Fig. 3.15a.

3.8 I mportant Characteristics of Amino Acids

1. Stereoisomerism
Amino acids carry chiral carbon atom except glycine. So amino acids have

two isomers. The (d) amino acids carry amino group on the right-hand side,
while (l) amino acids have amino group on the left-hand side of the chiral car-
bon as in Fig. 3.15b.
2. Optical Isomerism

All amino acids except glycine rotate the plane polarized light into right- (d)
and left-hand side (l) as in Fig. 3.15.

56 3  Protein and Amino Acids

a structure of amino acid b stereoisomerism in amino acid H

Hydrogen Atom H

H H2N C COOH HOOC C NH2
R R
H2N C COOH

Amino group R Carboxylic L - Amino acid D - Amino acid
Group

Side chain

cH H H

H3N+ C COOH H3N+ C COO– H2N C COO–

R R R
Acidic PH Isoelectric PH Alkaline PH

Cation Zwitterion Anion

Fig. 3.15 Zwitterion

3. Zwitterions
Amino acids possess (COOH) and (NH2) groups. The carboxylic group

donates proton and acts as acid. The amino group accepts proton and acts as
base. This is amphoteric property. Amino acids with amphoteric property are
called as ampholytes as in Fig. 3.15c.

So the carboxylic group becomes negatively charged, while amino group
becomes positively charged. At a particular pH, amount of negative and positive
charge equals each other, and the net charge on amino acid is zero. Amino acid
behaves as dipole ion and is called zwitterion. It is a neutral dipolar ion.

The pH at which amino acid carries no charge and it does not migrate toward
any electrode in an electric field is called as isoelectric pH.

3.9 C lassification of Amino Acids

Depending on various criteria, amino acids can be classified in different ways:

• Classification depending on position of amino group
• Classification depending on proteinogenic property
• Classification depending on polarity of side chain
• Classification depending on chemical structure of side chain
• Classification depending on nutritional value
• Classification depending on chemical property

3.9.1 Classification Depending on Position of Amino Group

Amino acids can be subclassified into three groups depending upon position of
amino group.

3.9  Classification of Amino Acids 57

Table 3.1  Showing name and nature of 20 amino acids Nature of amino acid
Essential
Serial number Name of amino acid Abbreviation Essential
Essential
1 Methionine Met Essential
2 Valine Val Essential
3 Isoleucine Ile Essential
4 Leucine Leu Essential
5 Phenylalanine Phe Essential
6 Threonine Thr Semi-essential
7 Tryptophan Trp Semi-essential
8 Lysine Lys Nonessential
9 Histidine His Nonessential
10 Arginine Arg Nonessential
11 Alanine Ala Nonessential
12 Asparagine Asn Nonessential
13 Aspartic acid Asp Nonessential
14 Cysteine Cys Nonessential
15 Glutamic acid Glu Nonessential
16 Glutamine Gln Nonessential
17 Proline Pro Nonessential
18 Serine Ser
19 Tyrosine Tyr
20 Glycine Gly

• Alpha amino acids
–– Amino group is attached to a carbon atom immediately next to carbox-
ylic group and is called as alpha amino group. Carbon atom is called as (C1)
or alpha (α) carbon.
Example: All 20 amino acids have alpha amino groups in Table 3.1

• Beta amino acids

–– Amino group is attached to a carbon atom next to alpha carbon and is called

beta (β) amino group, and carbon is called as (C2 ) or beta (β) carbon.
• Gamma amino acids

–– Amino group is attached to a carbon atom next to beta carbon and is called

gamma amino group, and carbon is called as (C3 ) or gamma (γ) carbon.

3.9.2 C lassification Depending on Proteinogenic Property

According to proteinogenic property, amino acids can be subclassified into two
groups:

1 . Proteinogenic amino acids
The amino acids that can be utilized for synthesis of proteins in living organ-

isms are called as called proteinogenic amino acids. These amino acids are coded
by DNA through triplet codons and also called as standard amino acids.
Example: 20 amino acids as in Table 3.1

58 3  Protein and Amino Acids

Out of 20 amino acids, 9 proteinogenic amino acids are essential as they cannot
be synthesized by body tissues and have to be supplemented with diet, while 1
proteinogenic amino acid is semi-essential as its physiological need is raised dur-
ing periods of pregnancy and active growth of children. The remaining ten pro-
teinogenic amino acids are synthesized in the body and called as nonessential.

Example: Essential and nonessential amino acids are proteinogenic as in
Table 3.1.
2. Non-proteinogenic amino acids

The amino acids that cannot be incorporated into structure of proteins during
their biosynthesis in body tissues are called non-proteinogenic amino acids.
They exist in cytoplasm either as metabolites or structural components of non-
protein compounds. They are also called as nonstandard amino acids.

Example: Gama amino butyric acid, hydroxyproline, ornithine, citrul-
line, carnitine, glycine

3.9.3 C lassification Depending on Polarity of Side Chain

Amino acids are subclassified into two groups depending on affinity of side chain to
water molecules:

1 . Hydrophobic amino acids
The hydrocarbon side chains do not react with water molecules and are

termed as non-polar side chains. Such amino acids are called as hydrophobic or
non-polar amino acids.

Example: valine, alanine, tyrosine, phenylalanine, proline, tryptophan as
in Table 3.1
2. Hydrophilic amino acids

The hydrocarbon side chains have high affinity for water molecules and are
termed as polar side chains. Such amino acids are called as hydrophilic or polar
amino acids. The side chains in these amino acids can be either positively charged
or negatively charged.

Example: serine, asparagine, threonine, glutamine, histidine, tyrosine in
Table 3.1

3.9.4 C lassification Depending on Nutritional Value

Amino acids are necessary nutritional components tissues. Depending on their
nutritional value, amino acids can be subclassified into three groups:

1. Essential amino acids
These amino acids are not synthesized by body tissues and have to be supple-

mented with diet to fulfill their physiological demand. They are also called as
indispensable amino acids.

Example: as listed in Table 3.1

3.9  Classification of Amino Acids 59

2. Semi-essential amino acid
It is also called as conditionally essential amino acid as its essentiality in the

body is determined by developmental stage of children and pregnancy and lacta-
tion among women. The amino acid has to be supplemented with diet during
these conditions.

Example: as listed in Table 3.1
3. Non-essential amino acids

These amino acids are synthesized in the body and do not have to be supple-
mented with diet.

Example: as listed in Table 3.1

3.9.5 Classification Depending on Chemical Structure of Side
Chain

Amino acids can be subclassified into different groups depending on chemical
structure of hydrocarbon side chains.

1. Simple amino acid
The side chain has the simplest structure in these amino acids as in Fig. 3.16.

Example: Glycine, alanine
2. Branched chain amino acids

The side chains are short and branched as in Fig. 3.17. Example: Valine,
isoleucine, leucine

Fig. 3.16 Simple Protein H
amino acids H H2N – C – COOH

H2N – C – COOH CH3
H Alanine

Glycine H
H2N – C – COOH
Fig. 3.17 Branched H H
chain amino acids CH2
H2N – C – COOH HC – CH3 H2N – C – COOH
CH – CH3 CH3 CH – CH3
CH3 Leucine CH2
CH3
Valine
Isoleucine

60 3  Protein and Amino Acids

Fig. 3.18  Hydroxy amino HH
acids
H2N –∝C – COOH H2N – C – COOH
CH2 – OH CH2 – OH

Serine CH3

Threonine
Hydroxy amino acids

Fig. 3.19 Sulfur-containing HH
amino acids
H2N – C – COOH H2N – C – COOH

CH2 CH2

SH CH2

Cysteine S CH3

Methionine

Sulfur containing A. Acids

Fig. 3.20 Amide-containing H H
amino acids H2N – C – COOH H2N – C – COOH

CO – NH2 CH2
Asparagine CH2
CO – NH2
Glutamine

Amide Group containing A. Acids

3. Hydroxy amino acids

The amino acids have hydroxyl groups in side chains as in Fig.  3.18.
Example: Serine, threonine
4. Sulfur-containing amino acids

The amino acids contain sulfur atoms in the groups attached in side chains
as in Fig. 3.19. Example: Cysteine, methionine
5. Amide groups containing amino acids

The amino acids possess carboxy-amide groups (R–CO–NH2–R) in the side
chains as in Fig. 3.20. Example: Asparagine, glutamine

Abovementioned groups of amino acids have an open chain structure and
possess one amino and one carboxylic group. So they are called as aliphatic
monoamino-monocarboxylic acids. These amino acids are neutral in
reaction.

3.9  Classification of Amino Acids 61

6. Monoamino-dicarboxylic acids

The amino acids contain an additional carboxylic group in the side chain and

are called as monoamino-dicarboxylic acids as in Fig. 3.21. These are acidic
amino acids. Example: Aspartic acid, glutamic acid
7. Diamino monocarboxylic acids

The amino acids contain additional amino group in the side chain and are

called as diamino monocarboxylic acids as in Fig. 3.22. These are basic amino
acids. Example: Arginine, lysine, and a derived amino acid called as
hydroxylysine

Above two groups of amino acids have open chain structure and called as
aliphatic amino acids. The amino acid histidine is dibasic monocyclic carbox-

ylic acid but it has a heterocyclic structure.
8. Diamino dicarboxylic acids

These amino acids contain carboxy-amide group (–RCO–NH2–) in the side
chain as in Fig. 3.20. Amino acids have two amino and two carboxylic groups.
These amino acids are neutral. Example: Asparagine, glutamine

Fig. 3.21 Monoamino-dicarboxylic H H
acid
H2N –∝C – COOH H2N –∝C – COOH
β CH2 β CH2
COOH γ CH2

Asparatic Acid COOH
Glutamic Acid

Mono amino dicarboxylic Amino acids

Fig. 3.22 Diamino-monocarboxylic HH
acids
NH2 ∝C COOH HN2 ∝C COOH
β CH2 β CH2

γ CH2 γ CH2

δ CH2 δ CH2

NH ε CH2

C = NH+ NH3+

NH2 Lysine

Arginine

Diabasic amino acids
(Diamino monocarbaxolic acids)

62 3  Protein and Amino Acids

9. Aromatic amino acids
The amino acids contain aromatic rings in the side chains as in Fig. 3.23.

They are neutral. Example: Phenylalanine, tyrosine
10. Heterocyclic amino acids

The amino acids possess heterocyclic rings in the side chains as in Fig. 3.24.
It contains dissimilar atoms like nitrogen, sulfur, or oxygen in the ring struc-
ture. Example: Tryptophan, histidine. The amino acid tryptophan is neu-
tral and histidine is basic in nature.
11. Amino acids containing amino group in side chain

The amino acids contain amino group (–NH) in side chains as in Fig. 3.25.
These amino acids are basic in nature. Example: Proline, hydroxyproline.
The nitrogen is a constituent atom of five-membered ring in these amino acids.
It is free to form peptide bond.

However, these amino acids lack a free amino group (–NH2). Hydroxyproline
is a non-proteinogenic amino acid. It is derived by hydroxylation of proline
during posttranslational modifications. It contains a hydroxyl group attached to
gamma carbon.

Benzene H2 Phenol H2
Ring C Group C
CH NH2 CH NH2
COOH HO COOH
Tyrosine
Phenyl alanine
Aromatic amino acids

Fig. 3.23  Aromatic amino acids

Fig. 3.24 Heterocyclic Indole CH2
amino acids group CH NH2
COOH
N
Tryptophan

Imidazole CH2
group N CH NH2
HN

COOH
Histidine

Fig. 3.25 Proline Pyrrolidine group

COOH
NH
Proline

3.9  Classification of Amino Acids 63

3.9.6 Classification Depending on Chemical Property

Amino acids can be subclassified into three groups based on the reaction in aqueous
medium.

• Acidic amino acids

–– These amino acids have one amino and two carboxylic groups. These are
acidic in nature. Example: Monoamino dicarboxylic acids

• Basic amino acids

–– These amino acids have two amino and one carboxylic group. These are basic
in nature. Example: Arginine, histidine, lysine, and a derived amino acid
named as hydroxylysine

• Neutral amino acids

–– These amino acids have one amino and one carboxylic group. These are neu-
tral in reaction. Example: Aliphatic amino acids, hydroxy amino acids,
aromatic amino acids, heterocyclic amino acids, sulfur-containing amino
acids, amide-containing amino acids (dibasic dicarboxylic acids)

Essential amino acids can be memorized as “TT named PVM is ILL
today,” where TT stands for threonine and tryptophan, while PVM stands for
phenylalanine, valine, and methionine, whereas ILL means isoleucine, leucine,
and lysine.

• Selenocysteine is a rare amino acid and is considered the 21st amino acid.
In it, cysteine contains selenium instead of sulfur. It is coded by termina-
tion codon named as UGA among prokaryotes, eukaryotes, and humans. It
contains trace element, selenium having an antioxidant property. Example:
Glutathione peroxidase enzyme contains selenocysteine.

• Pyrrolysine is another rare amino acid and considered as 22nd in number.
It is coded by UAG codon. It is present in methane-producing archaebac-
teria. Pyrrolysine amino acid is absent in the body of humans.

• Tyrosine amino acid is necessary for biosynthesis of catecholamines like
thyroxin, adrenaline, and nor-adrenaline. It is necessary for melanin pig-
ment synthesis.

• Methionine is coded by AUG codon and it starts the translation. Activated
methionine, S-adenosyl methionine (SAM), is a methyl donor to lipids,
proteins, and nucleic acids.

• Histidine undergoes decarboxylation to form histamine. It initiates allergy
and immune reaction.

• Trytophan is necessary for synthesis of nicotinic acid and serotonin.
• Glycine is necessary for synthesis of heme.

64 3  Protein and Amino Acids

3.10 Applied Biochemistry

3.10.1 Arginine

• It is an essential amino acid. Its oral health benefit has been tried. Kleinberg
(2002) proved that topical preparation containing calcium carbonate and argi-
nine bicarbonate gets deposited over the open dentinal tubules. The arginine
molecule helps to block open dentinal tubules and minimizes dental
hypersensitivity.

3.10.2 Casein Phosphopeptides (CPP)

• It has been demonstrated that milk phosphor-protein, casein has a role in the remin-
eralization of dental enamel. Casein phosphopeptides (CPP) are extracted from milk
casein by tryptic digestion. Reynolds (1997) observed that topically administered
CPP complexes are absorbed into the dental plaque. It has affinity for calcium and
phosphate from saliva and results into an increase in localized concentration of cal-
cium and phosphate over tooth surfaces.

• Casein phosphopeptides (CPP) possess sequences of (-Pse-Pse-Pse-Glu-Glu-),
where Pse indicates “phosphoseryl” residue. These residues stabilize calcium
and phosphate ions in oral cavity. These ions are precipitated into amorphous
calcium phosphate (ACP) complex. The CPP molecule combines with ACP to
form complexes, CPP-ACP.

• The ACP molecules over tooth surfaces can be converted into octacalcium phos-
phates. It is an intermediate of hydroxyapatite crystals in dental enamel.

• The CPP-ACP nanoclusters are formed at pH between 5 and 9.
• The CPP-ACP complexes can inhibit growth of Streptococcus mutans in dental

plaque.
• The complexes raise oral pH which can prevent dental demineralization.
• The ACP nano-complexes can be transformed into hydroxyapatite crystals and

help to remineralize the incipient dental carious lesion.

• Configuration of a molecule means the arrangement of atoms or groups
within a molecule in three-dimensional space. The isomers are called as
configurational isomers. For example: d-amino acid and l-­amino acid

• Conformation of a molecule means the rotation of atoms or groups about a
bond within a molecule in three-dimensional space. The isomers are called
as conformational isomers or rotamers. For example, peptide bond has cis
and trans conformers in polypeptide chain.

• Carnitine is an amino acid found in skeletal muscles, heart muscles, liver,
and kidneys. It is synthesized in the liver from methylation of lysine. It is
chemically, β-hydroxy-γ-N-trimethylaminobutyric acid

Suggested Readings 65

• Carnosine is a dipeptide made up of alanine and histidine. It is found in
skeletal muscles and brain tissues.

• A tripeptide, creatine, is synthesized by glycine, arginine, and
methionine.

• A tripeptide and antioxidant, glutathione, is synthesized by glycine, glu-
tamic acid, and cysteine.

• Bradykinin is a peptide of nine amino acids. It is a vasodilator.
• Angiotensin I is a peptide of ten amino acids. It is a potent vasoconstrictor.

It is converted into angiotensins II and III by cleavage of two and three
amino acids from angiotensin I, respectively.

Suggested Readings

Kleinberg I (2002) SensiStat. A new saliva-based composition for simple and effective treatment
of dentinal sensitivity pain. Dent Today 21:42–47

Reynolds EC (1997) Remineralization of enamel subsurface lesions by casein phosphopeptide-
stabilized calcium phosphate solutions. J Dent Res 76:1587–1595

Branden C, Tooze J (1999) Introduction to protein structure. Garland, New York
Murray RF, Harper HW, Granner DK, Mayes PA, Rodwell VW (2006) Harper’s illustrated bio-

chemistry. Lange Medical Books/McGraw-Hill, New York
Van Holde KE, Mathews CK (1996) Biochemistry. Benjamin/Cummings, Menlo Park, CA

Plasma Proteins 4

Plasma proteins constitute important organic component in plasma. They are
also called as serum proteins. Plasma proteins are comprised of simple as well as
conjugated proteins. The average concentration of plasma proteins is 7.4 g%, and it
varies between 6.5 g% and 8.4 g% under normal condition of health.

Amount of plasma proteins and relative proportion of individual proteins in
plasma are affected by diseases. Therefore, plasma proteins have diagnostic and
prognostic significance.

4.1 Classification of Plasma Proteins

A large number of plasma proteins have been identified and separated from plasma.
Various methods have been utilized to separate plasma proteins. The concentration of dif-
ferent proteins in plasma is highly variable. Today, around 100 plasma proteins have been
identified. Majority of proteins exist in plasma in trace amount and belong to subclasses of
major plasma proteins. It is difficult to classify plasma proteins. A list of plasma proteins
is given based on their fractions in electrophoretic method of separation (Table 4.1).

4.2 P lasma Proteins

Individual plasma proteins are described in the following portion as below:

4.3 A lbumin

• Characteristics
–– Albumin is a globular protein. It is protein made up of single polypeptide
chain. It has 610 amino acids.

© Springer Nature Singapore Pte Ltd. 2019 67
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_4

68 4  Plasma Proteins

Table 4.1  Showing different types of plasma proteins

Type of plasma protein Concentration of plasma protein
3.5–5.5 g/dl
1. Albumin 2.5–3.5 g/dl
2. Globulins 0.1–0.4 g/dl
60–140 mg/dl
(α1)-Globulins <1 μg/dl
(α1)-Acid glycoprotein (orosomucoid) 200–400 mg/dl
(α1)-Fetoprotein 0.4–0.8 g/dl
(α1)-Antitrypsin 30 mg/dl
(α2)-Globulins 30–200 mg/dl
Ceruloplasmin 0.5–1.2 g/dl
Haptoglobin 200–350 mg/dl
50–100 mg/dl
(β)-Globulins <1 mg/dl
Transferrin 0.7–1.5 g/dl
Hemopexin 150–250 mg/dl
C-reactive protein 600–1600 mg/dl
60–179 mg/dl
(γ)-Globulins 3 mg/dl
IgA 10–70 μg/dl
IgG
IgM 200–400 mg/dl
IgD 5–10 mg/dl
IgE 10–25 mg/dl
7 mg/dl
Other proteins 1 mg/dl
Fibrinogen 1–20 μg/l
Prothrombin
Transthyretin (pre-albumin)
Transcortin
Thyroxine-binding globulin
Transcobalamin I

–– Its molecular weight is 69,000.
–– Albumin constitutes nearly 50–70% of the total plasma proteins.
–– Its normal serum concentration varies between 3.5 and 5.5 g% in adults.
–– Its isoelectric pH is 4.7.
–– Albumin can be precipitated with full saturation of ammonium sulfate.
–– It is synthesized in the liver.
• Functions of Albumin
–– It exerts colloidal osmotic pressure inside plasma in vessels. Albumin is a

macromolecular organic compound. It cannot egress from capillary endothe-
lium. So albumin maintains 70–80% of the total colloidal osmotic pressure in
vessels. In a capillary, COP ranges from 25 to 30 mm Hg, and albumin con-
tributes to COP about 22 mm Hg.
–– Colloidal osmotic pressure or oncotic pressure is the force that draws the
water inside the plasma from interstitial fluid. It helps to maintain water con-
tent of plasma.

4.5  α1-Globulins 69

–– Albumin helps in the transportation of calcium ions, unconjugated bilirubin,
nonesterified fatty acids, and thyroid hormones.

–– Albumin is a transporter of acidic and neutral drugs like warfarin sodium,
penicillin, diazepam, acetyl salicylic acid, and furosemide.

• Clinical Significance
–– A decrease in the concentration of serum albumin is found in liver disease,
protein energy malnutrition, and glomerulonephritis. It results into movement
of fluid from vascular compartment into interstitial spaces. This leads to
edema formation in the body. Generally, it occurs when the serum albumin
concentration falls below 2.5 g%.

4.4 Globulins

• Characteristics
–– Globulins are globular group of proteins. They are insoluble in water.
–– They have molecular weight ranging from 90,000 to 1,300,000.
–– Globulins can be differentiated α-globulins, β-globulins, and γ-globulin frac-
tions with the help of electrophoresis.

4.5 α1-Globulins

4.5.1 Alpha-1: Acid Glycoprotein

• Characteristics
–– It is also called as orosomucoid protein.
–– Its normal serum concentration is 60–140 mg/100 ml.
–– It is synthesized in the liver.

• Clinical Significance
–– It is a carrier of basic drugs like quinidine, propranolol, and morphine.
–– It transports steroidal hormone like progesterone.
–– It is a biomarker for acute inflammation in the body, so it is called as acute-­
phase protein.

4.5.2 Alpha-1 Fetoglobulin

• Characteristics
–– It is present in fetal blood circulation in pregnancy.
–– In adults, its normal concentration is <1 μg/100 ml.

• Clinical Significance
–– This protein is a tumor marker for hepatocellular carcinoma.

70 4  Plasma Proteins

4.5.3 A lpha-1 Antitrypsin

• Characteristics
–– It is a serum trypsin inhibitor, and it inhibits the activity of proteases.
–– Its normal concentration is between 200 and 400 mg/100 ml in adults.
–– It is synthesized in the liver.
–– It is an important acute-phase reactant protein. Its concentration increases in
inflammation and infection as acute injury, liver cirrhosis, hepatocellular car-
cinoma, malignancy, and burns.
–– It protects the tissues from lytic activity of elastase enzyme. It is secreted by
neutrophils, and the enzyme degrades elastin protein in lung and liver tissues.

• Clinical Significance
1 . Lung disease
• Alpha-1 antitrypsin deficiency is a genetic disorder. It is due to presence of
defective alleles such as PiM, PiZ, and PiF. The alleles PiZ in homozygous
state (ZZ) is responsible for severe deficiency of alpha-1 antitrypsin in
blood. The persons with genotypes (PiMM) and (PiZZ) have normal and defi-
cient alpha-1 antitrypsin levels.
• Persons with ZZ genotype have higher susceptibility to chronic
obstructive pulmonary disease and liver cirrhosis.
• Cigarette smoking in ZZ genotype persons predisposes to emphysema and
COPD. Smoking oxidizes methionine 358 residue in alpha-1 antitryp-
sin and renders it inactive.
• Therefore, elastase disrupts lung tissues in the absence of alpha-1 antitryp-
sin as in Fig. 4.1.
2. Liver cirrhosis
• Deficiency of alpha-1 antitrypsin leads to juvenile liver cirrhosis.
3 . Diagnostic Tool
• Alpha-1 antitrypsin has a diagnostic role as tumor marker in malignancy of
gonads.

Fig. 4.1  Flow chart ZZ genotype
showing proteolysis of Absent or low alpha-1 Antitrypsin level

lung tissues

Inhibition of formation of elastase-alpha-1 Antitrypsin complex

Proteolysis of lung tissues by active elastase enzyme

4.6  α2-Globulins 71

4.6 α 2-Globulins
4.6.1 Ceruloplasmin

• Characteristics
–– It is a glycoprotein containing eight copper atoms as cofactor.
–– Its normal serum concentration is 30 mg/100 ml in adults.
–– It is synthesized in the liver.
–– Ceruloplasmin contains about 90% of the total serum copper.

• Clinical Significance
–– It is an important ferroxidase enzyme. It helps in conversion of ferrous ion
into ferric ion.
–– Its serum concentration is decreased in liver disease and mineral deficiency.
–– In Wilson’s disease, copper accumulates in tissues of the liver and brain. It is
an autosomal recessive trait. It is characterized by edema of the legs and abdo-
men, yellow discoloration of the skin, anxiety, emotional disturbance, and
behavior change.
–– In Menkes disorder, deficiency of copper occurs in the body. It is an X-linked
recessive trait. It is characterized by brittle hairs, muscular weakness, growth
deficiency and damage to brain development, and seizures. The disease starts
early in infancy, and generally, baby dies by the age of 2–3 years.

4.6.2 Haptoglobin

• Characteristics
–– Haptoglobin is composed of two light chains (alpha) and two heavy chains
(beta) linked covalently with each other by disulfide bridges.
–– It is synthesized in the liver.
–– Its normal serum concentration is between 30 and 200 mg/100 ml in adults.

• Clinical Significance
–– In normal healthy persons, breakdown of RBCs in blood vessels is a common
event. About 10% of total erythrocytes can undergo hemolysis.
Hemoglobin is released from ruptured erythrocytes within blood vessels.
–– Free heme is highly toxic to tissues of blood vessels. Ferrous iron (Fe++) in
heme undergoes Fenton reaction to produce reactive oxygen species (ROS) in
blood vessels. These can damage lipid layer, protein, and DNA.
–– Haptoglobin binds with free hemoglobin through its alpha chain and
forms a haptoglobin-hemoglobin complex. This complex cannot cross
through glomerular filtration.
–– Haptoglobin prevents loss of free hemoglobin in urine.
–– Haptoglobin-hemoglobin complex is captured by macrophages and spleen.
Free hemoglobin undergoes biodegradation.
–– Haptoglobin has antioxidant and cytoprotective functions.

72 4  Plasma Proteins

–– Haptoglobin is an acute-phase protein. Its serum level increases in acute
inflammation and infection.

–– Haptoglobin helps in diagnosis of hemolytic anemia. Its serum concen-
tration decreases in hemolytic anemia.

4.7 β-Globulins

4.7.1 T ransferrin

• Characteristics
–– Transferrin is an iron-containing glycoprotein in plasma.
–– It is synthesized in the liver.
–– Its normal serum concentration is 200–350 mg/100 ml in adults.
–– Transferrin is a carrier of iron in plasma.
–– Transferrin is made up of a single polypeptide chain called as “apo-­
transferrin” which binds with two ferric ions and is called as transferrin.

• Clinical Significance
–– Transferrin helps in the distribution of iron in ferric state. It delivers iron
to bone marrow for biosynthesis of hemoglobin.
–– Transferrin has a positive role in providing innate immunity.
–– Serum transferrin concentration is increased in iron deficiency anemia.
–– Serum transferrin concentration is decreased in liver cirrhosis, glomerulone-
phritis, protein-energy malnutrition, acute infection, and burns.

4.7.2 C -Reactive Protein

• Characteristics
–– It is a beta globulin protein present in plasma.
–– It is a pentameric protein. It is made up of five polypeptide chains.
–– Its normal concentration is less than 1 mg/100 ml in adults. Aging, pregnancy,
inflammation, and burns increase serum level of C-reactive protein.
–– It is synthesized in the liver.
–– This protein has a capability to react with antigenic polysaccharide of group
C present in pneumococci, so-called C-reactive protein.
–– C-reactive protein has the ability to bind with phosphocholine in the plasma
membrane of dead cells and bacteria. C-reactive protein-phosphocholine
complex activates complement system and helps in the activation of T lym-
phocytes and macrophages.

• Clinical Significance
–– It is an acute-phase reactive protein. Its serum concentration increases in
acute inflammation and infection. So it is a non-specific biomarker for inflam-
mation and infection. It is helpful in the prognosis of a disease.
–– C-reactive protein is a better indicator of inflammation than erythrocyte sedi-
mentation rate.