Comprehensive Biochemistry for Dentistry Textbook for Dental Students

8.11  Ribonucleic Acid (RNA) 175

Fig. 8.25 T-RNA Carrier
molecule end
OH
Amino acid
binding 3'
site

A
C
5' C

Amino acid ΤΨU
receptor arm Loop

Enzyme Stem
site

IV

D loop I III Ribosomal
Anticodon site
II
arm CCG Variable arm

Anticodon
loop

m-RNA
recognition

site

GG G

Codon m-RNA

base. The 3′ end contains a sequence of three bases as CCA (cytosine-
cytosine-­adenine). It is called as CCA tail. It has a free OH group at the tip as
in Fig. 8.24.
Activated amino acid attaches by its COOH group to OH group of CCA
tail.
–– D arm (loop)

This arm contains four base pairs in stem and seven unpaired bases in loop. D
loop contains a modified base called as dihydrouridine (5,6-dihydrouracil

base), a modified base with two hydrogen atoms in uracil.
This arm contains enzymes which are essential for attaching amino acid
with tRNA.
–– TΨC arm (loop)
This arm is located opposite to D arm. Its stem contains five paired bases. Its
loop contains a modified base called as pseudouridine.

176 8  Nucleic Acids

This arm is helpful in recognition of ribosomes.
–– Variable arm
• This arm is without a stem. Its loop contains five to six unpaired bases as in
Fig. 8.25.

Function
• The tRNA molecule carries anticodons to recognize codons of mRNA.  It is

essential for recognizing amino acids.
• It also helps to transfer activated amino acids to surface of ribosomes for protein

synthesis.

Ribosomal RNA (rRNA)
It is a type of ribonucleic acid that is structural component of ribosomes.

Occurrence

• It is present in small and large subunits of ribosomes in cytoplasm.

Synthesis of rRNA

• The rRNA is synthesized by RNA polymerase I (in eukaryotes) in nucleolus and
RNA polymerase (in prokaryotes) in cytoplasm.

Types of rRNA

• There are seven types of rRNA molecules such as:
–– Eukaryotes have 80S ribosomes.
–– Its 60S large subunit contains 28S rRNA, 5.8S rRNA, and 5S rRNA.
–– Its 40S small subunit contains 18S rRNA.
–– Prokaryotes contain 70S ribosomes.
–– Its 50S large subunit contains 23S rRNA and 5S rRNA.
–– Its 30S small subunit contains 16S rRNA.

Structure
• The rRNA molecule constitutes about 80% of total RNA of cell.
• It has highly variable length and its shape is highly folded.
• Structurally, rRNA is made up of polyribonucleotides containing nitrogenous

bases, ribose sugar, and phosphate residues.
• It has a single-stranded structure. In some regions, molecule is highly coiled to

form helix.
• In a helical region of molecule, bases are present in complementary pairs which

are held by hydrogen bonds.
• In non-helical region of molecule, bases are unpaired.

8.12  Translation (Protein Synthesis) 177

Function
• The rRNA forms subunits of ribosomes. They are helpful in protein synthesis.

8.12 Translation (Protein Synthesis)

Definition
Translation is a biological process in which genetic message contained in
mRNA directs the synthesis of polypeptide chain in ribosomes.

8.12.1 Site of Occurrence

• Protein synthesis occurs on the surface of ribosomes either in cytoplasm or rough
endoplasmic reticulum.

8.12.2 P rerequisite of Translation

• Amino acids
Amino acids are raw material for protein synthesis. Human body makes use of
20 amino acids for biosynthesis of different types of proteins. Essential amino
acids are supplemented in diet, and nonessential amino acids are synthesized in
the body.

• Ribosomes
–– Ribosomes are called as protein factories. They are made up of rRNA and
proteins. Ribosomes exist in the cytoplasm in the form of two subunits such
as large and small subunits.
–– Before the initiation of protein synthesis, both subunits of ribosomes assem-
ble to form a ribosome in the presence of Mg++ ions. Further, multiple ribo-
somes group together on the strand of mRNA, and the structure is called as
polyribosomes or polysomes. Each ribosome is separated by a distance of
340 Å. The number of ribosomes in polysome is dependent on the length of
mRNA strand.
–– Large subunit of ribosome has three sites that perform different func-
tions such as:
A site: It is aminoacyl-tRNA site. This site attaches to tRNA carrying acti-
vated amino acid as in Fig. 8.25.
P site: It is peptidyl-tRNA site. It attaches to tRNA holding elongating poly-
peptide chain.
The tRNA that carries first amino acid methionine or formylmethionine
attaches to the P site of ribosome.
E site: It is the exit site. This site releases tRNA which enters cytoplasm and
binds with another activated amino acid as in Fig. 8.25.

178 8  Nucleic Acids

• RNA
Ribonucleic acids serve multiple important functions during protein synthesis.
–– mRNA carries genetic message.
–– tRNA carries anticodons to recognize and transfer activated amino acids.
–– rRNA is the structural basis of ribosomes.

• DNA
DNA is the master macromolecule that controls protein synthesis.

8.12.3 Mechanism of Translation

Protein synthesis is a complex biological process that is regulated by mRNA
molecule. It can be divided into three stages such as:

I nitiation of Polypeptide Chain
Depending upon the events involved in polypeptide biosynthesis, initiation stage
can further be subdivided into the following stages such as:

Phosphorylation of Amino Acids

• The mRNA codes for amino acids. A coded amino acid undergoes phosphor-
ylation to form amino acid-AMP complex with the release of pyrophos-
phate molecule. This complex is called as activated amino acid as in
Fig. 8.26.

• Reaction is catalyzed by aminoacyl-tRNA synthetase enzyme in the presence of
Mg++ ions. Each amino acid is activated by a separate enzyme.

(P site) EPA (A site)
Peptidyl-tRNA 60S amino acyl-tRNA
binding site
binding site 40S Large submit of
(E site) ribosome

Exit site for t RNA Small sub unit of
RNA binding site ribosome

Fig. 8.26  Sites on ribosome

8.12  Translation (Protein Synthesis) 179

Formation of tRNA-Amino Acid Complex

• The tRNA carries anticodons to recognize the coded amino acid.

• The COOH group of activated amino acid attaches to OH group of CCA sequence
at acceptor arm of tRNA. This process is called as charging of tRNA. The reac-

tion is catalyzed by aminoacyl-tRNA synthetase.
• Reaction produces tRNA-amino acid complex with release of a molecule of

AMP and aminoacyl-tRNA synthetase as in Fig. 8.26.

Aminoacyl-tRNA synthetase

Amino acid + ATP Amino acid–AMP Complex + Pyrophosphate

Mg++ ions

Aminoacyl-tRNA synthetase

Amino acid-AMP Complex + tRNA tRNA-amino acid Complex

+ AMP + Aminoacyl-tRNA synthetase

Formation of Translation Initiation Complex

• The subunits of ribosomes remain separated in cytoplasm.
• At the time of polypeptide synthesis, small subunit of ribosome binds with

leader segment on 5′ end of mRNA.
–– In prokaryotes, there is base pairing between ribonucleotides of rRNA and

mRNA.
–– In eukaryotes, 5′ cap end of mRNA helps in attachment of small subunit of

ribosome to leader segment.
• The mRNA carries initiation codon (AUG) which codes for methionine in

eukaryotes and formylmethionine in prokaryotes. The tRNA which carries acti-
vated methionine and formylmethionine has anticodons. There is hydrogen
bonding between anticodons of tRNA and codons of mRNA that helps to bind
initiator tRNA to mRNA as in Fig. 8.27.
• Large subunit of ribosome binds with small subunit in the presence of Mg++ ions.
• An assembly of ribosome, mRNA, and tRNA-amino acid complex is called
as translation initiation complex.
• Initiation factors and Mg++ ions are essential for the formation of translation
initiation complex.
• The tRNA-amino acid (methionine or formylmethionine) complex is located
at P site of ribosome. This marks the initiation of polypeptide chain synthe-
sis as in Fig. 8.27.

180 Amino acid + ATP 8  Nucleic Acids
ppi
Activated amino acid Amino acMygl t+RN+ A syn
3' A thesis
C
C Amino acid - AMP-Enzyme
complex
5'
3' A
C
C

5'

AMP

Amino acyl tRNA
tRNA

Fig. 8.27  Charging of tRNA

Elongation of Polypeptide Chain
It is the successive addition of coded amino acids to the first amino acid on the sur-
face of ribosome to form a polypeptide chain.

Elongation of polypeptide chain can be described in the following steps such as:

• Peptide bond formation
–– The second tRNA-amino acid complex enters ribosome at A site. The tRNA
contains anticodons which help in its pairing with codons on mRNA through
hydrogen bonding.
–– The bond between COOH group of 1st amino acid and OH group in CCA of
1st tRNA is split. The COOH group of 1st amino acid at P site forms a peptide
bond with NH2 group of 2nd amino acid at A site. Reaction is catalyzed by
peptidyl transferase enzyme (ribozyme).
–– The 2nd tRNA located at A site carries a dipeptide (methionine amino acid/
formylmethionine amino acid).
–– The 1st tRNA is located at P site without amino acid as in Fig. 8.28.

• Translocation of tRNA
–– It is the movement of tRNA-amino acid complex from A site to P site on the
surface of ribosome.

8.12  Translation (Protein Synthesis) 3' 181

Initiation mRNA binding site
codon Small sub unit
AUG ribosome
Initiation
5' factor
m-RNA
3'
Initiation
codon
AUG

5'
Small sub unit
ribosome

[mRNA - small subunit ribosome]

Methionine Amino acid

Fist tRNA
Anticodon on tRNA

Large subunit

P Initiation
UAC codon
EA mRNA

5' AUG 3'

Small
subunit

Fig. 8.28  Showing formation of translation initiation complex

–– The tRNA dipeptide moves from A site to P site. GTP is hydrolyzed to pro-
vide energy for the translocation of tRNA.

–– The 1st tRNA (uncharged) moves from P site to E site on ribosome. It is dis-
charged from E site to cytoplasm. It is again charged with another amino acid
as in Figs. 8.28 and 8.29.

182 8  Nucleic Acids

5' P AAG Methionine (MET)
mRNA Arginine (ARG)
EA
UAC 3'
AUG

5' PA Methionine
Arginine
E
UAC AAG 3'
AUGUUC

2nd tRNA

tRNA released 3rd tRNA
for reuse Lysine

GGC

EP A 3'

5' ME T A A G
AUG UUC GGC

EP A Release Polypeptide
factor released

5' 3' 5' 3'
UAA

Fig. 8.29  Elongation of polypeptide chain

One cycle of elongation of polypeptide chain includes:

• Attachment of successive tRNA-amino acid complex at A site on ribosome
• Formation of peptide bond between already existing amino acid residue at P site

and a successive amino acid residue at A site
• Translocation of uncharged tRNA from P site to E site
• Translocation of tRNA-growing peptide complex from A site to P site

This cycle takes about 0.1 s to complete. With every successive cycle, peptide
chain elongates by one amino acid residue.

8.12  Translation (Protein Synthesis) 183

Methionine (MET) Arginine (ARG) MethioAnrginineine (ARG)

5′ P 3′ GGC5′ P 3′
m RNA EA AAG E

UAC UAC AAG

Aug Aug UUC

t RNA released for 2nd t RNA] 3rd t RNA
reuse Lysine

EPA 3′
5′ MET-ARC

AUG UUC GGC

Fig. 8.30  Diagram showing release of Nascent Polypeptide chain

As the ribosome moves in 5′ → 3′ direction on mRNA, it leads to expression of
all codons in the coding region of mRNA.  Therefore, all coded amino acids are
bonded by peptide bonds to form a polypeptide chain.

Elongating polypeptide chain is kept attached to a particular ribosome.

Termination of Polypeptide Chain
• Ribosome reaches termination codon region at 3′ end of mRNA.  Termination

codon stops addition of successive amino acid to elongating chain.
• A release factor attaches at A site of ribosome. It hydrolyzes a bond between

polypeptide chain and tRNA at P site of ribosome.
• Polypeptide chain is released from mRNA as in Fig. 8.30.
• Ribosome detaches from termination codon of mRNA.  It dissociates into two

subunits as in Fig. 8.30.

184 8  Nucleic Acids

A newly synthesized polypeptide chain is called nascent polypeptide chain.
It has a primary protein structure as in Fig. 8.30.

8.12.4 P ost-translational Modification of Polypeptide Chain

It is an elaborate biochemical process of altering nascent polypeptide chain after the
synthesis of polypeptide chain is complete.

Types of Post-translational Modifications

1. Proteolytic Splitting
• Nascent proteins have larger size (precursor) in comparison to active form of
protein. For example, parathormone and insulin are synthesized in precursor
forms. The proteins are proteolyzed in the Golgi body and release active pro-
teins. This process is also called as trimming.

2. Covalent Modification
• Covalent modification is the alteration in chemical structure of nascent pro-
tein through addition or removal of functional groups. It occurs by the fol-
lowing reactions such as:
–– Hydroxylation
It is the addition of hydroxyl groups in the nascent protein. In synthesis of
collagen, proline and lysine amino acids are hydroxylated into hydroxypro-
line and hydroxylysine. These reactions require ascorbic acid as coenzyme.
–– Glycosylation
It is the addition of glycosyl group to proteins containing serine, threonine,
and asparagine amino acid residues.
–– Iodination
It is the addition of iodine atom to tyrosine ring during synthesis of thyroxine.
–– Phosphorylation
It is the addition of phosphate group to proteins containing serine, threo-
nine, and tyrosine residues.
–– Carboxylation
It is the addition of CO2 molecule to glutamic acid residue in clotting factors.

Suggested Readings

Alberti KGMN (ed) (1978) Recent advances in clinical biochemistry. Churchill Livingstone,
London

Berg JM, Tymoczko JL, Stryer L, Clarke ND (2002) Biochemistry. W. H. Freeman, New York
Calladine CR, Drew HR, Luisi BF, Travers AA (2003) Understanding DNA: the molecule & how

it works. Elsevier Academic, Amsterdam
Conn EE, Stump PK (1969) Outline of biochemistry, 2nd edn. Wiley, New Delhi
Davidson JN (1965) Biochemistry of nucleic acids, 5th edn. Wiley, New York
Korenberg A (1980) DNA replication. W. H. Freeman, New York

Enzymes 9

9.1 D efinition

Enzymes are defined as highly specific biomolecules capable of catalyzing bio-
chemical reactions.

Commonly, enzymes are polypeptide in nature except a few RNA molecules
(Ribozymes) which possess enzymatic property.

Enzymes are synthesized by the cells of the body. They have catalytic potential
and increase the rate of a biochemical reaction.

9.2 C haracteristics of Enzymes

• Enzymes are proteinaceous in nature.
• They are heat labile.
• They form colloidal solution.
• They have high specificity.
• They can accelerate a biochemical reaction.
• They can be denatured by heat, UV rays, X-rays, and alcohol.
• Enzymes can be simple proteins.
• Examples: lactase, sucrose, maltase, and malate isomerase
• Enzymes can be conjugated proteins. Such enzymes are called as

holoenzymes.
–– Holoenzyme has a protein part and nonprotein part as in Fig. 9.1.
–– Protein part of enzyme is called apoenzyme and nonprotein part is called as

prosthetic group.
Examples:
Aminotransferase + Pyridoxal phosphate
Glyceraldehyde-3-phospho dehydrogenase + NAD+
Glucokinase + Mg++

© Springer Nature Singapore Pte Ltd. 2019 185
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_9

186 9 Enzymes

Carbonic anhydrase + Zinc++
Alcohol dehydrogenase + Zinc++
• Enzymes can be monomeric and oligomeric in structure.
Examples:
Monomeric enzyme: lactase and glucokinase
Oligomeric enzymes: Alcohol dehydrogenase (dimer)
Lactate dehydrogenase (tetrameric)
• Multienzyme Complexes
These are stable clusters of more than one enzymes which are non-covalently
linked with each other. The complex catalyzes a biochemical reaction in a cas-
cade manner as in Fig. 9.2.
Pyruvate dehydrogenase complex catalyzes the conversion of pyruvic acid into
acetyl CoA-SH.
• Multienzyme: It is the enzyme that performs multiple catalytic functions. It is
due to presence of different domains on the enzyme. These domains have sepa-
rate catalytic function.

For example, bacterial DNA polymerase-1 has following properties:5′–3′
Deoxyribonucleotides polymerization activity3′–5′ Exonuclease activity

Fig. 9.1 Holoenzyme HOLOENZYME

APOENZYME + PROSTHETIC GROUP

(protein part) (non-protein part)

CO-ENZYME OR CO-FACTOR

Pyruvate dehydrogenase complex
=

Pyruvatedehydrogenase+Dihydrolipoyldehydrogenase+Dihydrolipoyl transacetylase
+

Thiamine pyrophosphate+Lipoic acid+CoA-SH+FAD+NAD+Mg++
Fig. 9.2  Pyruvate dehydrogenase complex

9.3  Nomenclature and Classification of Enzymes 187

9.3 N omenclature and Classification of Enzymes

Enzymes act on the substances and transform them into products. The substances
which are transformed are called as substrates. The enzymes are named by addition
of suffix “ase” to the name of a substrate. For example, sucrose is converted into
glucose and fructose by enzyme sucrose. Enzymes like trypsin, chymotrypsin, and
pepsin are the exception to this rule of nomenclature of enzymes. Some enzymes
exist in inactive forms and are called as zymogen, for example, fibrinogen and pep-
sinogen. This method of nomenclature creates ambiguity.

The International Union of Biochemistry and Molecular Biology (IUBMB)
in 1964 adopted a system of enzyme classification. This system was based on the
type of chemical reaction which an enzyme can catalyze. It provided an Enzyme
Commission number (EC number) for every enzyme. According to IUBMB system
of enzyme classification,

• Each enzyme is described by EC number. It is followed by four digits sepa-
rated by points.

• First digit represents the enzyme class.
• Second digit represents subclass.
• Third digit represents sub-subclass.
• Fourth digit represents serial number of enzyme.

Example: Aminotripeptidase has nomenclature as EC3.4.11.4

• EC 3 represents hydrolase enzyme.
• EC 3.4 represents hydrolase enzyme that acts on peptide bond.
• EC 3.4.11 represents hydrolase enzyme that cleavages amino acid from

amino-terminal.
• EC 3.4.11.4 represents hydrolase enzyme that act on tripeptides.

According to suggestions of Nomenclature Committee by IUBMB, enzymes
can be categorized into six main classes, as described below:

• Class 1. Oxidoreductases
These enzymes catalyze oxidation and reduction reactions.

AH + B → A + BH (reduction)
A + O → AO (oxidation)
Examples: Lactate dehydrogenase, alcohol dehydrogenase, and glutathione
reductase
• Class 2. Transferases
These enzymes catalyze transfer of a group from one substrate to another.

AD + C → A + DC
Examples: Hexokinase, alanine transaminase, and aspartate transaminase
• Class 3. Hydrolases
These enzymes catalyze hydrolysis of peptides and glycosidic or ester bonds
by addition of water molecule.

188 9 Enzymes

AB + H2O → AOH + BH
Examples: Trypsin, pepsin, and esterase
• Class 4. Lyases
These enzymes catalyze cleavage of a large substrate without addition of
water.
Examples: Aldolase and fumarase
• Class 5. Isomerases
These enzymes catalyze isomerization of substrate.
Examples: Phosphohexoisomerase and retinal isomerase
• Class 6. Ligases
These enzymes catalyze the addition of two substrates.
Examples: DNA ligase and glutamine synthetase
• Six classes of enzymes can be remembered by OTH-LIL.

9.4 E nzyme Specificity

Definition
Enzyme specificity is the ability of enzyme to act upon a substrate and catalyze
a particular biochemical reaction.

It is the property of an enzyme. Enzyme specificity determines the ability of
enzyme to act upon one or more substrates. Higher the specificity, fewer the sub-
strates on which enzyme can act. It is dependent on three-dimensional structure of
active sites on the surface of enzyme.

Types of Enzyme Specificity

1 . Absolute Specificity
Absolute specificity is a property of an enzyme when it acts upon only one par-
ticular substrate. It is a rare phenomenon. Enzyme with absolute specificity can
catalyze only a specific biochemical reaction.
Examples
• Urease catalyzes cleavage of urea.
• Lactase acts on lactose.

2. Group Specificity
Group specificity is a property of an enzyme when it acts on molecules contain-
ing specific functional groups.
Examples:
Trypsin acts on polypeptides which contain arginine and lysine.
Pepsin hydrolyzes peptide bonds among hydrophobic amino acids like, phe-
nylalanine, tryptophan, and tyrosine.
Hexokinase acts on 6-carbon containing monosaccharides.
Chymotrypsin hydrolyzes peptide bonds among aromatic amino acids.

3. Bond Specificity
Bond specificity is a property of an enzyme when it acts on specific type of
chemical bonds.

9.5  Mechanism of Enzyme Action 189

Examples:
Protease catalyzes hydrolysis of peptide bonds.
Lipase cleavages ester bonds.
Glucosidase acts on glycosidic bonds.
Group specificity and bond specificity are collectively called as relative
specificity.
Absolute specificity and relative specificity together constitute substrate
specificity.
4. Stereochemical Specificity

Stereochemical specificity is a property of an enzyme when it acts on specific stereo-

isomer. A substrate can be either laevorotatory or dextrorotatory, depending on its

optical property. Stereochemically active enzymes exhibit selectivity for optically

active substrate. Stereochemical specificity is also called as optical specificity.
Example:
d-amino acids and l-amino acids undergo oxidative deamination by d-­
amino acid oxidase and l-amino acid oxidase enzymes.
5. Reaction Specificity

Reaction specificity is a property of an enzyme when it catalyzes only a specific

biochemical reaction of a substrate. Any substrate can undergo various types of

biochemical reactions. Each reaction of a substrate is catalyzed by a particular

enzyme.
Examples:
Oxaloacetate undergoes condensation reaction with acetyl-CoA in the pres-
ence of citrate synthase enzyme.
Oxaloacetate is converted into aspartate by transaminase enzyme.
Oxaloacetate is decarboxylated and phosphorylated into phosphoenolpyru-
vate by PEP-carboxykinase enzyme.

9.5 Mechanism of Enzyme Action

Enzymes are organic molecules synthesized by living cells. They accelerate bio-
chemical reactions. Generally, the rate of enzyme-catalyzed reactions is around
103–1016 times faster than nonenzymatic reactions. Enzymes are highly specific and
can differentiate between optical isomers.

Several theories have been proposed to explain the mechanism of enzyme
action which are described below:

9.5.1 T heory of Activation Energy

Characteristics of Theory

• A biochemical reaction is accompanied by a continuous change in the energy
system.

190 9 Enzymes

• In ground state:
–– All molecules of a substrate must possess specific amount of kinetic energy.
–– Substrate molecules must undergo collision with each other.
–– Collisions must be in proper orientation. A molecule with its reactive side
must collide with the reactive side of another molecule. Otherwise, collisions
will be unproductive.

• Ground state is a stable state and has the lowest energy. Free energy of sub-
strate molecules is higher than product molecules in ground state. So the reaction
moves in a forward direction.

• Transition state:
–– It is a state of maximum energy along a reaction coordinate. It is a measure of
reaction progress in a reaction pathway.
–– Old bonds in substrate molecules are broken; new bonds are formed.
–– It is an unstable state.

• The difference in energy between ground state to transition state is called as
activation energy. It is the minimum amount of energy needed to transform all
molecules of a substrate from ground state to transition state as in Fig. 9.3.

• The rate of reaction is dependent on the magnitude of activation energy. The
higher the magnitude of activation energy, the lower the rate of a reaction.

• Enzyme binds with substrate and forms an enzyme-substrate complex.
Enzyme helps to orient the colliding molecules. So enzyme lowers the activa-
tion energy. Now more substrate molecules reach the transition state in a given
time period and are transformed into product. This accelerates the rate of
reaction.

LOWERING OF ENERGY ACTIVATION
ACTIVATION ENERGY
ENERGY
ACTIVATION
WITH ENZYME ENERGY

REACTION
WITHOUT ENZYME

PRODUCTS

TIME

REACTANTS

REACTION
WITH ENZYME

Fig. 9.3  Diagram showing Activation Energy of Chemical Reaction

9.5  Mechanism of Enzyme Action 191

• Enzyme does not change the energy level of substrates and products. It does not

alter the equilibrium constant of a reaction as in Fig. 9.3.
• Examples:

–– Hydrolysis of sucrose by acid requires 26,000 cal/mol, whereas sucrase-­
induced hydrolysis requires 10,000 cal/mol.

–– Nonenzymatic decomposition of hydrogen peroxide requires 18,000 cal/
mol, whereas catalase-induced decomposition requires 2000 cal/mol.

9.5.2 E nzyme-Substrate Complex Theory

In 1903, Victor Henri suggested that enzyme (E) binds with substrate (S) to form
enzyme-substrate (ES) complex.

Later on, in 1913, Leonor Michaelis and Maud L.  Menten promulgated
Michaelis-Menten Enzyme Kinetics Theory of enzyme action.

Postulates of Michaelis-Menten Enzyme Kinetics Theory

• Enzyme (E) binds with substrate (S) to form an enzyme-substrate complex
(ES complex).

• This complex is formed through non-covalent forces like van der Waal’s forces,
ionic and hydrophobic interactions between enzyme and substrate.

• The ES complex is unstable and dissociates immediately to form product (P) and
enzyme as in Fig. 9.4.

9.5.3 Lock and Key Model of Enzyme Action

It was propounded by Emil Fischer in 1894. This model is also named as
“Fischer’s template theory.”

Active Substrate Products
sites S Enzyme

E

Enzyme Enzyme
substrate
complex

Fig. 9.4  Diagram showing Enzyme-Substrate Complex

192 9 Enzymes

Postulates

• In this model, lock is analogous to enzyme, while key is analogous to
substrate.

• This model describes that enzyme has predetermined shape and enzyme exhibits
rigidly in its shape.

• It provides a pre-shaped and rigid template to substrate molecule.
• As a key with proper shape fits into the lock, similarly, a substrate with compli-

mentary conformation can fit into the enzyme and forms enzyme-substrate com-
plex as in Fig. 9.5.

Limitations of Lock and Key Model

1 . This model could not explain competitive and noncompetitive inhibition of
enzyme.

2 . This model failed to explain the allosteric modulation of enzyme activity.

9.5.4 Induced-Fit Model

Introduction
Despite limitations, Fischer’s model was accepted for over 50 years by the scientist
community. The model explained scientific observations at that time.

In 1930, Haldane suggested that catalysis takes place in a small region of
enzyme. He called the region as “active site.”

In 1959, Daniel E.  Koshland proposed induced-fit model for enzyme
action.

Active Compatible Active
site substrate site Substrate

Enzyme E S ES Enzyme E A No E- S Complex
Analog [Incompatible
E - S Complex substrate]

Fig. 9.5  Diagram showing Lock and Key Model

9.5  Mechanism of Enzyme Action 193

Postulates of Induced-Fit Model

• This model is analogous to “hand in glove,” where hand represents substrate
and glove represents enzyme. As the hand approaches a folded glove, it opens up
and confirms proper hand-in-glove fit.

• This model assumes that enzymes are flexible and active sites are not
pre-shaped.

• The substrate molecule induces conformational changes in the active site. It
helps to orient catalytic amino acid residues in proper position. It optimizes
enzyme-substrate binding. It is necessary for substrate catalysis as in Fig. 9.6.

Active Site of Enzyme

• Enzyme is a macromolecule. Active site is a small region of enzyme where sub-
strate molecule binds.

• Active site is located in a cleft of enzyme. During folding of polypeptide chain,
internal cavities and surface clefts are formed.

• Active site has a three-dimensional structure possessing substrate-binding site
and catalytic site.

• A substrate binding site has large number of hydrophobic groups of amino acid
residues. These groups help in recognition of substrate. Substrate-binding site
attaches with substrate molecule through non-covalent forces.

• Catalytic site may have sulfhydryl group of cysteine and phenolic group of tyro-
sine. These residues bring about breakage of old bonds and formation of new
bonds in substrate molecule. These residues help to lower the activation energy.

• A substrate induces proper alignment of amino acid residues, while, a substrate
analog induces improper alignment of residues.

Enzyme E S ES
Enzyme - Substrate
Active Substrate Complex
site

Fig. 9.6  Diagram showing Induced-Fit Model

Rate of reaction194 9 Enzymes

9.6 Factors Regulating Enzyme Action

Enzyme activity is regulated by several factors. Therefore, rate of an enzymatic
reaction is affected by following factors.

9.6.1 E ffect of Enzyme Concentration

• Rate of enzymatic reaction is directly proportional to enzyme concentration, pro-
vided the substrate concentration is infinite.

• Enzyme is the limiting factor in enzymatic reactions.
• Availability of active sites is increased by increasing the enzyme concentration.

More numbers of substrate molecules occupy active sites and are converted into
product.
• Graphic representation between rate of reaction and enzyme concentration
is “straight line” as in Fig. 9.7.

9.6.2 E ffect of Substrate Concentration

• In the initial phase, rate of reaction is directly proportional to substrate concen-
tration. This point is called as ½ Vmax.

• At half maximal velocity (½ Vmax), about half of the enzymes are occupied by
substrate. Further increase in substrate concentration leads to full saturation of
enzymes. The rate of reaction attains maximal velocity (Vmax).

• Later on, reaction rate decreases and attains the steady state. Afterward, reaction
rate does not increase on increasing the substrate concentration.

• Graphic representation between rate of reaction and substrate concentra-
tion provides a “hyperbolic curve” as in Fig. 9.8.

Fig. 9.7  Diagram showing
effect of enzyme
concentration on rate of
reaction

Concentration of Enzyme

9.6  Factors Regulating Enzyme Action 195

Fig. 9.8  Diagram showing Vmax
effect of substrate
concentration on rate of
reaction

Rate of reaction V

Km
Concentration of Substrate

9.6.3 Effect of Product Concentration

• Substrates are converted into products in reversible reaction. As the concentra-
tion of products increase, the rate of reaction is slowed down. Further increase in
products concentration can lead to an equilibrium stage.

• Additional increase in product concentration can reverse the reaction rate.

9.6.4 Effect of Temperature

• Enzymes are thermolabile. The enzymatic activity is affected by temperature.
• A rise in temperature results into an increase in rate of reaction. Temperature

increases the kinetic energy of substrate molecules. They can easily reach transi-
tion state.
• A rise in 10 °C temperature doubles the rate of enzymatic reaction. This is called
as temperature coefficient (Q10).
• At a particular temperature, rate of enzymatic reaction is maximum and is called
as “optimum temperature.” In the human body, optimum temperature for enzy-
matic activity is 37 °C.
• Further increase in temperature results into decrease in rate of reaction. It is due
to denaturation of enzyme molecules at high temperature.
• Graphic representation between rate of reaction and temperature provides
a “bell-shaped graph” as in Fig. 9.9.
• Human body cannot withstand temperature changes over a wide range. In fever,
significant metabolic changes are observed.

9.6.5 E ffect of pH

• The pH of medium affects the rate of enzymatic reaction.

196 9 Enzymes

Fig. 9.9  Diagram showing
effect of temperature on
rate of reaction

100% Rate of Reaction Optimum Temp

0°C 37°C 75°C
0°C Temperature (0C)

• A rise in pH of medium results into increase in rate of reaction, and it becomes
maximum at “optimum pH.” Enzymes in the human body exhibit maximum
activity at pH between (4 and 9). For example, optimum pH of pepsin, salivary
amylase, and trypsin are 1.6, 6.8, and 8, respectively.

• After optimum pH, rate of reaction decreases.
• The pH affects the orientation of catalytic groups of active site and substrate.

Very low pH inactivates hydrogen bonds in enzyme.
• Graphic representation between rate of reaction and pH of medium is a

“bell-shaped” graph as in Fig. 9.10.

9.6.6 E ffect of Coenzymes and Cofactors

• Coenzymes are organic molecules and cofactors are metallic ions. Most of the
human body enzymes require cofactors and coenzymes for their activity.

• Coenzymes like FAD and NAD and cofactors like calcium, magnesium, and
manganese are necessary for enzyme-substrate formation.

• Rate of enzymatic activity increases in the presence of cofactors and
coenzymes.

9.6.7 Effect of Inhibitors

• The inhibitors decrease the rate of reaction. These substances attach on active
sites, and binding of substrate molecules is decreased.

• Heavy metals like mercury, gold, and cadmium decrease rate of reaction.
• Mercury inactivate the free –SH group of enzymes. It inhibits enzymatic

activity.

9.7  Enzyme Inhibition 197

Fig. 9.10 Diagram 100%
showing effect of pH of
medium on rate of reaction

100% Rate of Reaction Optimum pH

0% 7.4 14
0 pH of Medium

9.7 Enzyme Inhibition

9.7.1 D efinition

Enzymes undergo inactivation and denaturation by several chemical substances.
The substances which hinder enzymatic activity are called as “inhibitors,” and the
phenomenon is called as “enzyme inhibition.”

Depending on the nature of inhibitors and the mechanism involved, enzyme inhi-
bition can be three types.

9.7.2 C ompetitive Enzyme Inhibition Features

• The chemical structure of inhibitor resembles closely with structure of sub-
strate. Inhibitor is a structural analog of substrate molecule.

• Inhibitor has high affinity for active sites on which substrate binds. Inhibitor
competes with substrate for binding to active sites on enzyme.

• Inhibitor combines with enzyme and forms enzyme-inhibitor complex (EI com-
plex). This condition results into decrease in number of substrate molecules
which occupy active sites.

• The degree of competitive inhibition depends on the relative concentration of
inhibitor and substrate as in Fig. 9.11.

• This type of inhibition is also called as “reversible inhibition.” An increase in
substrate concentration helps to terminate competitive inhibition.

• In competitive inhibition, Michaelis constant (Km) is increased. The maximal
velocity (Vmax) remains unchanged.

198 9 Enzymes

I S Substrate S

Competitive Inhibited I
inhibitor E E E-I Complex

Enzyme Enzyme - inhibitor
complex

Fig. 9.11  Competitive Enzyme Inhibition

• Examples:
–– Succinic acid dehydrogenase oxidizes succinic acid into fumaric acid.
Substances like malonic acid and glutamic acid resemble structurally to
succinic acid and inhibit enzyme activity.
–– Bacteria utilize para-aminobenzoic acid (PABA) to synthesize folic acid.
Sulfonamide drug resembles structurally to PABA and binds to enzyme,
dihydrofolate synthetase. Thus drug inhibits the enzymatic activity.
–– Therapeutics makes use of competitive inhibition to treat methanol pois-
ing. Accidental ingestion of methanol has serious health effects. It is
metabolized by alcohol dehydrogenase enzyme. Ethanol is infused in
patient. It competes with methanol for the same enzyme and inhibits
metabolism of methanol in the human body.

9.7.3 Noncompetitive Enzyme Inhibition Features

• Noncompetitive inhibitor does not structurally resemble substrate.
• Noncompetitive inhibitor binds to sites other than active sites on the enzyme.

These sites are called as allosteric sites.
• Inhibitor binds with enzyme covalently and deforms the three-dimensional struc-

ture of enzyme. The enzyme is inactivated and cannot perform catalysis.
• Noncompetitive inhibition cannot be terminated by increasing substrate concen-

tration. So it is an “irreversible inhibition.”
• In noncompetitive inhibition, Michaelis constant (Km) remains the same. The

maximal velocity (Vmax) is decreased as in Fig. 9.12. Examples:
–– Fluoride ion inhibits activity of enolase enzyme. It interrupts glycolysis.
–– Cyanide is an inhibitor to enzyme cytochrome oxidase.
–– Mercury and iodoacetate inactivate: SH free groups in enzymes and

inhibit the activity of sulfhydryl group containing enzymes like cysteine.

9.7.4 Uncompetitive Enzyme Inhibition Features
• Inhibitor does not structurally resemble to substrate.

9.7  Enzyme Inhibition 199

Active Substrate E-S Complex
site

S S
E
Enzyme E
Change in
active site Product
formed

Active
site

Enzyme E S E S
I I
No product
formed

Domain
Non-competitor
inhibitor

Fig. 9.12  Noncompetitive Enzyme Inhibition

• Inhibitor does not bind with free enzyme. But it has high affinity for enzyme-­
substrate complex and binds with ES complex.

• It is an irreversible inhibition. Inhibitor prevents product formation.
• In uncompetitive inhibition, Vmax and Km are decreased.

Example:
–– l-phenylalanine can inhibit human placental alkaline phosphatase

enzyme activity.
–– Lithium can prevent activity of enzyme inositol monophosphatase in

brain by uncompetitive inhibition.

9.7.5 S uicide Inhibition Features

• It is also called as “mechanism-based inhibition.” It is an irreversible
inhibition.

• Inhibitor is a substrate analog.
• It binds to an active site of enzyme. It is modified into a highly active inhibitor

by the enzyme. Thereafter, the modified inhibitor binds with active site cova-
lently and irreversibly, and there is formation of inhibitor-enzyme complex.

200 9 Enzymes

Example:
–– Drug allopurinol exhibits suicide inhibition of xanthine oxidase enzyme.

It is used in treatment of gout.
–– 5-Flurouracil shows suicide inhibition of thymidylate synthase enzyme.
–– Zidovudine shows suicide inhibition of HIV-1 reverse transcriptase

enzyme.
–– Acetylsalicylic acid is a suicide inhibitor of cyclooxygenase enzyme.

Allosteric Enzyme Modulation Features

• Allosteric enzyme is also called as regulatory or key enzyme. This enzyme
regulates a metabolic pathway in the body. Allosteric enzymes exhibit difference
in structure and activity from simple nonregulatory enzymes.

• Allosteric enzymes have an active site and an allosteric site. These are
located on different domains of the same enzyme.

• A substance which binds to allosteric site of an enzyme is called as modulator.
Depending on the structure of modulator, an allosteric enzyme can be homo-
tropic and heterotrophic as in Fig. 9.13.

• In homotropic enzyme, structure of substrate and modulator is identical, while
in heterotrophic enzyme, structure of modulator is different from structure of
substrate.

• Allosteric enzymes exhibit a sigmoidal graph between rate of reaction and sub-
strate concentration, while it is hyperbolic in simple nonregulatory enzymes.

Fig. 9.13  Allosteric Enzyme Modulation

E Catalytic site
Allosteric site
Enzyme
E-S complex
S Allosteric activator
E

A

S Absence of
E-S Complex
E Allosteric
I inhibitor

9.8  Isoenzymes or Isozymes 201

• Modulator which enhances rate of reaction is called as stimulatory modulator as

in Fig. 9.13.

• Modulator which inhibits the rate of enzymatic reaction is called as inhibitory

modulator, and process is called as allosteric inhibition.

• In allosteric inhibition, modulator does not resemble structurally to substrate.

• Allosteric inhibitor binds to allosteric site of enzyme. It modifies the three-­

dimensional shape of active site. It results into failure of substrate to bind to

active site. Allosteric inhibition is irreversible in nature as in Fig. 9.13.

• In allosteric inhibition, Vmax is decreased and Km is enhanced.
Example:
–– Phosphofructokinase is inhibited by ATP.
–– Glutamate dehydrogenase is inhibited by ATP.
–– Hexokinase is inhibited by ATP.
–– Citrate synthase is inhibited by ATP.
–– Pyruvate carboxylase is inhibited by ADP.
–– Carbamoyl phosphate synthetase II is inhibited by UTP.

9.8 I soenzymes or Isozymes

9.8.1 Definition

As recommended by a committee appointed by the International Union of
Biochemistry in 1964,

Isoenzymes are the multiple forms of an enzyme existing in a single species which may
differ variously but catalyze a single biochemical reaction.

Example:

• Lactate dehydrogenase (LDH) has five isoenzymes.
• Creatine phosphokinase (CPK) has three isoenzymes.

9.8.2 Occurrence

• Isoenzymes are widely distributed among unicellular organisms, insects, plants,
amphibians, birds, and mammals.

• In humans, isoenzymes are present in serum and tissues.

9.8.3 Structure of LDH Isoenzymes

• LDH isoenzyme is made up of four polypeptide chains and it is tetramer. Each
polypeptide chain is described as either “H” or “M.”

• An isoenzyme of LDH contains definite number of H and M subunits. Depending
on different proportions, LDH shows five combinations.

202 9 Enzymes

Isoenzymes of LDH
Lactate dehydrogenase (LDH) enzyme exhibits 5 isoenzymes as LDH-1, LDH-2,
LDH-3, LDH-4, and LDH-5. These isoenzymes are described as follows:

LDH-1 has four “H” subunits and designated as (H4). It is predominantly found
in myocardial tissues.

LDH-2 has three “H” and one “M” subunits and designated as (H3 M1). It is
found in reticuloendothelial tissues.

LDH-3 has two “H” and two “M” subunits and designated as (H2 M2). It is found
in lung tissues.

LDH-4 has one “H” and three “M” subunits and designated as (H1 M3). It is
found in renal, pancreatic, and placental tissues.

LDH-5 has four “M” subunits and designated as (M4). It is predominantly found
in the liver and skeletal tissues.

Normal Value

• Normal serum concentration of LDH varies from 60 to 250 IU/L.
• Its concentration rises in acute myocardial infarction, damage to skeletal tissues,

acute hepatitis, pernicious anemia, and carcinoma.

9.8.4 F unction of LDH

• All five isoenzymes have the same function.
• LDH brings about conversion of lactate to pyruvate. It is necessary for produc-

tion of energy through Cori’s cycle.

[E] + [S] [ES] [E] + [P]

(Enzyme) (Substrate) (Enzyme-substrate (Enzyme) (Product)
Complex)

• LDH is the key enzyme in anaerobic respiration. It also catalyzes conversion of
pyruvate into lactate in absence of oxygen.

All five Isoenzymes have same function

LDH brings about conversion of lactate to pyruvate. It is necessary for production of
energy through Cori’s cycle.

Lactic acid Pyruvic acid

NAD+ NADH + H+

LDH is the key enzyme in anaerobic respiration. It also catalyzes conversion of pyruvate
into lactate in absence of oxygen.

Pyruvic acid Lactic acid

NADH + H+ NAD+

9.9 Ribozymes 203

9.8.5 C linical Importance

• Isoenzymes of lactate dehydrogenase have diagnostic value.
• In normal health, serum of an individual contains predominantly LDH-2

isoenzyme.
• In acute myocardial infarction (AMI), cardiac muscles are damaged. They

release LDH-1 isoenzyme in blood circulation and level of LDH-1 rises in serum.
It is helpful in diagnosis of AMI.
• In acute hepatitis, LDH-5 level predominates in serum.
• In muscle fatigue, concentration of lactate rises in skeletal tissues. It results in
acidosis. As a consequence, level of LDH-5 rises in serum.
• In malignancy, serum concentration of LDH increases.

9.9 R ibozymes

Definition

Ribozymes are ribonucleic acid molecules which can catalyze biochemical reactions.

Historical Aspect

• In 1967, Woese C., Crick F., and Orgel L. suggested the catalytic property of
RNA.

• In 1989, Cech T.R. and Altman S. discovered ribozyme and shared a Nobel Prize.
• In 1982, Kelly Kruger et al. coined the term Ribozyme.

Characteristics and Function of Ribozymes

• Ribozymes have highly diverse structure. Ribozymes are categorized into two
groups based on their size.

• Large ribozymes are composed of RNase P, group I, and group II introns. Large
ribozyme has a size varying from a few hundred nucleotides to 3000 nucleotides.

• Small ribozymes have size varying from 35 to 150 nucleotides.
• Ribozymes carry out RNA splicing. It is the editing of nascent mRNA transcript

into mature mRNA. This process cleavages introns and ligate exons in mRNA
transcript. Introns are the noncoding regions in mRNA transcript. They do not
code proteins. Exons are the coding regions of RNA.
• Helps in splicing of unwanted ribonucleotides from primary RNA transcript.
• Ribozymes require divalent magnesium ion (Mg++) for catalytic action.

Natural Ribozymes
RNase P, Group I self-splicing introns, Group II self-splicing introns (Spliceosome),
Hairpin ribozyme, and Hammerhead ribozyme

204 9 Enzymes

9.10 Lysozyme

Definition
Lysozyme is an antimicrobial, proteinaceous substance which provides non-­
specific immunity against pathogens. It is also called as “muramidase” or “N-
acetylmuramide glycanhydrolase.”

Occurrence

• Lysozyme is distributed widely in plants and animals.
• In humans, lysozyme is abundantly found in secretions like tear, saliva, breast

milk, and mucus. Cytoplasmic granules of neutrophils and macrophages secrete
lysozyme.
• Egg white is rich in lysozymes.

Structure

• Lysozyme is composed of a single polypeptide chain of 129 amino acid
residues.

• Its molecular weight is 15,000.
• Under normal physiological condition, polypeptide chain of lysozyme is folded

to form a globular structure. It can unfold and refold rapidly. The polypeptide
chain possesses six sub-sites named as A, B, C, D, E, and F which bind to
substrate.
• Catalytic residues are located in between sub-sites D and E.

Function

• Lysozyme is an integral part of innate immune system.
• Lysozyme is a hydrolase. It cleavages 1,4 beta glycosidic bond between N-­

acetylmuramic acid N-acetyl-d-glucosamine residues in peptidoglycans in bac-
terial cell wall. It kills bacteria.
• Lysozyme in human milk provides innate immunity to infants.
• Lysozymes in tears are antibacterial.
• Serum lysozyme level is a biomarker for diagnosis and prognosis of “sarcoid-
osis” disease.

Suggested Readings

Fersht A (1984) Enzyme structure and mechanism. W. H. Freeman, New York
Lipscomb WN (1983) Structure and catalysis of enzymes. Annu Rev Biochem 52:17–34

Suggested Readings 205

Plowman K (1971) Enzymes kinetics. McGraw-Hill, New York
Tanner NK (1999) Ribozymes: the characteristics and properties of catalytic RNAs. FEMS

Microbiol Rev 23(3):257–275
Walsh C (1979) Enzymatic reaction mechanism. W. H. Freeman, New York
White A, Handler P, Smith EL (1964) Principles of biochemistry, 3rd edn. The Blakiston Division,

McGraw-Hill, New York

Hormones 10

10.1 H ormones

The human body has a neurohumoral system. It plays a dominant role in communi-
cation and coordination among tissues. This system is comprised of a network of
neurons and hormones.

Hormones bind to hormone receptors located on target cells. They initiate a
chemical message which in turn is followed by a cascade of molecular events.
Hormones can influence physiological functions of organs, behavior of organism,
and metabolism of the human body.

10.1.1  Definition

Hormone is defined as an organic molecule that is synthesized in minute quan-
tity by specified tissues and transported by circulation to distant target tissues
to regulate their biochemical and metabolic functions.

Hormones act as the first chemical messenger and signaling molecules.
The word hormone is derived from the Greek word Hormacin which means
urge or excite.
Hormones are secretions of endocrine glands. These glands release secretion
directly into blood circulation. These glands are devoid of ducts.

10.2 C omparison and Contrast

Hormones

• They are synthesized by endocrine glands.
• They belong to a diverse chemical nature.

© Springer Nature Singapore Pte Ltd. 2019 207
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_10

208 10 Hormones

• They are produced in small quantity.
• They are translocated from site of synthesis to site of action.
• They are exhausted in metabolic reaction.
• They act as the first chemical messengers to regulate biological, metabolic, and

physiological functions of body.
• Hyper- or hyposecretion of hormones is manifested as hormonal disorder.

Enzymes

• They are synthesized by exocrine glands.
• They are protein in nature.
• They are also produced in small quantity.
• They are not translocated from site of synthesis.
• They remain unchanged in enzymatic reaction.
• They act as biocatalyst. Enzymes increase the rate of reaction.
• Deficiency of enzyme also manifests as a disorder.

Vitamins

• Vitamins are nutritionally important chemical substances (micronutrients). They
are supplemented with diet.

• Vitamins have diverse chemical nature.
• Vitamins are required in small quantity.
• They are distributed by circulation.
• Vitamins act as coenzyme in metabolic and enzymatic reactions.
• Deficiency of vitamins also manifests as disorder.

10.3 Classification of Hormones

10.3.1 Depending Upon Chemical Structure

Hormones can be classified into three groups as follows:

• Peptide/Protein Hormones
These hormones are peptide in nature. These hormones are hydrophilic. They are
transported in circulation in unbound state. They have short duration of action.
Peptide hormones are unable to pass across plasma membrane of target cells.
Structurally, they are either polypeptides or short peptides.
Examples: Insulin, glucagon, calcitonin, pituitary hormones, and
parathormone

10.3  Classification of Hormones 209

• Steroid Hormones

These hormones are lipid in nature. They are lipophilic. They are transported in

circulation in bound state with protein carriers. They have longer duration of

action. These hormones can pass through plasma membrane of target cells.
They are derived from cholesterol. Structurally, these hormones contain cyclo-
pentano-perhydro-phenanthrene nucleus, also called as sterane nucleus.
Examples: Adrenocorticosteroids, androgens, progesterone, and estrogens
• Amino Acid Derivatives

These hormones are derived from tyrosine amino acids. They bind with protein

carriers in blood circulation. They have longer duration of action.
Examples: T3 and T4 hormones (thyroid hormones), adrenaline, and nor-
adrenaline (catecholamines)

Peptide Hormones
Peptide hormones are made up of small chains of amino acids. They are pro-
tein in nature.

Peptide hormones along with secretory glands are enlisted as follows:

Peptide hormones along with secretory glands are enlisted as follows:

Antidiuretic Hormone

Oxytocin Hypothalamus - Pituitary Gland

Parathormone Parathyroid Gland

Calcitonin Thyroid Hormone

Insulin

Glucagon Pancreas (Islets of Langerhans)
Somatostatin

Human Chorionic Gonadotropin Placenta

Steroid Hormones
Steroid hormones are derived from cholesterol and have cyclopentano-per-
hydro-phenanthrene nucleus (sterane). These hormones are lipid in nature.

Steroidal hormones along with their secretory glands are enlisted as follows:

210 10 Hormones
Steroidal hormones along with their secretory glands are enlisted as follows:

Aldosterone Adrenal Cortex (Corticosteroids)
Cortisol
Corticosterone
11-Deoxycorticosterone

Testosterone Gonads (Sex Steroids)
Dihydrotestosterone
Dehydroepiandrosterone
Androstenedione
Estrogen
Progesterone

Amino Acid Derivative Hormones
The following hormones are derived from tyrosine amino acid.

Amine hormones along with their secretory glands are enlisted as follows:

Adrenaline Adrenal Medulla
Nor-adrenaline
Dopamine

The above hormones are called as catecholamine hormones.

Thyroxine (T4) Thyroid Gland
Tri-iodothyronine (T3)

10.3.2  D epending Upon Nature of Site of Action

• Trophic Hormones
These hormones act on endocrine glands. They control growth of target endo-
crine gland.
Examples: Anterior pituitary hormones (TSH controls proliferation of thy-
roid gland and gastrin controls proliferation of enterochromaffin cells in
gastric mucosa)

10.4  Mechanism of Action of Hormones 211

• Non-trophic Hormones
These hormones act directly on target cells and influence cell functions.
Examples: Insulin, glucagon, and thyroxine

10.4 Mechanism of Action of Hormones

Hormones are ligands (endogenous or exogenous molecules that bind to specific
biomacromolecules). Hormones act through receptors (biomacromolecules).

Depending upon structure and location, there are different receptors. Therefore,
hormonal action is mediated through receptor-specific mechanisms which are
described as follows:

1 . Intracellular receptor-based mechanism of hormonal action:
• Type I nuclear receptor-based mechanism of hormonal action
• Type II nuclear receptor-based mechanism of hormonal action:

2. Cell-surface receptor-based mechanism of hormonal action
• cAMP as 2nd messenger-based mechanism of hormonal action
• cGMP as 2nd messenger-based mechanism of hormonal action
• Inositol 1,4,5-triphosphate as 2nd messenger-based mechanism of hor-
monal action
• Diacylglycerol as 2nd messenger-based mechanism of hormonal action
• Calcium as 2nd messenger-based mechanism of hormonal action
• Tyrosine kinase receptor-based mechanism of hormonal action

10.4.1  I ntracellular Receptor-Based Mechanism of Hormonal
Action

Definition
A receptor which is situated inside the cell is called as intracellular receptor.

Nuclear receptor is a type of intracellular receptor. It can bind with DNA and
control gene expression. Therefore, nuclear receptors belong to the family of
transcription factors. In human genome, 48 genes for nuclear receptors have been
identified.

Structure of Nuclear Receptor
Nuclear receptor is a transcription factor, and it has a molecular weight of nearly
70,000 daltons. It has the following domains discussed below.
N-terminal Domain

• This domain exhibits extreme variability in size and sequence of amino acid resi-
dues among different receptors. It modulates the process of gene transcription.

212 10 Hormones

DNA-Binding Domain

• DNA-binding domain has strictly conserved sequence. It contains two zinc fin-
gers (small supersecondary protein structures having coordination with zinc
ion). Zinc finger helps in binding with specific sequence of DNA, which is called
as hormone response element (HRE).

Hinge Region

• This domain is flexible and links DNA-binding domain with ligand-binding
domain (LBD).

Ligand-Binding Domain

• This domain has variable sequence. This domain attaches to ligand. It provides
surface for ligand-induced dimerization of receptor.

• It attaches with coactivator protein (protein which stimulates transcription of
RNA) and corepressor protein (protein which represses gene expression).

C-terminal Domain

• This domain has variable amino acid sequence in different receptors as Fig. 10.1.

Fig. 10.1 Structural N – terminal
Organization of (Nuclear domain

receptor)

DNA – Binding (DBD)
domain

Hinge region

Ligand binding (LBD)
domain

C – Terminal
domain

10.4  Mechanism of Action of Hormones 213

Based on the type of nuclear receptor, the two types of mechanisms of hormonal
action are as follows:

1 . Type I nuclear receptor-based mechanism of hormonal action
2 . Type II nuclear receptor-based mechanism of hormonal action

T ype I Nuclear Receptor-Based Mechanism
It is described in the following steps.

Association of Receptor with Heat Shock Proteins (HSP)
In the absence of hormone (ligand), type I nuclear receptor is located in cytosol.
It is linked with heat shock proteins (proteins synthesized cell in stressful condi-
tions). Examples: Glucocorticoids and mineralocorticoids hormones

Dissociation of HSP
Lipid-soluble hormones (steroid hormones) can rapidly pass through plasma mem-
brane by simple diffusion. Within cytoplasm, hormone can either move in free
state or in bound state with carrier protein. It is transported intracellularly to recep-
tor to form hormone-receptor complex. Receptor undergoes conformational
changes, and it releases heat shock proteins.

Hormone-Receptor Dimerization
Hormone-receptor complex is a monomer. It binds with a similar monomer to form
a dimer (homodimerization).

Translocation of Receptor Dimer
Dimer is translocated to nucleus. Within the nucleus, the receptor binds directly to

hormone response element of DNA. Hormone response element is a short and spe-

cific sequence of DNA. It is located in promoter region of gene. It binds with hor-

mone-receptor complex.
Promoter region is a short sequence of about 100–1000 nucleotide base pairs

in length, located on sense strand of DNA towards 5′ side. It activates
transcription.

Receptor-DNA complex recruits coactivator proteins. They transcribe down-

stream DNA to mRNA which regulates synthesis of proteins as in Fig. 10.2.

Proteins control cellular functions (like proliferation, differentiation, maturation,

metabolism, survival, and apoptosis).
Examples of hormones: Androgens, estrogens, progesterone, and

glucocorticoids

T ype II Nuclear Receptor-Based Mechanism
Type II nuclear receptor is located in the nucleus. In absence of ligand, it is associ-

ated with corepressor proteins.

214 10 Hormones

Hormone (steroid hormones)
(ligand)

Cell membrane

HSP

Heat Cytosol
shock nuclear receptor - hormone
protein complex
(HSP)
Receptor
Type I dimerization
nuclear
receptor HRA Coactivator RNA polymerase
DBD mRNA Nuclear membrane
DBD

LBD

DNA

mRNA

Protein
synthesis

Cell Function

Fig. 10.2  Mechanism of action of Type I nuclear receptor

Lipid-soluble hormone like thyroid hormone can easily pass through membrane
by facilitated diffusion. Within cytoplasm, it is translocated into nucleus. Hormone
binds with ligand-binding domain (LBD) of receptor to form hormone-receptor
complex (H-R complex). Receptor undergoes conformational changes and releases
corepressor proteins.

Hormone-receptor complex undergoes heterodimerization with retinoid X recep-
tor. The heterodimer attaches to HRE of DNA. Receptor recruits coactivator pro-
teins and RNA polymerase as in Fig. 10.3.

Heterodimer activates promoter of gene and transcribe mRNA. It controls pro-
tein synthesis which in turn regulates cell functions.

Examples: Thyroid hormone receptor, retinoid X receptor, and retinoic acid
receptor

10.4  Mechanism of Action of Hormones 215

Hormone Cell membrane
(Thyroid hormone)

Cytoplasm

Hormone responsive DBD Coactivator Nuclear pore
element [HRE] LBD
Type II N.R. mRNA Receptor
DNA dimerization
Nucleus
Corepressor
Corepressor Ribosome
mRNA
detached Protein
synthesis
mRNA
Cell Function
RNA polymerase

Fig. 10.3  Hormonal mechanism of action based on type II nuclear receptor

10.4.2  C ell-Surface Receptor-Based Mechanism of Hormonal
Action

C yclic AMP as 2nd Messenger-Based Mechanism
Cyclic AMP (cAMP)

• It is described as 3′,5′-cyclic adenosine monophosphate or 3′,5′-cyclic ade-
nylic acid. It is composed of adenine, ribose sugar, and phosphate group.

• Cyclic AMP differs from AMP in attachment of phosphate group. In cAMP,
phosphate group is attached at 3′and 5′carbon positions in cyclic manner in
ribose sugar, while in AMP, phosphate group is attached either attached at 5′C or
3′C in ribose sugar.

216 10 Hormones

• cAMP is a hydrophilic molecule and acts as 2nd messenger in cytosol.
• It is synthesized from ATP by action of adenylate cyclase enzyme and Mg++ ions.

It is hydrolyzed by 3′,5′ nucleotide phosphodiesterase enzyme.

Adenylate cyclase
ATP cyclic AMP + Pyrophosphate

Mg++

Phosphodiesterase

Cyclic AMP + H2O 5AMP

Mechanism

Activation of G-Protein-Coupled Receptor

• G-protein-coupled receptors:
–– They belong to the largest family of plasma membrane receptors (cell-surface
receptor or transmembrane receptor). They are integral proteins of plasma
membrane (proteins that span across the entire membrane).
–– The receptor has extracellular domain (N-terminal), transmembrane
domain (made up of seven transmembrane alpha-helices) and intracel-
lular domain (C-terminal).The seven transmembrane alpha-helices are
linked to three extracellular loops and three intracellular loops.
–– G-protein-coupled receptors are involved in the following three signal
transduction pathways as:
cAMP-dependent pathway
IP3-dependent pathway
DAG-dependent pathway

• Hormone binds with either N-terminal tail or extracellular loop or seven trans-
membrane alpha-helices (ligand-binding domain). Hormone induces conforma-
tional change in receptor molecule. Receptor enters in activated state.

Activation of G-Protein

• G-protein
–– G-protein is called as guanine nucleotide-binding protein. It acts as intracel-
lular molecular switch. It is “inactive” when bound to GDP and becomes
“active” when attached to GTP molecule.
–– It relays signal from external stimuli to interior of cell.
–– G-protein is a heterotrimeric protein. It is composed of α-, β-, and γ-subunits.
The β- and γ-subunits are tightly folded to form a stable dimer (Gβγ).
–– The Gα-subunit is associated with GDP in “inactive state.” The G-protein-
coupled receptor is bound to Gα-subunit of G-protein.

10.4  Mechanism of Action of Hormones 217

Cell membrane Hormone
Cytosol (1st messenger)

G-Protein
coupled
receptor

Adenyl cyclase cAMP G-Protein (GS)
ATP
GDP
2nd messenger GTP
Activated
Phosphorylates G-Protein
protein
Cell function

Protein kinase Activated
(Inactive) Protein kinase

Fig. 10.4  CAM as second messenger based hormonal action

• Activated G-protein-coupled receptor binds with GTPase enzyme. This enzyme
in turn catalyzes dissociation of GDP from Gα-subunit of G-protein and attach-
ment of GTP molecule with Gα-subunit. This exchange of GDP with GTP on
Gα-subunit is brought about by activated G-protein-coupled receptor.
Therefore, it is called as guanine nucleotide exchange factor.

• G-protein enters in active state. It results into dissociation of Gα-subunit from
G-protein and G-protein-coupled receptor. Gα-GTP-subunit is released to affect
the functioning of target proteins as in Fig. 10.4.

Synthesis of cAMP

• Gα-GTP activates adenylate cyclase enzyme. It is located in the inner side of
the plasma membrane.

• Adenylate cyclase catalyzes conversion of ATP molecule into 3′,5′-adenosine
monophosphate (cAMP) molecule. Enzyme requires Mg++ for its activity.

• The cAMP acts as second messenger.

Role of cAMP

• Protein kinase A is an intracellular protein. It is a tetramer. In inactive state,
it is made up of two regulatory subunits represented as 2R associated with two
catalytic subunits represented as 2C.

• Cyclic AMP molecule attaches to protein kinase A and brings about separa-
tion of 2R subunits from 2C subunits. The 2C catalytic subunits are released and
influence phosphorylation of proteins in cytosol.

218 10 Hormones

• Catalytic subunit of protein kinase A induces phosphorylation of other proteins
in cytosol. It catalyzes transfer of phosphate group from ATP to serine and threo-
nine residues in protein molecules.

• Overall, protein kinase A-induced phosphorylation of proteins controls cell func-
tioning as shown in Fig. 10.4.
Example:
In the liver and skeletal tissues, glucagon induces release of cAMP and activa-
tion of PK-A. Protein kinase A converts inactive phosphorylase kinase into active
form. It in turn causes phosphorylation of phosphorylase B (inactive) into phos-
phorylase A (active). This phosphoprotein regulates breakdown of glycogen in
the liver and skeletal muscles.

C yclic GMP as 2nd Messenger-Based Mechanism of Hormonal Action
The cGMP acts as second messenger in response to hormonal action. Synthesis of
cGMP follows analogous pathway as synthesis of cAMP after binding of hormone
to cell-surface receptor. It is briefly described as follows:

• Hormone (first messenger) binds to surface receptor located on external surface
of membrane.

• Hormone-receptor complex activates guanylyl cyclase enzyme located in the
inner surface of the membrane.

• Activated guanylyl cyclase enzyme acts on GTP as a substrate. GTP molecule is
bound to membrane.

• GTP is converted into cGMP which acts as second messenger. Protein kinase G
is an enzyme which is dependent on cGMP for its activity. Increase in cytosolic
concentration of cGMP activates protein kinase G.

• Activated protein kinase G in turn phosphorylates cellular proteins which control
cell functions.
Example: Phototransduction in rods is regulated by cGMP.

Inositol 1,4,5-Triphosphate (IP3) as 2nd Messenger-Based Mechanism
of Hormonal Action
• Inositol 1,4,5-triphosphate (IP3) is an organic compound. It is composed of

inositol ring and three phosphate residues which are attached to carbon atoms at
1,4,5 positions in the ring. It is a hydrophilic and polar molecule.
• Phosphatidylinositol (PI) is a class of phospholipids. It is located on the inner
portion of phospholipid bilayer in the plasma membrane. It has inositol as an
alcohol.
PI is phosphorylated by kinase enzyme in presence of ATP to form phosphati-
dylinositol-4-phosphate (PIP). It further undergoes phosphorylation by kinase
in the presence of ATP to form phosphatidylinositol-4,5-bisphosphate (PIP2).
PIP and PIP2 are the plasma membrane-bound phosphoinositides.

10.4  Mechanism of Action of Hormones 219

Mechanism
Activation of G-Protein-Coupled Receptor

• Hormone (first messenger) attaches to G-protein-coupled receptor. It is located
on the plasma membrane.

• Hormone induces conformational changes in receptor. It in turn activates plasma
membrane-bound Gq protein (a class of G-protein).

Activation of Phospholipase C

• Phospholipase C is a plasma membrane-bound enzyme. It exists in six isomeric
forms. It can cleavage phosphatidylinositol-4,5-bisphosphate.

• The α-subunit of Gq protein has the ability to activate phospholipase C enzyme
(PLC-β isozyme).

Cleavage of Phosphatidylinositol-4,5-Bisphosphate

• Phospholipase C (PLC-β) catalyzes hydrolysis of phosphodiester bond in
phosphatidylinositol-4,5-bisphosphate.

• There is formation of inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG)
molecules.

• DAG is a hydrophobic molecule. It remains associated with plasma membrane.

Release of IP3 (2nd Messenger)

• Inositol-1,4,5-triphosphate is a polar and hydrophilic molecule. It is released into
cytosol.

• It diffuses freely through the cytoplasm of cell. It moves toward smooth endo-
plasmic reticulum (SER) in a cell or sarcoplasmic reticulum in muscle fiber as in
Fig. 10.5.

Increase in Cytosolic Calcium Concentration

• Membrane of SER contains IP3receptors. These receptors are ligand-gated
calcium channels. Binding of IP3 molecule on its receptor results into the open-
ing of calcium channels. Thus Ca++ ions are released from SER into cytosol.

• Concentration of calcium in cytosol is increased from 0.1 μ moles to 1.0 μ
moles. Under normal condition, calcium concentration of cytosol is kept low

(0.1 μ moles) due to active export of calcium ions from cell by calcium pump.
• IP3 can additionally act on calcium channels in plasma membrane and induce

further increase in calcium concentration in the cell.

220 Hormones 10 Hormones

Phosphatidylinositol PIP2 Hormone
-4-phosphate [PIP] G-protein
DAG coupled receptor
Phosphatidyl G-protein
-inositol [PI] Ca++
DAG
Cell membrane Protein kinase C

Cytosol Hydrolysis Calcium channel
Activation I IP3 gated
Inositol
1,4,5- Triphosphate phospholipase C DAG
Ca++Ca++ Activated
IP3 protein kinase C
IP3 Ca++Ca++Ca++
Ca++Ca++Ca++

Smooth Ca++
endoplasmic

reticulum

Protein
phosphorylation

Regulation of
cell function

Fig. 10.5  DAG & IPT as second messengers based hormonal action

Activation of Protein Kinase

• Protein kinase is present in cytosol and involved in phosphorylation of proteins.
It transfers high-energy phosphate group from ATP to either tyrosine residue
(tyrosine kinase) or serine/threonine residues (protein kinase C).

• Protein kinase contains C1 and C2 domains at N-terminal. These domains can
bind with diacylglycerol and Ca++ ions, respectively. Protein kinase is kept inac-
tive due to binding of pseudosubstrate region (short sequence of amino acid resi-
dues) to catalytic domain.

• Cytosolic calcium concentration regulates activity of protein kinase in cyto-
plasm. Due to ↑ in Ca++ concentration, it binds at C2 domain of protein kinase and
dislodges pseudosubstrate region.

• Protein kinase is switched on as shown in Fig. 10.5.

Regulation of Cell Functions

• Activated protein kinase C catalyzes phosphorylation of cell proteins. The nature
of substrate protein is highly variable. It depends on the type of cell.
Phosphorylated proteins regulate cell functions.

10.4  Mechanism of Action of Hormones 221

Examples:
1. Protein kinase C activation in smooth muscle fibers in GIT causes con-

traction of muscle fibers.
2. Protein kinase C activation in neurons causes nerve cell excitation.
3. It induces secretion of saliva in salivary glands.

D iacylglycerol as 2nd Messenger-Based Mechanism of Hormonal
Action
• Diacylglycerol (DAG) is an organic compound. It is composed of two fatty acid

residues attached to glycerol through ester bonding. DAG exists as 1,2-diacylg-
lycerol and 1,3-diacylglycerol. It is a lipophilic and nonpolar molecule.

• It is produced from hydrolytic cleavage of PIP2 by PLC-β. It is a hydrophobic
molecule and remains bound to plasma membrane.

• DAG attaches to C1 domain at N-terminal of protein kinase C. It activates enzyme
which in turn regulates cell functions.

• DAG acts synergistically with calcium ions on different domains on PK-C.

C alcium as 2nd Messenger-Based Mechanism of Hormonal Action
Calcium Ions

• Calcium ions mediate through allosteric modulation of cell proteins. Calcium
ions act as a ubiquitous 2nd messenger. It controls a wide variety of bio-
chemical reactions and physiological functions.

• Low cytosolic calcium concentration.

–– Calcium ion is an important cation of extracellular fluid compartment. Its

concentration in ECF is 132 meq/L, while concentration of calcium ions in

cytosol is very low (0.1 μM). It is due to calcium pump in cytoplasm which
actively exports calcium ions from cytoplasm to ECF. Additionally, calcium

ions are transported into the smooth endoplasmic reticulum leading to deple-

tion of calcium in cytosol.
• Increase in cytosolic calcium concentration.

–– Phospholipase C catalyzes splitting of PIP2 into diacylglycerol and IP3 mole-
cules. DAG molecule remains in membrane, and IP3 molecule is transported
to cytosol. It binds with IP3-gated calcium channels in SER. Thus, calcium
ions are released into cytosol.

–– Indirectly, calcium channels in the membrane are opened, and calcium ions

are transported inside the cell.
–– These activities result into ↑ in calcium ions in cytosol to a level 0.5–

1.0 μM. Rise in calcium concentration triggers signals for various physi-
ological functions.

222 10 Hormones

Targets of Calcium Ions
Protein Kinase C is the target of intracellular calcium ions.

• Calcium ions bind at C2 domain of protein kinase C.  It leads to activation of
PK-C.

• Activated PK-C in turn phosphorylates other proteins and regulates cell
functions.
Example: Calcium ions in matrix of mitochondria regulate activity of isoci-
trate dehydrogenase enzyme. It is a key regulatory enzyme in TCA cycle.

Calmodulin
• Calmodulin is calcium-modulated protein.
• It is a calcium ion-binding protein. It is made up of 148 amino acid residues.

Calmodulin has four calcium-binding regions. Each one is 12 amino acid resi-
dues long.
• Binding of calcium ions activate calmodulin. It undergoes conformational
changes which affect affinity of calmodulin to proteins.
• Activated calmodulin modulates protein kinases and phosphatases.
Example: Calmodulin regulates excitation-contraction coupling in smooth
muscle fibers. Calmodulin activates calcium-bound myosin light-chain
kinase enzyme. This enzyme in turn phosphorylates head of myosin light
chain. It causes excitation-contraction coupling and contraction of smooth
muscles.
Calcium ions as second messenger regulate secretion, contraction, transmis-
sion of nerve impulse at synapse, fertilization, regulation of nuclear pore, blood
clotting, and transcription.

First Messenger

• It is an extracellular molecule, for example, peptide hormone.
• It is unable to pass through the plasma membrane. It acts as a ligand and

binds with cell-surface receptor. It activates receptor (induces conformation
change in structure).
• Activated receptor brings about release of 2nd messenger by cell.

Second Messenger

• It is an intracellular biomolecule for signaling.
• It receives a signal (chemical message) from cell-surface receptor and relays

it to target molecule in cytosol or nucleus.

T yrosine Kinase Receptor-Based Mechanism of Hormonal Action
Tyrosine kinase receptor is a cell-surface or transmembrane receptor. It is a
glycoprotein.

10.5  Hormones from Hypothalamus 223

Structure

• Most of the tyrosine kinase receptors are made up of single polypeptide chain
(monomer). However, insulin receptor is comprised of two alpha-chains which
are extracellular in position. These chains are interlinked by disulfide bridges.
Two alpha-chains are again attached to two transmembrane beta-chains by disul-
fide bridges.

• Tyrosine kinase receptor has three domains as:
–– Extracellular N-terminal region which contains many domains including
ligand-binding domain
–– Transmembrane helix
–– Cytoplasmic C-terminal region which has tyrosine kinase catalytic domain

Mechanism

• Hormone binds with ligand-binding domain of tyrosine kinase receptor on cell
membrane.

• Hormone-receptor complex induces receptor dimerization on membrane.
• Receptor dimer induces conformation change in the tyrosine kinase domain and

activates it.
• Activated tyrosine kinase catalyzes phosphorylation of tyrosine residues of the

other monomer in a dimer. It is called trans-autophosphorylation.
• Phosphorylated tyrosine kinase controls cell functions.
• For example, tyrosine kinase acts as a receptor for insulin, growth factors

like epidermal growth factor (EGF), platelet-derived growth factor (PDGF),
fibroblast growth factor (FGF), and nerve growth factor (NGF).

10.5 Hormones from Hypothalamus

Hypothalamus synthesizes factors that regulate release of hormones from pitu-
itary gland either through stimulation or inhibition of hormonal release.

These factors are peptides in nature and are called as hypothalamic
hormones.

Depending on action, they are designated as releasing hormones or inhibiting
hormones (statins).

Types of Hypothalamic Hormones
Six types of hypothalamic hormones are as follows:

• Thyrotropin-releasing hormone (TRH)
TRH is a tripeptide composed of pyroglutamate (derivative of glutamate), pro-
line, and histidine. It acts on the anterior pituitary and stimulates release of thy-
roid-stimulating hormone (TSH).

224 10 Hormones

• Corticotropin-releasing hormone (CRH)
CRH is a peptide of 41 amino acid residues. CRH acts on anterior pituitary and
stimulates release of adrenocorticotropic hormone (ACTH).

• Gonadotropin-releasing hormone (GRH)
GRH is a peptide of ten amino acid residues. It acts on the anterior pituitary and
stimulates release of gonadotropins (LH, FSH).

• Growth hormone-releasing hormone (GRH)
GRH is a peptide composed of 44 amino acid residues. It acts on anterior pitu-
itary and stimulates release of growth hormone. GRH is also named as
somatotropin.

• Growth hormone release-inhibiting hormone (GRIH)
GRIH is a peptide of 14 amino acid residues. It acts on anterior pituitary and
inhibits release of growth hormone. GRIH is also named as somatostatin.

• Prolactin release-inhibiting hormone (PRIH)
PRIH is dopamine. It acts on anterior pituitary and inhibits release of prolactin.
The hypothalamus controls the release of hormones and may be regarded as
master hormonal regulator.

10.6 H ormones from Pituitary Gland

Anterior Pituitary Hormones
The anterior pituitary is called as adenohypophysis. It produces tropins or tropic
hormones. These hormones act on endocrine glands and regulate their functions.

10.6.1  Prolactin

Prolactin is also called as mammotropin or lactogenic hormone or luteotropic
hormone.

Structure

• Prolactin is a polypeptide hormone.
• It is made up of 199 amino acid residues.
• Prolactin is homologous to growth hormone in sequence of amino acid

residues.

Secretion

• Prolactin is secreted by lactotrophs, acidophilic cells of the anterior pituitary
gland. Prolactin release-inhibiting hormone inhibits secretion of prolactin.

• Secretion of prolactin is stimulated by estrogen hormone. It is secreted in preg-
nancy and lactation.

10.6  Hormones from Pituitary Gland 225

Normal Serum Value

• In males, 2–10 ng/ml
• In pregnant females, 9–200 ng/ml
• In nonpregnant females, 2–20 ng/ml

Metabolic Functions
Mammotropic Effect

• Prolactin promotes growth of mammary gland and development of alveolar ducts
in breast. It promotes increase in size of breast in pregnancy.

Lactogenic Effect

• It enhances formation of milk proteins after child birth. It stimulates secretion of
milk.

Luteotropic Effect

• Prolactin maintains growth of corpus luteum.
• It stimulates secretion of progesterone from corpus luteum.

10.6.2  Thyroid-Stimulating Hormone
Structure

• Protein component is composed of α- and β-subunits.
• α-Subunit is made up of 92 amino acids. It is identical to gonadotropins (LH,

FSH, and HCG). β-Subunit is made up of 112 amino acids. It is biologically
active. Carbohydrate moiety is made up of oligosaccharides which are non-cova-
lently linked to α- and β-subunits.

Secretion

• TSH is a glycoprotein.
• It is secreted by basophilic cells of anterior pituitary gland.

Normal Serum Value
• Its normal value is 0.5–5 mIU/L.