Comprehensive Biochemistry for Dentistry Textbook for Dental Students

378 13  Metabolism of Proteins and Amino Acids

Amino acid pool is comprised of nearly 90–100 g of amino acids. The amino
acid pool represents a small fraction of total body proteins which constitute
nearly 12 kg in a hypothetical person of 70 kg weight.

Amino acids differ from carbohydrates and lipids in terms of storage in tissues.
Carbohydrates are stored in skeletal muscles and the liver in the form of glycogen.
Lipids are stored in adipose tissues in the form of triglycerides. Unfortunately,
amino acids are not stored in body tissues.

Amino acid pool is maintained by following two factors:

1. Sources providing free amino acids to amino acid pool
2. Sources utilizing free amino acids from amino acid pool

13.1.1 S ources Providing Free Amino Acids into Amino Acid Pool

• Dietary proteins
–– Proteins in diet are important source of amino acids to amino acid pool.
–– It has been observed that 35–50 g of tissue proteins are wasted daily from the
body.
–– It is recommended that about 35–50 g of dietary proteins should be consumed
per day. It helps to augment the protein loss.
–– Dietary proteins are hydrolyzed in gut to release free amino acids.
–– Free amino acids are absorbed by mucosa of the small intestine.
–– Dietary proteins supply free amino acids to the pool. They also provide essen-
tial amino acids to the body.
–– It represents positive nitrogen balance.
–– However, daily recommended intake of proteins should be around 50 g per
day. Protein intake in developed countries is much higher (50–100 g per day).

• Proteolysis of tissue proteins
–– Tissue proteins are continuously degraded and resynthesized through a well-­
regulated mechanism. It is called as turnover of tissue proteins. The rates at
which different proteins are catabolized and resynthesized are highly variable
and depend on type of cell.
–– Glucocorticoids regulate proteolysis of tissue proteins. The proteolytic
enzymes are contained within lysosomes in cells. The mobilized amino acids
from tissue proteins enter amino acid pool and thereafter are transported to
the liver for resynthesis of proteins.
–– The following are pathological conditions:
Acute infections like diarrhea, influenza, typhoid fever
Chronic conditions like tuberculosis, malabsorption syndrome, celiac disease
Burns
Proteolysis of tissue proteins is tremendously increased in diseases.
Released amino acids are transported into the liver where they are resynthe-
sized into acute-phase C-reactive protein and alpha-antitrypsin.

13.1  Amino Acid Pool 379

However, synthesis of plasma proteins like albumin and globulin by the liver
in diseased state declined.
Glucocorticoids regulate proteolysis of skeletal muscles in burns, diarrhea,
and typhoid conditions. On an average, protein loss is about 0.5–1.0 g per kg
body weight per day.
• Biosynthesis of nonessential amino acids
–– Nonessential amino acids are synthesized from the intermediate metabolites
in various metabolic pathways. For example, aspartate and alanine are synthe-
sized by transamination reaction.
–– Amino acid like cysteine is synthesized from essential amino acid, methio-
nine (methionine → homocysteine + serine → cystathione → cysteine).
–– In normal health, input to amino acid pool is in equilibrium with output.
–– Amino acid pool is in dynamic equilibrium.
–– Body has positive nitrogen balance.

13.1.2 S ources Utilizing Free Amino Acids from Amino Acid Pool

Amino acid pool is depleted by the following three sources:

• Biosynthesis of tissue proteins
• Structural proteins like actin, myosin, albumin, and hemoglobin are synthesized

from amino acids of AA pool.
• Biosynthesis of functional proteins like enzymes and hormones is performed

from amino acid pool.
• Biosynthesis of nonprotein nitrogenous substances like urea, creatine, creati-

nine, and xanthine takes place from amino acid pool.
–– Catabolism of amino acids from amino acid pool takes place by deamina-

tion and decarboxylation as in Fig. 13.1.
• Conversion of amino acids into glucose and fatty acids.

A cell can utilize fixed number of amino acids from amino acid pool for its func-
tions. Conversely, it furnishes the same number of amino acids to the amino acid
pool. Such a cell is considered to be in dynamic equilibrium and human body
has nitrogen balance.

Catabolism of Amino Acids
Catabolism of amino acids is essentially focused on the removal of nitrogen
from amino acids. The alpha amino group prevents oxidation of amino acids.
After liberation of nitrogen, carbon skeleton (alpha-keto acid) of amino acid is
metabolized through various pathways.

Three important biochemical mechanisms are described as follows:

1. Transamination
2 . Deamination
3. Transdeamination

380 13  Metabolism of Proteins and Amino Acids

METABOLISM OF AMINO ACIDS

Formation of Daily Tissue protein Total Body
non-essential dietary proteins break down Tissue-proteins
amino acids 300–400 gms/day 10–12 kg in adults
(50–80g) day

Digestion
Absorption

Loss of Amino Acid Pool Tissue protein
amino acids (30g/day) 100g synthesis

in urine 300–400 gms/day

Urea uric acid
creatinine

ATP Synthesis Synthesis Formation of
(10% of calories
requirement 1 day) of Non-Protein

Carbohydrates Nitrogenous

substances
(Purine pyrimidine, Creatline

porphyrins)

Fig. 13.1  Sources and utilization of Amino acids from pool

13.2 Transamination

Definition
Transamination is a biochemical process in which an amino group (-NH2) from
the amino acid is transferred to the α-keto acid to form new amino acid and
α-keto acid.

Characteristics of Transamination

• Transamination involves transfer of an amino group from donor amino acid to
recipient alpha-keto acid. It is an intermolecular transfer of amino group as in
Fig. 13.2.

• In transamination, ammonia is not formed.
• Donor amino acid is converted into new alpha-keto acid. Recipient alpha-keto

acid is converted into new amino acid.
• Transamination is a reversible process.
• In transamination, α-amino group of amino acid is transferred to α-keto acid.

Exception: δ-amino group of ornithine is transaminated.
• Alpha-keto acids, namely, glutarate, oxaloacetate, and pyruvate, act as recipient

of amino group.
• Few amino acids, namely, proline, hydroxyproline, lysine, and threonine, do not

undergo transamination.

13.2 Transamination 381

H COOH H COOH
H2N C COOH + C=0
H2N C COOH + C=0 Transaminase
Pyridoxal R1 R2
R1 R2 phosphate

a - amino acid a - Keto acid PYRIDOXAL New New
PHOSPHATE a - amino acid a - keto acid
(Donor) (Recipient)

AMINO ACID AMINO ACID
(DONOR) (NEW)

TRANSAMINASE TRANSAMINASE

KETO ACID PYRIDOXAMINE KETO ACID
(NEW) PHOSPHATE (RECIPIENT)

ALANINE

ALANINE TRANSAMINASE PYRUVATE
+
+
α – KETO GLUTARATE
serum glutamate GLUTAMATE

pyruvate transamianse

ASPARTATE

ASPARTATE TRANSAMINASE OXALOACETATE

++

α – KETO GLUTARATE serum glutamate GLUTAMATE

oxaloacetate transaminase

Fig. 13.2  Transamination (a) mechanism of transamination (b) alanine transaminase (c) aspartate
transaminase

Site of Occurrence

• Primarily, transamination occurs in the liver, heart, kidneys, and brain tissues.
• However, transamination can take place in all body tissues.

Enzymes for Transamination

• Transamination is catalyzed by transaminase enzyme. It is also called as ami-
notransferase. Transaminases are widely distributed in cytoplasm and mito-
chondria of cells. Transaminases can be two types:

• Group transaminases
These enzymes catalyze transamination of a group of structurally related amino
acids.

• Substrate-specific transaminases
These enzymes catalyze specific amino acids.

382 13  Metabolism of Proteins and Amino Acids

Example:
1. Alanine aminotransferase (ALT) or also called as serum glutamate pyruvate

transaminase (SGPT)
2. Aspartate aminotransferase (AST) or also called as serum glutamate oxalo-

acetate transaminase (SGOT)

Coenzyme for Transamination
Pyridoxal phosphate acts as coenzyme in transamination. It is attached by covalent
bonding to ε-amino group of lysine residue in transaminase enzyme.

Mechanism
Transamination occurs in two steps as follows:

• Step I
Formation of pyridoxamine phosphate
–– Donor amino acid interacts with pyridoxal phosphate bound to transaminase
enzyme. Amino group is transferred to pyridoxal phosphate and results in
formation of pyridoxamine phosphate.
–– Donor amino acid is converted into new alpha-keto acid as in Fig. 13.2a.

• Step II
Transfer of amino group from pyridoxamine phosphate to recipient alpha-­
keto acid
–– Pyridoxamine phosphate transfers its amino group to recipient alpha-keto
acid. There is formation of new amino acid. Pyridoxal phosphate is regener-
ated as in Fig. 13.2a.

Mechanism of Substrate-Specific Transaminases

• Alanine Aminotransferase (ALT)
ALT is abundantly found in the cytoplasm of hepatocytes. It is also present in the
heart and kidneys. Alanine transaminase catalyzes transfers of alpha-amino
group of alanine amino acid to alpha-ketoglutarate. Transamination results in the
formation of glutamate as new amino acid and pyruvate as new alpha-keto acid
as in Fig. 13.2b.

• Aspartate Aminotransferase (AST)
AST is abundantly found in cardiac muscle fibers and liver cells. It is found in
the cytoplasm and mitochondria. It is also present in the kidneys and intestine.
Aspartate transaminase catalyzes transfer of alpha-amino group from aspartate
to alpha-ketoglutarate to form glutamate as new amino acid and oxaloacetate as
new alpha-keto acid as in Fig. 13.2c.

13.3 Deamination 383

Biological Significance

• Transamination provides carbon skeleton of amino acids after removal of
amino group. It is called as alpha-keto acids. Carbon skeleton of amino acids
is variously catabolized as follows:
–– Transamination (ketogenic amino acid) provides acetyl CoA. It serves as

a precursor for cholesterol synthesis. Example: leucine
–– Transamination (glucogenic amino acid) provides pyruvate. It is utilized in

TCA cycle and gluconeogenesis. Example: alanine, glycine, cysteine, and serine
–– Transamination (glucogenic amino acid) provides oxaloacetate. It is uti-

lized in TCA cycle. Example: aspartic acid and asparagine
–– Transamination (glucogenic amino acid) provides alpha-ketoglutarate. It

is utilized in TCA cycle. Example: glutamate, glutamine, arginine, histidine,

proline, and hydroxyproline
–– Transamination (glucogenic amino acid) provides succinyl CoA. It is uti-

lized in TCA cycle. Example: valine, methionine, and threonine
• Transamination helps to synthesize nonessential amino acids. For example,

pyruvate is converted into glutamate.
• Transamination helps to convert surplus amino acids into another amino

acid which is low in amino acid pool. It helps to normalize the amount of nones-

sential amino acids.
• For clinical significance, transamination helps in the diagnosis and progno-

sis of liver and heart diseases. Transaminases are intracellular enzymes. Their

plasma concentration is maintained in a reference range. In liver diseases, hepa-

tocytes are damaged. Transaminases are released in blood circulation, and their

plasma concentration is elevated. This act is utilized in diagnosis and determin-

ing the outcome of disease.

13.3 D eamination

Definition
Deamination is a biochemical process of conversion of amino acid into alpha-­
keto acid with the liberation of ammonia.

Site of Occurrence
Deamination occurs in the liver, kidneys, and brain tissues.

13.3.1 T ypes of Deamination

Deamination is of two types which are as follows:

O xidative Deamination
It is the release of ammonia through breakdown of amino acid in the presence
of molecular oxygen.

384 13  Metabolism of Proteins and Amino Acids

Enzymes for Oxidative Deamination
Two enzymes are present in cells which deaminate D-amino acids and L-amino
acids. They are as follows:

• d-Amino acid oxidase (DAAO)
–– This enzyme catalyzes oxidative deamination of d-amino acids.
–– Enzyme is a flavoprotein (Fp)-dependent enzyme. It requires FAD as coen-
zyme for its activity.
–– In a latest research, DAAO has been shown to be associated with control of
glutamate secretion in CNS. Concentration of DAAO in plasma was found to
be much higher in schizophrenia patients as revealed in postmortem
examination.

• l-Amino acid oxidase (LAAO)
–– This enzyme catalyzes oxidative deamination of l-amino acids.
–– Enzyme is a flavoprotein (Fp). It requires FMN as coenzyme for its activity.
–– LAAO is limited to tissues of the liver and kidneys.

Mechanism of Oxidative Deamination
It occurs in the following two steps:

• Dehydrogenation of amino acid
Amino acid undergoes dehydrogenation to form α-imino acid. Hydrogen atoms
are accepted by flavoprotein (Fp), and it is reduced into FpH2 (FADH2 and
FMNH2) as in Fig. 13.3.

• Spontaneous hydrolysis of α-imino acid
The α-imino acid is an intermediate metabolite. It is an unstable compound. It
undergoes spontaneous hydrolysis (breakdown of a bond by addition of water
molecule) to form α-keto acid with loss of a molecule of ammonia (NH3) as in
Fig. 13.3.

• Decomposition of H2O2
Reduced flavoprotein (FpH2) is further oxidized into (Fp) by supply of molecular
oxygen. This activity results into formation of H2O2. It is toxic to tissues, and it
is decomposed into water and oxygen by catalase enzyme as in Fig. 13.3.

N on-oxidative Deamination
It is the deamination of amino acids in the absence of molecular oxygen in cells.
Non-oxidative deamination takes place by substrate-specific enzymes.

Depending on types of amino acids, non-oxidative deamination has the follow-
ing three types:

Deamination of Hydroxy Amino Acids
Hydroxy amino acids like serine, homoserine, and threonine undergo non-oxidative
deamination. Reaction is catalyzed by serine dehydratase enzyme (l-hydroxy
amino acid dehydratase). It is located in cytoplasm of the liver.

Enzyme requires pyridoxal phosphate as coenzyme.

13.3 Deamination 385

H2O

Catalase O2
H2O2

H FMN FMNOH2
R C COOH α

H2N R C COOH
L - Amino acid
L - Amino acid oxidase HN
O
RC α - Imino acid
H2O
COOH
NH3
α - Keto acid

Fig. 13.3  Oxidative deamination

Fig. 13.4 Non- H2O
oxidative deamination
H2C–OH (Amino acid
of hydroxy amino acids dehydratase)
H–C–N–H
H – Pyridoxal H–C–H
––
COOH Phosphate C–N–H
Serine
H

NH3 H2O COOH

CH3 Imino Acid
C=O

COOH
Pyruvic Acid

• Serine is converted into imino acid with release of water molecule. Imino
acid undergoes hydrolysis to form pyruvate with release of a molecule of
ammonia as in Fig. 13.4.

386 13  Metabolism of Proteins and Amino Acids

Deamination of Sulfur-Containing Amino Acids
Sulfur-containing amino acids like cysteine and homocysteine undergo non-­
oxidative deamination as in Fig. 13.5. Reaction is catalyzed by cysteine desulfhy-
drase enzyme.

Enzyme requires pyridoxal phosphate as coenzyme.

• Cysteine is converted into imino acid with release of H2S.  Imino acid
undergoes hydrolysis to form pyruvate with release of a molecule of
ammonia.

Deamination of Histidine

• Histidine is converted into urocanic acid with release of a molecule of ammo-
nia. Reaction is catalyzed by histidase enzyme as in Fig. 13.6.

Fig. 13.5  Non-oxidative H2C∼SH Desulfhydrase
deamination of sulfur-
containing amino acids HC–N–H H2S
(Cysteine & Homo H H3C
Cysteine ) C = NH
COOH COOH

Imino Acid
H2O
NH3

Pyruvic acid

Fig. 13.6  Non-oxidative Histidine Histidase Urocanic acid
deamination of Histidine NH3

13.4 Transdeamination 387

13.4 Transdeamination

Definition

The α-amino groups from most of the l-amino acids are transferred to α-ketoglutarate.
Reactions are catalyzed by group transaminases and substrate-specific transaminases.

There is formation of glutamate. It is the highly abundant amino acid in amino acid pool.

Glutamate rapidly undergoes oxidative deamination to form α-ketoglutarate with
release of a molecule of ammonia.

Glutamate is the center stage of amino acid catabolism. It is either oxida-
tively deaminated or acts as donor of amino group in synthesis of glutamine
amino acid.

Transdeamination is the sequential transamination of l-amino acids (transfer of

amino group to α-ketoglutarate) to generate glutamate which in turn successively
proceeds to oxidative deamination to regenerate α-ketoglutarate with liberation
amino group in the form of a molecule of ammonia in body tissues.

Site of Occurrence
It occurs in most of body tissues.

Enzymes

Transamination occurs by transaminases.
Oxidative deamination of glutamate occurs by l-glutamate dehydrogenase

enzyme.

• l-Glutamate dehydrogenase
It is a zinc-dependent metalloenzyme.
It is found in mitochondria of most of body tissues.
Enzyme is substrate specific (acts on l-glutamate).
Enzyme requires NAD+ or NADP+ as coenzyme.
It is an allosteric enzyme. Its activity is inhibited by ATP and NADH. Its activity
is stimulated by ADP.

Mechanism

• Step I
Synthesis of l-Glutamate
–– Transamination of l-amino acids brings about synthesis of l-glutamate in
tissues.

• Step II
Dehydrogenation
–– l-Glutamate undergoes dehydrogenation with loss of two hydrogen atoms.
Reaction is catalyzed by l-glutamate dehydrogenase in the presence of
NAD+ as coenzyme. Hydrogen atoms are accepted by coenzyme, and it is
reduced into NADH2. l-Glutamate is converted into α-imino-glutarate as in
Fig. 13.7.

388 13  Metabolism of Proteins and Amino Acids

Alpha Amino Acids Transamination

α-ketoglutarate

COOH ––– –

COOH H–C–H
H–C–H H–C–H
H–C–NH2
H–C–H
α C= O COOH

COOH L-Glutamate

α-ketoglutaric acid NAD+ L-glutamate
NADH2
–– – – Dehydrogenase
(Deamination)

NH3 COOH

H2O H–C–H–– –
H–C–H
α C – NH

COOH

α-imino-glutaric acid

Fig. 13.7  Transdeamination

Hydrolysis
–– The α-imino-glutarate is an unstable compound. It undergoes rapid hydrolysis

to form α-ketoglutaric acid with loss of a molecule of ammonia.
–– Transdeamination is an important pathway to catabolism of N of amino acids

with disposal of ammonia as in Fig. 13.7.

13.5 Urea Cycle

Definition
Urea cycle is the cyclic biochemical pathway in which ammonia is converted
into urea.

Ammonia is a nitrogenous compound. It is produced through different catabolic
pathways. Ammonia is transformed into urea which represents the main excretory
form of nonprotein nitrogenous compound in the body.

13.5  Urea Cycle 389

Characteristics of Urea Cycle

• Urea cycle is also called as Krebs-Henseleit cycle.
• Urea cycle is called as ornithine cycle.
• Urea cycle involves five enzyme-catalyzed reactions.
• In urea, first nitrogen atoms are derived from ammonia, and second nitrogen

atom is derived from alpha-amino group of aspartic acid as in Fig. 13.9.

Site of Occurrence

• Urea cycle takes place in the liver.
• In the kidneys, incomplete urea cycle takes place. Kidneys lack arginase enzyme

for the formation of urea from arginine.

13.5.1 S teps in Urea Cycle

Urea cycle can be described in the following five steps:
Mitochondrial reactions

1. Formation of carbamoyl phosphate
2 . Synthesis of citrulline

Cytosolic Reactions

3. Synthesis of argininosuccinate
4 . Synthesis of arginine
5 . Synthesis of urea

F ormation of Carbamoyl Phosphate
• This reaction occurs in mitochondrial matrix of hepatocytes.
• Ammonia molecule undergoes condensation with a molecule of CO2 to form

carbamoyl phosphate. Two ATP molecules are utilized in reaction.
• Reaction is catalyzed by carbamoyl phosphate synthetase-I (CPS-I) enzyme.

–– Carbamoyl phosphate synthetase-I is located in mitochondria of liver cells. It
catalyzes urea synthesis.

–– Carbamoyl phosphate synthetase-II is located in cytosol of liver cells. It cata-
lyzes synthesis of pyrimidine.

• Carbamoyl phosphate synthetase-I is an allosteric enzyme. It is the key regula-
tory enzyme in urea synthesis as in Fig. 13.8.

S ynthesis of Citrulline
• This reaction occurs in mitochondrial matrix of liver cells.
• Carbamoyl group is transferred to amino group of ornithine molecule. There is

formation of citrulline.

390 13  Metabolism of Proteins and Amino Acids

NH+4 2A+p 2ADp + Pi
CO2
Mg++

Carbamoyl
phosphate
synthetase I

H–N–H
C=O

O H2N
O–P–O CO
NH
O CH2
CH2
H N+H2 CARBAMOYL PHOSPHATE HC H
CH2 CH2
CH2 TRANSCOARRNBIATMHOINY CH-NH3
O CH2 COO–
NH2 C NH2 (UREA) CITRULLINE
HC NH+3
–COO– ATP
AMP+Pi
ARGINASEORNITHINE COO–
ARGISNYONTSHUACSCEINATE CH2
E LASEH2OUREA CH NH3+
CYCLE COO–
+
COO– ASPARTATE
NH 2
H 2N C NHCHC2HC2HC2 H-CNOH+O3– CH2 N+H2 CO2

H C NH C

COO– NH

CH2
CH2

CH2
CH – N+H3
COO–

ARGINO
SUCCINATE

ARGININE COO– MALATE OAA
TCA
CH
CYCLE
CH
COO–

FUMARATE

Fig. 13.8  Urea cycle

13.5  Urea Cycle 391

H2N-C=O-NH2

1st N-atom from Ammonia

C-atom from CO2 2nd N-atom from Aspartic acid
Fig. 13.9  Sources of atoms in Urea

• Reaction is catalyzed by ornithine carbamoyl transferase enzyme. It is also
called as ornithine transcarbamylase.

• Citrulline is transported across mitochondrial membranes to cytosol. It occurs by
a carrier protein as in Fig. 13.8.

• Citrulline is excreted in milk.

S ynthesis of Argininosuccinate
• This reaction occurs in cytosol of liver cells.
• Citrulline undergoes condensation with a molecule of aspartic acid to form

argininosuccinate as in Fig. 13.8.
• Reaction is catalyzed by argininosuccinate synthetase enzyme.
• One molecule of ATP is hydrolyzed to provide two high-energy phosphate bonds.

ATP is converted into AMP.

S ynthesis of Arginine
• This reaction occurs in cytosol of liver cells.
• Argininosuccinate is cleavaged into a molecule of arginine and fumarate as in

Fig. 13.8.
• Reaction is catalyzed by argininosuccinate lyase enzyme.
• Fumarate has inhibitory effect on argininosuccinate lyase. However, it is pre-

vented by channelizing fumarate into mitochondria to take part in TCA cycle. It
is converted into malate and oxaloacetate. Transamination of oxaloacetate fur-
ther regenerates aspartic acid.

Synthesis of Urea
• This reaction occurs in cytosol of liver cells.
• Arginine undergoes cleavage to form a molecule of urea and ornithine as in

Fig. 13.8.
• Reaction is catalyzed by arginase enzyme.
• Ornithine reenters mitochondrial matrix for reuse.

392 13  Metabolism of Proteins and Amino Acids

13.5.2 Biological Significance

Ammonia Detoxification

• Urea cycle is highly involved in conversion of toxic ammonia into nontoxic urea.
It is excreted in urine. Therefore, urea cycle helps to regulate ammonia plasma
concentration.

Synthesis of Arginine

• Arginine is a conditionally essential amino acid. It is synthesized by the liver,
kidneys, and intestinal mucosa. In kidneys, arginine cannot be decomposed into
urea due to the lack of arginase enzyme. Therefore, arginine from the kidneys
and intestine can be utilized in synthesis of proteins.

13.5.3 Regulation of Urea Cycle

Dietary Intake of Proteins

• Protein-rich diet results into increase in concentration of glutamate. It is a sub-
strate for ammonia synthesis. Hence, synthesis of urea is increased after con-
sumption of protein-rich diet.

N-Acetyl Glutamate (NAG)

• N-Acetyl glutamate activates allosterically the CPS-I enzyme. It is the rate-­
limiting step in urea synthesis.

Arginine and Glutamate Concentration

• N-Acetyl glutamate is synthesized from glutamate and acetyl CoA by N-acetyl
glutamate synthetase enzyme. Both arginine and glutamate activate allosterically
synthesis of N-acetyl glutamate synthetase enzyme.

Fumarate

• Fumarate is an inhibitor of argininosuccinate lyase enzyme.

13.5.4 Clinical Significance
Disorders of Urea Cycle
In healthy person, plasma concentration of urea ranges between15 and 40 mg/100 ml.
The deficiency of enzymes involved in urea cycle disturbs normal synthesis of urea.

Suggested Readings 393

The following conditions are caused by genetic deficiency of any one of the enzymes
as follows:

Hyperammonemia Type I

• Disorder is caused by inherited deficiency of CPS-I. There is increased plasma
ammonia level (hyperammonemia).

Hyperammonemia Type II

• Disorder is caused by deficiency of ornithine transcarbamylase. There is
increased plasma ammonia level (hyperammonemia).

Citrullinemia

• Disorder is caused by deficiency of argininosuccinate synthetase. There is
increased plasma ammonia level (hyperammonemia).

Argininosuccinic Aciduria

• Disorder is caused by argininosuccinate lyase. It is a rare condition. There is
increased plasma ammonia level (hyperammonemia).

Hyperargininemia

• Disorder is caused by deficiency of arginase. There is increased plasma ammonia
level (hyperammonemia).

Suggested Readings

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

Baron DN (1982) A short textbook of chemical pathology, 4th edn. Wiley, New York
Conn EE, Stump PK (1969) Outline of biochemistry, 2nd edn. Wiley, New Delhi
Harper HA (1979) Review of physiological chemistry, 17th edn. Lange Medical, New York
Kleiner IS, Orten JM (1966) Biochemistry, 7th edn. Mosby, St. Louis
Latner AL, Cantarow A, Trumper M (1975) Clinical biochemistry, 7th edn. W.  B. Saunders,

Philadelphia
Lehninger AL (1978) Biochemistry, 2nd edn. Kalyani, Ludhiana
Murray RK et al (1999) Harper’s biochemistry. Lange Medical, New York
Murray RK et al (2003) Harper’s illustrated biochemistry, 26th edn. Lange Medical, New York
Oser BL (ed) (1965) Hawk’s: physiological chemistry, 14th edn. McGraw-Hill, New York

Digestion and Absorption 14
of Carbohydrates

14.1 D efinition of Digestion

Digestion is the physicochemical process of conversion of nondiffusible form of food
into diffusible form in the alimentary canal.

Carbohydrates are important component of human diet. A healthy adult person
requires around 2800 cal/day. Calorific value of carbohydrates is 4 cal/g. It is rec-
ommended that around 60% of calories should be obtained from carbohydrates. A
person should consume around 450 g of carbohydrates per day. However, carbohy-
drates are the major dietary component in economically weaker section of society.
Carbohydrates provide 90% of daily calories.

14.2 D ietary Sources

Plant Sources of Carbohydrates

• Cereals like rice, wheat, maize, oats, and millets are source of starch.
• Fruits like banana, mango, apple, grapes, orange, and dates are rich in starch,

fructose, and glucose.
• Vegetables like potatoes, sweet potatoes, carrots, beets, and cassava are rich

sources of starch.
• Honey is rich in fructose.
• Jaggery is a highly concentrated product from sugarcane juice. It contains around

50–60% sucrose. Jaggery is a traditional dessert in India and Asia.
• Cold drinks and beverages contain fructose and sucrose.
• Nondigestible carbohydrates like cellulose and pectin in vegetables and fruits.

© Springer Nature Singapore Pte Ltd. 2019 395
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_14

396 14  Digestion and Absorption of Carbohydrates

Animal Sources of Carbohydrates

• Milk contains lactose.
• Meat, sardines, salmons, sea food, and eggs are almost carbohydrate free.
• Shellfish contains 5% carbohydrates.
• Organ liver contains 5% carbohydrates.

14.3 D igestion of Carbohydrates

Digestion is an enzymatic hydrolytic process. Carbohydrate digestion starts in oral
cavity, and it is completed in the small intestine. It occurs in different portions of
alimentary canal.

14.3.1 D igestion in Oral Cavity

• Oral cavity contains saliva. It is rich in “salivary amylase.” This enzyme is also
called as “ptyalin.”

• Salivary amylase acts on dietary starch and glycogen. It requires chloride ions
and pH 6.7 for catalysis of polysaccharides.

• Starch and glycogen are converted into dextrin, maltose, and maltotriose.
• Dietary substances are masticated by teeth, and this physical process helps in

proper mixing of salivary enzyme with food particles.
• Partially digested food is called as “bolus,” and it is swallowed and enters the

stomach.
• In the stomach, activity of salivary amylase stops due to decline in pH of the

medium (1.5–3).

Salivary amylase

Dietary Starch and Glycogen Dextrin + Maltose + Maltotriose
Cl- ions / pH (6.7)

14.3.2 D igestion in the Stomach

• Gastric juice does not contain enzyme for carbohydrate digestion.
• Hydrochloric acid in the stomach may hydrolyze sucrose into glucose and

fructose.

14.3.3 Digestion in the Duodenum

• Duodenum part of the small intestine is important for digestion. It receives pan-
creatic juice from the pancreas. It is rich in “pancreatic amylase.”

14.4  Absorption of Carbohydrates 397

• The enzyme splits starch, glycogen, and dextrin into maltose.
• Its optimum activity requires Cl ions as cofactor and pH 7.4.

Dietary Starch, Glycogen and Dextrin Pancreatic amylase Maltose
Chloride ions/pH 7.4

14.3.4 D igestion in the Small Intestine

• Intestinal juice contains disaccharide-splitting enzymes. The optimum pH is 7.4
for intestinal enzymes.

• Enzyme intestinal amylase cleavages alpha-1,4 glycosidic bonds in disaccha-
rides and oligosaccharides. It liberates glucose.

• Enzyme maltase acts on maltose molecules and liberates glucose molecules.
• Enzyme lactase splits lactose into glucose and galactose.
• Enzyme isomaltase cleavages alpha-1,6 glycosidic bonds in limit dextrin mole-

cules. It liberates glucose and maltose molecules.
• Enzyme sucrase splits sucrose molecules and liberates glucose and fructose.

14.4 Absorption of Carbohydrates

14.4.1 Definition

Absorption is a physiological process of passage of digested food from the lumen of the
alimentary canal into blood circulation.

14.4.2 R ate of Absorption of Monosaccharides

• Only monosaccharide form of carbohydrates is absorbed from lumen of the
small intestine.

• Dietary carbohydrates are digested into monosaccharides like glucose, galactose,
fructose, and mannose.

• Monosaccharides have different rate of absorption from intestinal lumen.
• C.F. Cori in 1933 studied rate of absorption of glucose and other monosaccha-

rides from lumen of small intestine in experimental albino rats. According to
Cori’s study:
–– Galactose absorption is the fastest among monosaccharides.
–– Galactose and glucose are the most rapidly absorbed monosaccharides.
–– Fructose and mannose have intermediate rate of absorption.
–– Xylose and arabinose have the slowest rate of absorption among all monosac-

charides in intestinal lumen. Comparative rate of absorption has been depicted
in Fig. 14.1.

398 14  Digestion and Absorption of Carbohydrates

ArabinoseMonosaccharides
Xylose
100 120
Mannose
Fructose
Glucose
Galactose

0 20 40 60 80
Rate of absorption

Fig. 14.1  Showing rate of absorption of monosaccharides

14.4.3 Mechanism of Monosaccharide Absorption

A bsorption by Simple Diffusion
Features

• It is the movement of sugar molecules from the region of higher concentration

(lumen of the intestine) to blood circulation through mucosal cells.

• Simple diffusion is dependent on concentration gradient.
• Small amount of all monosaccharides are absorbed by simple diffusion.
• Predominantly, arabinose and xylose are absorbed by simple diffusion.

A bsorption by Active Transport
Features
• It is a process of movement of sugar molecules against concentration gradi-

ent across the plasma membrane of the intestine in the presence of a protein
carrier and energy.
• Glucose and galactose are rapidly absorbed by active transport mechanism.

• Comparative higher rate of absorption of glucose and galactose is due to active

transport.

• Active transport is the chief process which is involved in absorption of monosac-

charides from the lumen of small intestine. Active transport of monosaccharides

can be explained by Crane hypothesis as described below:

Crane Hypothesis of Active Transport
Features
• In 1960, R. K. Crane proposed “sodium-glucose cotransport hypothesis” for

absorption of glucose through intestinal mucosa.

–– Brush border epithelium of the small intestine contains specialized protein
molecule which helps in transport of glucose. It is called as “carrier
protein.”

–– Carrier protein is a transmembrane protein molecule.

–– Active transport of carbohydrates (monosaccharides) requires the presence of

sodium ions in the lumen of small intestine.

14.4  Absorption of Carbohydrates 399
Enterocyte
Carrier Intestinal
(symporter) lumen

G Na+
Na+ Na+

Glucose Na+ ADP Glucose (G)
ATP Na+

k+ k+
Na+ – k+ –ATPase

Brush border
surface of enterocyte

Microvillus

Nucleus

Basolateral Cytoplasm
surface

Fig. 14.2  Digestion and absorption of carbohydrates

–– This is an ATP-dependent mechanism.
–– On the surface of brush border epithelium of small intestine, sodium ion

attaches to carrier protein molecule. This binding results in conformational
change in carrier protein molecule. Thereafter, glucose binds to carrier pro-
tein at another site.
–– Carrier protein molecule passes across plasma membrane of the small intesti-
nal epithelium and enters inside the epithelial cell (enterocyte) as in Fig. 14.2.
This method of active transport is called as Co-transport.
–– Crane sodium-glucose co-transport hypothesis explains the role of con-
comitant oral administration of sodium and glucose in oral rehydration
therapy in diarrhea and cholera.

Role of Glucose Transporters (GLUT) in glucose absorption

• Glucose transporters are the membrane-bound protein molecules. They help in
transport of glucose across the plasma membrane of cells.

• Glucose transporters are found in different body tissues.

400 14  Digestion and Absorption of Carbohydrates

• They are numbered from GLUT-1 to GLUT-14, depending upon the tissues in
which they are located.

Important Glucose Transporters

• GLUT-1
–– It is found in the RBC, retina, fetus, and blood-brain barrier.
–– It is helpful in the uptake of glucose.
–– GLUT-1 activity is independent of insulin.

• GLUT-2
–– It is found in epithelial cells of the small intestine, liver, and pancreas.
–– GLUT-2 helps in glucose transport through intestinal epithelial cells. GLUT-2
activity is independent of insulin.

• GLUT-4
–– It is found in adipose tissues and muscles.
–– GLUT-4 activity is under control of insulin hormone.

Absorption by Facilitated Transport
• It is a process of passive movement of sugar molecules in favor of concentra-

tion gradient across the plasma membrane of the intestine in the presence of
a transmembrane integral protein.
Features
• Fructose and mannose are absorbed by facilitated transport.
• It occurs in the presence of a transmembrane integral protein molecule.
• It occurs independent of ATP.
• It occurs in favor of concentration gradient.
Facilitated transport of monosaccharides from the lumen of small intestine can
be explained by ping-Pong model as described below:

Ping-Pong Model of Facilitated Transport
• Transmembrane integral proteins span across the plasma membrane of the

intestine.
• Transmembrane protein has two conformations.

–– Ping conformation
–– Pong conformation
• In pong conformation, transmembrane protein molecule is exposed to sugar
molecules in the lumen of the intestine. Sugar molecules bind to its extrinsic
surface. They are transmitted across the lipid bilayer and reach intrinsic surface
of transmembrane protein. Pong conformation of protein changes to ping
conformation.

Suggested Readings 401

• In ping conformation, sugar molecules are delivered to interior of cells. Again,
transmembrane protein mutates to pong conformation.

14.5 Clinical Significance

Lactose Intolerance

• This disorder is due to deficiency of lactase enzyme in intestinal epithelium. The
deficiency may be congenital and acquired.

• Congenital lactase deficiency is rare in occurrence. Infants cannot digest lac-
tose in milk. Its symptoms are:
–– Diarrhea
–– Distension of abdomen
–– Abdominal cramps and flatulence
–– Wasting

• Acquired lactase deficiency occurs due to environmental factors and struc-
tural changes in GIT.
–– Lactase activity is normal in early life.
–– It is a common condition among population in Southeast Asia.
–– Lactase activity is decreased in older age.
–– It results into diarrhea, flatulence, and abdominal cramps.

Suggested Readings

Baron DN (1982) A short textbook of chemical pathology, 4th edn. Wiley, New York
Conn EE, Stump PK (1969) Outline of biochemistry, 2nd edn. Wiley, New Delhi
Crane RK (1960) Intestinal absorption of sugars. Physiol Rev 40:789–825
Harper HA (1979) Review of physiological chemistry, 17th edn. Lange Medical, New York
Kleiner IS, Orten JM (1966) Biochemistry, 7th edn. Mosby, St. Louis
Murray RK et al (1999) Harper’s biochemistry. Lange Medical, New York
Murray RK et al (2003) Harper’s illustrated biochemistry, 26th edn. Lange Medical, New York
Oser BL (ed) (1965) Hawk’s: physiological chemistry, 14th edn. Mc-Graw Hill, New York
Ramasubbu N, Paloth V, Luo Y, Brayer GD, Levine MJ (1996) Structure of human salivary

α-amylase at 1.6 Å resolution: implications for its role in the oral cavity. Acta Crystallogr Sect
D Biol Crystallogr 52(3):435–446
Thorens B, Mueckler M (2010) Glucose transporters in the twenty-first century. Am J Physiol
Endocrinol Metab 298(2):E141–E145

Metabolism of Carbohydrates 15

15.1 Introduction

Carbohydrates are synthesized by green plants utilizing solar energy. Primary con-
sumers are mainly dependent on plant-based foods. Carbohydrates are the major
energy source in plant-based food substances. Living organisms derive energy eas-
ily from carbohydrates. Glucose is the prime carbohydrate in the human body as a
source of energy. Glucose is the major carbohydrate that is involved in carbohydrate
metabolism in humans.

15.2 G lycolysis

Definition
Glycolysis is defined as series of biochemical reactions involved in conversion
of glucose or glycogen into pyruvate or lactate with the release of energy.

Glycolysis is derived from Greek words:
Glycose means sugar.
Lysis means decomposition.

Site of Occurrence

• Glycolysis occurs in cytosol in almost all cells of the body.
• Brain tissues and erythrocytes are exclusively dependent on glycolysis for fulfill-

ing energy requirement.

© Springer Nature Singapore Pte Ltd. 2019 403
A. Gupta, Comprehensive Biochemistry for Dentistry,
https://doi.org/10.1007/978-981-13-1035-5_15

404 15  Metabolism of Carbohydrates

Characteristics of Glycolysis

• Glycolysis is also called as Embden-Meyerhof-Parnas pathway.
• It was described by Embden and Meyerhof in 1940.
• Glycolysis occurs in aerobic as well as anaerobic conditions.
• Glycolysis is chief glucose metabolism in erythrocytes for energy generation

(RBCs lack mitochondria; fatty acids cannot be oxidized; TCA cycle cannot
operate; glucose is anaerobically decomposed into lactate for energy).
• Glycolysis is the obligatory pathway to provide energy to brain tissues (glu-
cose is completely oxidized into CO2 and water). In conditions like fasting and
starvation, brain tissues depend on ketone bodies and lactic acid for energy
synthesis).

Steps in Glycolysis
Glycolysis occurs in a cascade of biochemical reactions. Each one is enzyme con-
trolled. These reactions are described in the following steps:

1 . Phosphorylation of glucose
• Glucose is freely permeable into liver cells. Glucose uptake in cardiac mus-
cles, adipose tissues, and skeletal muscles is regulated by insulin.
• Glucose undergoes phosphorylation to form glucose-6-phosphate.
• Reaction requires one molecule of ATP.  It donates phosphate group and is
converted into ADP.
• Mg++ ions act as cofactor in the reaction.
• Reactions are catalyzed by kinase enzyme whose nomenclature is based on
its presence in tissues as:
–– Glucokinase. This enzyme is found in the liver. It catalyzes phosphoryla-
tion of only glucose. Due to its high affinity to glucose, it is not inhibited
by glucose-6-phosphate.
–– Hexokinase. This enzyme is found in all body tissues. It phosphorylates
hexoses like glucose, mannose, or fructose. It is inhibited by
glucose-6-phosphate.
• Glucose-6-phosphate is the KEY compound across glucogenesis, gluconeo-
genesis, glycogenolysis, HMP shunt, and uronic acid pathway.
• Glucose phosphorylation is an irreversible step as in Figs. 15.1 and 15.2a, b, c.

2. Isomerization of glucose-6-phosphate
• Glucose-6-phosphate undergoes isomerization into fructose-6-phosphate
as in Figs. 15.1 and 15.2a, b, c.
• Reaction is catalyzed by phosphohexose isomerase and Mg++ ions.

3 . Phosphorylation of fructose-6-phosphate
• Fructose-6-phosphate undergoes phosphorylation into fructose-1,6
bisphosphate as in Figs. 15.1 and 15.2a, b, c.
• One ATP molecule is consumed in reaction and is converted into ADP. Mg++
ions act as cofactor.

15.2 Glycolysis 405

Metabolism of carbohydrates

Hexokinase Phosphohexo

Glucose or Isomerase Fructose – 6 – Phosphate
Glucose – 6 – Phosphate
Glucokinase

Mg++ Phosphofructo kinase ADP
ATP ADP

ATP

Glycerol Fructose – 1, 6 – Bis phosphate

ATP Glycerol kinase Aldose
ADP

Glycerol – 3 – P Phosphotriose
isomerase
Dihydroxy Glyceraldehyde – 3 – Phosphate
Acetone
Dehydrogenase Phosphate NAD+ Glyceraldehyde – 3 – P
NADH+ H+ Dehydrogenase

NAD+ NADH+ H+

1,3 – Diphospho glycerate

ADP Phosphoglycerate kinase

ATP

3 – Phospho glycerate

Phosphoglycerate
Mutase

2 – Phospho glycerate

Enolase

H2O Mg++

Phospho Enol Pyruvate

ADP Pyruvate kinase

ATP Mg++

Pyruvate Spontaneously (Enol) Pyruvate
(Keto)

Fig. 15.1  Showing glycolysis

• This is an irreversible step.
• Reaction is catalyzed by phosphofructokinase.

–– Phosphofructokinase. It is a key regulatory enzyme in glycolysis.

–– It is an allosteric enzyme. Its enzymatic activity is inhibited by ATP mol-

ecule and citrate molecule and activated by AMP molecule.

406 15  Metabolism of Carbohydrates

O= O =
C–H C–H =
H – C – OH ADT ATD H – C – OH
HO – C – H Mg++ HO – C – H
H – C – OH H – C – OH
H – C – OH Hexokinase H – C – OH
or

Glucokinase

O

6CH 2 – OH Phosphohexo 6CH2 – O – P – OH
Glucose isomerase OH

Glucose – 6 – Phosphate Glucose – 6 – P
CH2OH
C=O

HO – C – OH
H–C–H
H – C – OH

H – C – OH O

6CH2 – O – P – OH

OH
Fructose – 6 – P

O

CH2 – O – P – OH=
C – O OH
ATP ADP
mg++
Fructose – 6 – Phosphate HO – C – OH

Phosphofructo kinase H – C – OH

H – C – OH O

6CH2 – O – P – OH
OH

Fructose – 1,6, Bis Phosphate

Fig. 15.2 Glycolysis

15.2 Glycolysis 407

Fructose – 1, 6 – Bis phosphate
Aldolase

1 CH2 – OH = O
=1C – H
2 C=O O Phosphotriose H –2 C – OH O
isomerase 3 CH2 – O – P – OH
=
3 CH2 –O – P – OH = OH
Glyceraldehyde – 3 – P
OH
O
Dihydroxy acetone 1 COO – P – OH2
Phosphate H – C – OH OH
2O
Glyceraldehyde – P– P Glyceraldehyde – 3 – P 3CH2 – O – P – OH=
Dehydrogenase =
OH
O NAD+ NADH+H+ 1,3 – Bisphospho
HO – P – OH
Phospho glycerate glycerate
OH kinase
Inorganic COOH
phosphate
H – C – OH O
1,3 – Bis–phospho glycerate CH2 – O – P – OH
OH
ADP ATP 3 – Phospho glycerate==

Phospho glycerate 1 COOH O
mutase
3 – Phospho glycerate 2

HC – O – P – OH

3 CH2 OH OH

2 – Phospho glycerate

Fig. 15.2 (continued)

408 15  Metabolism of Carbohydrates

2 – phosphoglycerate Enolase COOH O
Phospho Enol pyruvate Mg++ C – O – P – OH

Enol pyruvate H2O H – C – H OH

Pyruvate Kinase Phospho enol
ADP ATP pyruvate

COOH
C=O
H–C–H
Enol pyruvate

Spontaneous COOH
C=O
H–C–H
H

Pyruvate

Fig. 15.2 (continued)

4 . Splitting of fructose-1,6 bisphosphate
• Fructose-1,6 bisphosphate (6C) compound is cleavaged into two com-
pounds, namely, dihydroxyacetone phosphate (3C) and glyceraldehyde-­
3-­phosphate (3C), as in Figs. 15.1 and 15.2a, b, c.
• Reaction is catalyzed by aldolase (lyase enzyme) as in Figs. 15.1 and 15.2a,

b, c.

• Dihydroxyacetone phosphate is a ketotriose, and glyceraldehyde-3-phosphate

is an aldotriose. These triose phosphates are interconvertible by phosphotri-

ose isomerase.

Dihydroxyacetone phosphate is converted into glyceraldehyde-3-phosphate.
It results into formation of two molecules of glyceraldehyde-3-phosphate
from one molecule of glucose.

• Bromohydroxyacetone phosphate resembles structurally to dihydroxyacetone

phosphate.

It competitively inhibits phosphotriose isomerase. This leads to inhibition of

glycolysis.
5 . Oxidative phosphorylation of glyceraldehyde-3-phosphate

• Glyceraldehyde-3-phosphate is oxidized and phosphorylated into
1,3-bisphosphoglycerate as in Figs. 15.1 and 15.2a, b, c.

15.2 Glycolysis 409

• Reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase. It
requires NAD+ as coenzyme. It is reduced to NADH with a release of one H+.

• Inorganic phosphate (Pi) provides phosphate group for phosphorylation.

• Each NADH passes through ETS and generates three ATP molecules

(3 × 2 = 6 ATP).
6 . Substrate-level phosphorylation

• 1,3-Bisphosphoglycerate is a high-energy phosphate molecule, and it is
converted into 3-phosphoglycerate as in Figs. 15.1 and 15.2a, b, c.

• Its high-energy phosphate group at first position is transferred to ADP to form
ATP. This process is called substrate-level phosphorylation (synthesis of
ATP without passing through electron transport chain, while synthesis of
ATP through oxidation of NADH and FADH in ETS is called as oxidative
phosphorylation).

• Reaction is catalyzed by phosphoglycerate kinase in the presence of Mg++
ions as cofactor.

7. Conversion of 3-phosphoglycerate
• 3-Phosphoglycerate is converted into 2-phosphoglycerate as in Figs. 15.1

and 15.2a, b, c.

• Reaction is catalyzed by phosphoglycerate mutase.

• Enzyme transfer phosphate group from third position to second position. It is

internal rearrangement of molecule.
8. Conversion of 2-phosphoglycerate

• 2-Phosphoglycerate is converted into phosphoenol pyruvate (PEP).

• Reaction is catalyzed by enolase in the presence of Mg++ ions as cofactor.

• There is loss of a water molecule.

• PEP has high-energy phosphate enolic group.
• Fluoride inhibits enolase (sodium fluoride is added in collected blood sam-

ple for blood-glucose estimation test; it prevents glycolysis in the sample)
9. Substrate-level phosphorylation

• Phosphoenol pyruvate is converted into pyruvate.

• High-energy phosphate group in PEP is transferred to ADP to form ATP.

• Reaction is catalyzed by pyruvate kinase.

Energetic in Glycolysis (One Molecule of Glucose)
Aerobic glycolysis

Glycolysis step Synthesis of Consumption of
ATP ATP
Phosphorylation of glucose 1 ATP
Nil 1 ATP
Phosphorylation of fructose-6-phosphate Nil
Nil
Oxidative phosphorylation of Nil
glyceraldehyde-3-phosphate 2 NADH
2 × 3 = 6 ATP
Conversion of 1,3-bisphosphoglycerate into
3-bisphosphoglycerate 2 ATP

Conversion of phosphoenol pyruvate into pyruvate 2 ATP Nil

Total synthesis Total consumption
+10 ATP −2 ATP

410 15  Metabolism of Carbohydrates

In aerobic glycolysis

• Net gain = 8 ATP

In anaerobic condition

• Two NADH molecules are produced in oxidative phosphorylation of

glyceraldehyde-3-phosphate.

• NADH molecule must be oxidized into NAD+ through electron transport chain in
mitochondria. It is the NAD+ coenzyme that can sustain glycolysis. However, in
anaerobic condition, NADH cannot be oxidized into NAD+. Additionally,
cytosolic pool of NAD+ is limited. These events lead to discontinuity in sup-
ply of NAD+ for glycolysis.

• In order to continue glycolysis in anaerobic condition, pyruvate is reduced

into lactate in cells.

• Reaction is catalyzed by lactate dehydrogenase in the presence of NADH

coenzyme.

• In the reaction, two NADH molecules are oxidized into two NAD+.

Lactate dehydrogenase

Pyruvate Lactate

NADH + H+ NAD+

• NAD+ generated in pyruvate reduction is utilized by cells in oxidative phos-
phorylation of glyceraldehyde-3-phosphate. This activity maintains glycoly-
sis in anaerobic condition.

Glycolysis step Synthesis of Consumption of
Phosphorylation of glucose ATP ATP
Phosphorylation of fructose-6-phosphate
Conversion of 1,3-bisphosphoglycerate into Nil 1 ATP
3-bisphosphoglycerate Nil 1 ATP
Conversion of phosphoenol pyruvate into pyruvate 2 ATP Nil

Net gain = 2 ATP (4 − 2 = 2) 2 ATP Nil

Total synthesis Total consumption
4 ATP 2 ATP

In anaerobic glycolysis

• Net gain = 2 ATP

15.2 Glycolysis 411

Site of Occurrence of Anaerobic Glycolysis
Erythrocytes

• Erythrocytes lack mitochondria. Glucose undergoes anaerobic glycolysis in
erythrocytes. They need continuous supply of glucose for energy need.

• Lactate accumulates in erythrocytes.
• It is transported to the liver for oxidation into pyruvate.

Skeletal muscles

• Skeletal muscles have high-energy demand owing to rapid contraction.
• In strenuous physical exercise, energy demand of tissues is increased. Oxygen

supply to contracting muscles is limited.
• In this relative anaerobic condition, anaerobic glycolysis occurs in skeletal mus-

cles. Accumulated lactate is cycled to the liver through blood circulation.

Regulation of Glycolysis
Glucokinase enzyme

• Glucokinase phosphorylates glucose into glucose-6-phosphate in the liver.
Glucokinase has low affinity to glucose molecules. It is controlled by feedback
mechanism. Decrease in blood glucose level inhibits glucokinase enzyme which
sequentially inhibits glycolysis. Adequate concentration of glucose promotes
glucokinase.

Hexokinase enzyme

• Hexokinase enzyme phosphorylates hexoses in tissues. Hexokinase has high
affinity for glucose. It is also controlled by feedback mechanism. Decrease in
glucose concentration activates hexokinase and sequentially promotes glycoly-
sis. This mechanism is responsible for supply of energy to the brain, cardiac
fibers, and skeletal fibers.

Phosphofructokinase enzyme

• Phosphofructokinase is the key regulatory enzyme in glycolysis. It is alloste-
rically inhibited by ATP and citrate molecules. It is activated allosterically by
AMP molecule.

Pyruvate kinase enzyme

• Pyruvate kinase is a key regulatory enzyme in glycolysis. It catalyzes dephos-
phorylation of phosphoenol pyruvate into pyruvate. It is an irreversible step. It is
activated by insulin and inhibited by glucagon hormone.

412 15  Metabolism of Carbohydrates

Insulin hormone

• Insulin hormone activates phosphofructokinase enzyme, pyruvate kinase
enzyme, and hexokinase enzyme (glucokinase).

Glucagon hormone

• Glucagon hormone inhibits phosphofructokinase enzyme, pyruvate kinase
enzyme, and hexokinase enzyme (glucokinase).

15.3 Pyruvate Metabolism

Pyruvic acid (pyruvate) is α-keto acid. It is produced in the body in the following
metabolic pathways:

• Pyruvate is produced in glycolysis.
• Pyruvate is produced by oxidation of lactic acid.
• Pyruvate is produced by transamination of amino acids.
• Pyruvate is produced as in Fig. 15.3.

Pyruvate is actively transported through inner membrane of mitochondria.
It enters the mitochondrial matrix. It undergoes oxidative decarboxylation.

Glycolysis Glucose

Glucose Gluconeogenesis P Lactate dehydrogenase

Lactate L. Dehydrogenase Lactate

Alanine Y
R Transamination
Oxalo U Alanine
acetate
Transamination V Gluconeogenesis
A
Glucogenic

Pyruvate carboxylase T Oxaloacetate amino acid
E decarboxylase Oxalo acetate

CO2

Pyruvate
dehydrogenase
complex

Acetyl coa

Fig. 15.3  Pyruvate metabolism

15.3  Pyruvate Metabolism 413

Oxidative Decarboxylation of Pyruvic Acid (Pyruvate)
In aerobic condition

• Pyruvate (3C) is oxidized and decarboxylated to form (2C) compound as acetyl

CoA-SH.
• Reaction occurs in the mitochondrial matrix.
• Reaction is catalyzed by pyruvate dehydrogenase.

Pyruvate dehydrogenase

• It is a multienzyme complex called as pyruvate dehydrogenase complex.

• It is found in the mitochondrial matrix.

• Its high concentration is found in cardiac muscles and kidneys.
• Pyruvate dehydrogenase complex is a cluster of three types of enzymes:

–– Pyruvate decarboxylase
–– Dihydrolipoyl dehydrogenase
–– Dihydrolipoyl transacetylase

• Pyruvate dehydrogenase requires six prosthetic groups for catalysis:
–– Lipoic acid
–– Thiamine pyrophosphate (TPP)
–– CoA-SH
–– NAD+
–– FAD
–– Mg++

Steps in Oxidative Decarboxylation of Pyruvate

1 . Enzyme-bound thiamine pyrophosphate activates pyruvate in the presence of Mg++.
2. Activated pyruvate undergoes decarboxylation by pyruvate decarboxylase bound

to TPP.  Active pyruvate is converted into alpha-hydroxyethyl thiamine pyro-
phosphate (active acetaldehyde).
3. Active acetaldehyde is converted into acetylated lipoate by lipoate transacetylase
bound to lipoic acid coenzyme.
4. Acetylated lipoate is converted into acetyl CoA and reduced lipoate in presence
of CoA-SH.
5 . FAD+ oxidizes reduced lipoate into oxidized lipoate with formation of FADH2.
6 . FADH2 transfers hydrogen atoms to NAD+ to form NADH + H+ which enters
ETS chain to generate two ATP molecules.

Energetic of Oxidative Decarboxylation of Pyruvate

• Two pyruvate molecules are produced by glycolysis.
• Oxidative decarboxylation of pyruvate generates two acetyl CoA molecules

and two NADH.
• Two NADH molecules generate six ATP molecules.
• Energetics in oxidative decarboxylation of pyruvate

–– +6 ATP

414 15  Metabolism of Carbohydrates

15.4 Citric Acid Cycle

Definition
Citric acid cycle is defined as a series of biochemical reactions occuring inside
mitochondria of aerobic organisms that are essential for synthesis of energy in the
form of ATP. It is termed as Citric acid Cycle because citric acid is the first metabo-
lite that is produced as a sequence of chemical reactions.

Site of Occurrence

• Citric acid cycle occurs in mitochondrial matrix in eukaryotes.

Characteristics of Citric Acid Cycle

• Citric acid cycle was described by Sir Hans Krebs in 1937.
• Citric acid is the first metabolite in the cyclic process, and it is named as citric acid

cycle. It is also called as Krebs cycle based on the name of the discoverer. It is addi-
tionally termed as tricarboxylic acid cycle as citric acid has three carboxylic groups.
• The cyclic pathway starts with synthesis of citric acid utilizing acetyl CoA and
terminates with regeneration of citric acid accompanied by production of CO2,
water, and ATP molecules.
• TCA cycle is the common metabolic pathway involving oxidation of lipids,
carbohydrates, and proteins. Amino acids are metabolized into intermediate
metabolites of TCA cycle. Fatty acids and glucose are oxidized to form acetyl
CoA.  Overall, diffusible form of macronutrients is converted into acetyl CoA
which in turn is oxidized into CO2 with release of energy through TCA cycle.
• TCA cycle is absolutely amphibolic pathway. It includes catabolic as well as
anabolic reactions as follows:
–– Anabolic reactions

Aspartate is synthesized from oxaloacetate.
Alpha-ketoglutarate can be used for synthesis of glutamate.
–– Catabolic reactions

Steps in Citric Acid Cycle

1. Condensation
• Oxaloacetate (4C) compound undergoes condensation with acetyl CoA to
form citric acid as in Fig. 15.4.
• Reaction is catalyzed by citrate synthetase.
• CoA-SH is released in the reaction.

2 . Isomerization of citric acid
• Citric acid undergoes isomerization into isocitric acid as in Fig. 15.4.
• Reaction is catalyzed by aconitase.

15.4  Citric Acid Cycle 415

• Process occurs in two steps:
–– Dehydration of citrate
This reaction is catalyzed by aconitase enzyme. Citric acid is dehydrated
to form cis-aconitate.
–– Hydration of cis-aconitate
Hydration of cis-aconitate forms isocitrate.

• Dehydration and hydration reactions are catalyzed by aconitase.
3 . Oxidative decarboxylation of isocitrate

• Isocitrate (6C) undergoes oxidation and decarboxylation to form
α-ketoglutarate (5C) as in Fig. 15.4.

• Reaction is catalyzed by isocitrate dehydrogenase in the presence of NAD+.
Process occurs in two steps:
–– Oxidation of isocitrate
Isocitrate is oxidized to form oxalosuccinate. Reaction delivers two hydro-
gen atoms which are accepted by NAD+. It is reduced in NADH + H+.
–– Decarboxylation of oxalosuccinate
Oxalosuccinate is decarboxylated to form α-ketoglutarate. A molecule of
CO2 is released.
Both reactions are catalyzed by isocitrate dehydrogenase.

4. Oxidative decarboxylation of α-ketoglutarate
• α-Ketoglutarate (5C) is oxidized and decarboxylated to form succinyl CoA
(4C) as in Fig. 15.4.
• Reaction is catalyzed by α-ketoglutarate dehydrogenase complex in presence
of NAD+ and CoA-SH.
• α-Ketoglutarate dehydrogenase complex is a multienzyme complex. Reaction
is similar to oxidative decarboxylation of pyruvate into acetyl CoA-SH.

5. Conversion of succinyl CoA into succinic acid
• Succinyl CoA-SH undergoes hydrolysis to form succinic acid as in Fig. 15.4.
• Reaction is catalyzed by succinate thiokinase.
• Thioester bond in succinyl CoA-SH is cleavaged to release high-energy phos-
phate group.
• Guanidine diphosphate (GDP) is phosphorylated into GTP with release of
CoA-SH.
• GTP molecule phosphorylates one molecule of ADP into ATP by nucleoside
diphosphokinase.

6 . Oxidation of succinic acid
• Succinic acid is oxidized to form fumarate as in Fig. 15.4.
• Reaction is catalyzed by succinate dehydrogenase in the presence of FAD+ as
coenzyme.
• Reaction releases a pair of hydrogen atoms which are accepted by FAD+.
• FADH2 generates two ATP molecules through oxidative phosphorylation in
ETS chain.
• Succinate dehydrogenase is exclusive enzyme that is attached to the mito-
chondrial membrane.

416 15  Metabolism of Carbohydrates

PYRUVATE Oxidative CH3 CO ∼ S. COA O
Decarboxylation H2 – C – C – OH
COOH ACETYL COA
C=O O
CH2 COA – SH HO – C – C – OH
COOH
Oxaloacetate Citrate synthase O
H2 – C – C – OH
Malate Dehydrogenase
NAD+ NADH+H+ CITRATE

H2O Aconitase
CH – COOH
C – COOH
CH2 – COOH

CIS – ACONITATE
H2O ACONITASE

H
HO C COOH

O H C COOH
HO – CH – C – OH
CH2 COOH
O
H – CH – C – OH NAD+ ISOCITRATE
NADH+H+
Malate Isocitrate
Dehydrogenase
HO2
O
O C – COOH
H – C – C – OH
Fumarase H – C – COOH
O
HO – C – C – OH CH2 – COOH

H Oxalo succinate
Fumarate
Mh++ Isocitrate

Dehydrogenase

CO2

O
C – COOH

H – C – CH

Succinate Dehydrogenase CH2 – COOH
FAD FADH2
∝ – Ketoglutarate

HO COA – SH ∝ – Keto
H – C – C – OH CON2AD+
H – C – C – OH glutarate
Dehydrogenase
H
+ ATP NADH+H+
GDP ADP
COA ∼ SH O

C ∼ COOH

Succinate H–C–H

GDP+Pi GTP

CH2 – COOH
Succinyl coa

Succinate thiokinase

Fig. 15.4  Citric acid cycle

15.4  Citric Acid Cycle 417

7. Hydration of fumarate
• Fumarate (4C) undergoes hydration into malate (4C) as in Fig. 15.4.
• Reaction is catalyzed by fumarase.

8 . Dehydrogenation of malate
• Malate (4C) undergoes dehydrogenation to form oxaloacetate (4C) as in
Fig. 15.4.
• Reaction is catalyzed by malate dehydrogenase in the presence of NAD+.
• It is reduced into NADH + H+.

Energetic of TCA Cycle

Steps in TCA cycle Synthesis of ATP
6 ATP (2 NADH × 3)
Oxidative decarboxylation of
isocitrate 6 ATP (2 NADH × 3)

Oxidative decarboxylation of 2 ATP
α-ketoglutarate
Conversion of succinyl CoA into 4 ATP (2 FADH × 2)
succinic acid 6 ATP (2 NADH × 3)

Conversion of succinate into fumarate

Conversion of malate into
oxaloacetate

Total gain of ATP in TCA cycle
• 24 ATP

Energetic of Glucose Catabolism
• Net gain in glycolysis

–– 8 ATP
• Net gain in oxidative decarboxylation of pyruvate

–– 6 ATP
• Net gain in TCA cycle

–– 24 ATP

Net gain = 38 ATP
Total energy production in glycolysis
1 ATP = 7300 cal
Oxidation of one molecule of glucose

• 7300 × 38 = 277400

Regulation of TCA Cycle
TCA cycle is regulated by the following factors which are described as follows:

418 15  Metabolism of Carbohydrates

Citrate synthase

• Citrate synthase catalyzes synthesis of citrate in TCA cycle. ATP molecule is an
allosteric inhibitor of citrate synthase enzyme.

ATP molecule

• Increased demand of ATP molecules in cell promotes TCA cycle. It is due to
close association between synthesis of NADH in TCA cycle and oxidation of
NADH in electron transport chain.

Hypoxia

• Decreased partial pressure of oxygen in circulation and tissue hypoxia result in
accumulation of NADH2 and FADH2 in cells. These coenzymes retard citric acid
cycle.

Isocitrate dehydrogenase

• ADP molecule is an allosteric stimulator of isocitrate dehydrogenase enzyme. It
promotes TCA cycle.

Toxins (arsenite, fluoroacetate, malonate)

• Toxins inhibit mitochondrial enzyme and retards TCA cycle. It is as follows:
–– Arsenic
Arsenic is a heavy metal. It is a noncompetitive inhibitor of alpha-ketogluta-
rate dehydrogenase enzyme.
–– Fluoroacetate
It is a noncompetitive inhibitor of aconitase enzyme.
–– Malonate
Malonate is a competitive inhibitor of succinate dehydrogenase enzyme.

15.5 S huttle System

Definition
Shuttle system is comprised of a pair of substrates which are interconvertible
through dehydrogenase enzymes.

Significance of Shuttle System

• NADH2 is produced in glycolysis. They are impermeable to mitochondrial mem-
brane. But oxidation of NADH2 occurs in mitochondrial matrix. Shuttle system
serves to transfer reducing equivalents from NADH2 molecule to mitochondrial
matrix across mitochondrial membranes.

15.5  Shuttle System 419

Types of Shuttle Systems
There are two types of shuttle systems operating in human tissues as follows:

• Alpha-glycerophosphate shuttle system
• Malate shuttle system

Steps in Working of Alpha-Glycerophosphate Shuttle System

• This shuttle system is less prominent in human body tissues. It is important in
liver tissues.

• In cytosol
–– Dihydroxyacetone phosphate is present. It is reduced into
alpha-glycerophosphate.
–– Reaction is catalyzed by alpha-glycerophosphate dehydrogenase enzyme.
This enzyme is NADH2-dependent. Coenzyme is oxidized into NAD+.
–– Alpha-glycerophosphate moves across mitochondrial membranes into matrix.

• In mitochondrial matrix
–– Alpha-glycerophosphate is oxidized into dihydroxyacetone phosphate.
Reaction is catalyzed by alpha-glycerophosphate dehydrogenase enzyme.
This enzyme in mitochondria is flavoprotein dependent.
–– Reducing equivalents are accepted by FAD and are converted into FADH2.
–– FADH2 enters electron transport chain and produces two ATP molecules.

Significance

• Alpha-glycerophosphate shuttle system is less significant for human tissues.
NADH2 produces two ATP molecules through this system.

• One molecule of glucose after entering into glycolysis and TCA cycle pro-
duces 36 ATP molecules in alpha-glycerophosphate shuttle system.

Steps in Working of Malate Shuttle System

• This shuttle system is highly important for human tissues for production of ATP
molecules.

• In cytosol
–– Oxaloacetate is reduced into malate. Reaction is catalyzed by malate dehy-
drogenase enzyme. It is NADH-dependent enzyme in cytosol. NADH2 mole-
cule provides reducing equivalents for reduction, and it is oxidized into NAD+.
–– Malate crosses the mitochondrial membranes and enters the matrix.

• In mitochondrial matrix
–– Malate is oxidized into oxaloacetate. Reaction is catalyzed by malate dehy-
drogenase enzyme. This enzyme in mitochondria is NADH-dependent.
–– NAD+ coenzyme accepts reducing equivalents and reduced into NADH2
which in turn enters ETC and releases protons.

420 15  Metabolism of Carbohydrates

Significance

• One molecule of glucose after entering into glycolysis and TCA cycle pro-
duces 38 ATP molecules in malate shuttle system.

• This is a universal shuttle system for the oxidation of NADH2 and synthesis of
ATP molecules.

15.6 H exose Monophosphate Shunt (HMP)

Definition
Hexose monophosphate shunt is an alternate carbohydrate metabolic pathway
for the oxidation of glucose.

• HMP shunt is also called as pentose-phosphate pathway (PP pathway) or
Warburg-D­ ickens-Lipman pathway.

Site of Occurrence

• HMP shunt chiefly occurs in tissues which are actively involved in synthesis of
lipid and nucleotides.

• It occurs in the liver, erythrocytes, adipose tissues, adrenal cortex, gonads, lactat-
ing mammary glands, cornea, and lens.

Characteristics

• HMP shunt is mainly an anabolic pathway unlike glycolysis which is a catabolic
pathway.

• HMP shunt involves oxidation of glucose like glycolysis; however NADH and
ATP are not generated in HMP. It serves to generate NADPH called as reducing
equivalents.

• In HMP shunt, hydrogen atoms are accepted by NADP coenzyme and not NAD+
coenzyme as in glycolysis.

• Oxidation of glucose results in formation of CO2 in HMP shunt, while it is not
formed in EMP pathway.

• HMP pathway and EMP pathway occur in cytosol of cells.

Steps in HMP Shunt
This pathway occurs in two separate phases:

• Phase A – conversion of hexose into pentose
• Phase B – conversion of pentose into hexose

15.6  Hexose Monophosphate Shunt (HMP) 421

Biochemical Reactions in Phase A

1. Phosphorylation of glucose
Glucose is phosphorylated into glucose-6-phosphate. Reaction is catalyzed by
hexokinase enzyme. One molecule of ATP is hydrolyzed into ADP to provide
high-energy phosphate to glucose as in Fig. 15.5.

2 . Oxidation of glucose-6-phosphate
Glucose-6-phosphate is dehydrogenated to form a cyclic ester called as
6-­phosphogluconolactone as in Fig. 15.5.
Reaction is catalyzed by glucose-6-phosphate dehydrogenase. Two hydrogen
atoms are liberated and accepted by coenzyme NADP. It is reduced into NADH2.

3 . Hydrolysis of 6-phosphogluconolactone
The cyclic ester, 6-phosphogluconolactone, is unstable. It undergoes nonenzy-
matic hydrolysis to form 6-phosphogluconic acid as in Fig. 15.5.

4. Oxidation of 6-phosphogluconic acid
Oxidation of 6-phosphogluconic acid forms ribulose-5-phosphate as in Fig. 15.5.
Reaction is catalyzed by 6-phosphogluconate dehydrogenase. Coenzyme NADP
accepts two hydrogen atoms and reduced into NADPH2.

5. Conversion of ribulose-5-phosphate
Ribulose-5-phosphate is catalyzed by two distinct enzymes. Ribulose-5-­
phosphate epimerase converts ribulose-5-phosphate into xylulose-5-p­ hosphate.
Another enzyme ribulose-5-phosphate isomerase converts ribulose-5-phosphate
into ribose-5-phosphate as in Fig. 15.5.

Biochemical Reactions in Phase B

1. Transketolation
Ribose-5-phosphate reacts with xylulose-5-phosphate to form sedoheptulose-7­ -­
phosphate and glyceraldehyde-3-phosphate as in Fig. 15.5.
Reaction is catalyzed by transketolase.

2 . Transaldolation
Sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate react together to
form erythrose-4-phosphate and fructose-6-phosphate as in Fig. 15.5.
Reaction is catalyzed by transaldolase.

3. Transketolation
Erythrose-4-phosphate reacts with xylulose-5-phosphate to form fructose-­6-­
phosphate and glyceraldehyde-3-phosphate as in Fig. 15.5.
Reaction is catalyzed by transketolase.
Fructose-6-phosphate and glyceraldehyde-3-phosphate are converted into
glucose-6-phosphate.

422 15  Metabolism of Carbohydrates

Glucose Kinase Glucose – 6 – P
6 molecules Mg++

ADP ADP NADP+ Glucose – 6 – P
Dehydrogenase

NADPH2 Mg++

6 – phosphoglucono
lactone

NADPH2 NADP+ H2O Hydrolase

Phosphogluconolactone

Dehydrogenase

Ribulose – 5 – Phosphate 6 – phosphogluconate

6 molecules CO2 6 molecules
Isomerase Epimerase
xylulose – 5 – Phosphate
Ribose – 5 – Phosphate 4 molecules
2 molecules
2 mol

Ribose – 5 – Phosphate + xylulose – 5 – Phosphate

(2 mol) ketoTlarasens (2 mol)

TPP 2 mol

Sedoheptulose – 7 – Phosphate + Glyceraldehyde

(2 mol) Transaldolase 3 – phosphate Transketolase
(2 mol)

Fructose – 6 – Phosphate + Erythrose – 4 – phosphate

(2 mol) (2 mol)

Fructose – 6 – Phosphate + Glyceraldehyde – 3 – phosphate
(2 mol) (2 mol)

Fig. 15.5  Hexose monophosphate shunt

Significance of Hexose Monophosphate Shunt

• Synthesis of NADPH
NADPH is the reducing equivalent which is produced in HMP pathway. It
donates hydrogen atoms in various anabolic reactions in tissues as:
–– Cholesterol synthesis
–– “De novo” synthesis of fatty acids

15.7 Gluconeogenesis 423

–– Synthesis of steroid hormones
–– Uronic acid pathway in synthesis of ascorbic acid
• Synthesis of pentoses
D-ribose is synthesized in HMP pathway. It is a structural component of nucleic
acid.
• Synthesis of CO2
Carbon dioxide is produced in HMP pathway. It is used in synthesis of purine
and fatty acids.
HMP shunt liberates fructose-6-phopshate and glyceraldehyde-3-phosphate
which are utilized in EMP pathway for synthesis of ATP.
• Maintenance of RBC integrity
In RBCs, reduced glutathione (G-SH in glutathione peroxidase) is helpful in decom-
position of hydrogen peroxide into water and oxygen. Glutathione is oxidized (G-S-
S-G) in reaction. NADPH is essential to convert oxidized glutathione into reduced
glutathione. NADPH prevents oxidative damage to erythrocytes by H2O2.
• Helpful in phagocytosis
NADPH is necessary for synthesis of superoxide radicals which destroy foreign
substance in macrophages.

15.7 Gluconeogenesis

Definition
Gluconeogenesis is an anabolic process in which glucose is synthesized from
non-carbohydrate substances.

Site of Occurrence

• The liver is the chief organ for gluconeogenesis.
• It also occurs in the kidneys.

Gluconeogenesis from Pyruvate
Pyruvate is produced in glycolysis. It is a metabolic product in transamination of
amino acids. Pyruvate is converted into glucose through the following steps:

• Pyruvate undergoes carboxylation to form oxaloacetate. Reaction is catalyzed by
pyruvate carboxylase. One molecule of ATP is hydrolyzed to ADP with release
of inorganic phosphate.

• Oxaloacetate undergoes simultaneously decarboxylation and phosphorylation to
form phosphoenol pyruvate. Reaction is catalyzed by phosphoenol pyruvate car-
boxykinase. One molecule of GTP is hydrolyzed into GDP with release of CO2.

• Phosphoenol pyruvate is reversibly converted into fructose-1,6-bisphosphate.
• Fructose-1,6-bisphosphate is converted into fructose-6-phosphate by fructose-­

1,6-bisphosphatase enzyme. This is a hydrolysis reaction with release of inor-
ganic phosphate.

424 15  Metabolism of Carbohydrates

• Fructose-6-phosphate is reversibly isomerized into glucose-6-phosphate by
phosphoglucoisomerase.

• Glucose-6-phosphate is converted into glucose by glucose-6-phosphatase enzyme.

Gluconeogenesis from Glucogenic Amino Acids

• Glucogenic amino acids undergo transamination and deamination. They are
metabolized into intermediates of TCA cycle.

• Intermediates are finally converted into oxaloacetate and pyruvate which enters
gluconeogenesis pathway.

Gluconeogenesis from Glycerol

• Mobilization of fat in adipose tissues releases fatty acids and glycerol. Glycerol
is transported to the liver by blood circulation.

• In the liver, glycerol is phosphorylated into glycerol-3-phosphate by glyceroki-
nase enzyme. Adipose tissues lack above enzyme.

• Glycerol-3-phosphate undergoes oxidation to form dihydroxyacetone phosphate
by glycerophosphate dehydrogenase enzyme. Coenzyme NAD+ is reduced into
NADH2.

• Dihydroxyacetone phosphate isomerizes into glyceraldehyde-3-phosphate by
phosphotrioisomerase.

• Glyceraldehyde-3-phosphate undergoes gluconeogenic reactions to form glu-
cose as in pyruvate to glucose conversion.

15.8 Gluconeogenesis from Lactic Acid (Cori’s Cycle)

Definition
It is metabolic pathway in which lactate synthesized in skeletal muscles is con-
verted into glucose in the liver.

The pathway was discovered by Carl Cori and Gerty Cori.

Site of Occurrence

• Cori’s cycle occurs in the liver and skeletal muscles.

Steps in Cori’s cycle
Steps in Cori’s cycle can be divided into two groups:

Steps in skeletal muscles

• Skeletal muscles store glycogen to serve a source of energy. Skeletal muscles are
actively contractile tissues as in Fig. 15.6.

• In skeletal muscles, glycogen is decomposed into glucose-1-phosphate which is
converted into glucose-6-phosphate by phosphoglucomutase enzyme. The
glucose-­6-phosphate undergoes glycolysis to generate ATP for muscular activity.

15.8  Gluconeogenesis from Lactic Acid (Cori’s Cycle) 425

Glucose Glucose Glycogenesis Glucose

Gluconeogenesis Glucose 6 – P
Liver
Skeletal muscle fibre
Pyruvate

COOH

Blood C=O

Pyruvate CH3

NADH2

Lactate Lactate NAD+
dehydrogenase
NAD+ dehydrogenase

NADH2 Lactate COOH

Lactate CHOH

CH3
Lactate

Fig. 15.6  Cori’s cycle

• Under normal muscular activity, supply of oxygen is adequate in muscles. The
glucose-6-phosphate is aerobically converted into pyruvate which enters TCA
cycle to produce ATP molecules.

• Under strenuous muscular contractility, oxygen supply to muscles is
decreased. Pyruvate molecule is reduced into lactate by lactate dehydroge-
nase enzyme in the presence of NADH2.

• Pyruvate reduction yields NAD+ molecules which are essential to maintain gly-
colysis in anaerobic condition.

• Accumulation of lactate in skeletal muscle can cause muscle cramps. To avoid it,
lactate diffuses rapidly into blood circulation. Lactate is transported to the liver.
Skeletal muscles are unable to convert lactate into glucose as in Fig. 15.6.

Steps in the liver

• In the liver, lactate is oxidized into pyruvate by lactate dehydrogenase enzyme in
the presence of NAD+ coenzyme.

• Pyruvate is converted into glucose.
• Liver glucose is either stored as glycogen or released into systemic circulation as

in Fig. 15.6.

426 15  Metabolism of Carbohydrates

Significance of Cori’s Cycle

• Cori’s cycle is an important pathway for gluconeogenesis. It is helpful in uti-
lizing and converting lactate (waste metabolite) into glucose.

• Cori’s cycle helps in prevention of lactic acidosis (accumulation of lactate in
tissues, type of metabolic acidosis)

15.9 G lycogenolysis

Definition
It is a biochemical process of breakdown of glycogen into glucose in body tissues.

Site of Occurrence

• Glycogenolysis occurs in the liver and skeletal muscles.

Steps in Glycogenolysis
Cleavage of alpha-1 → 4-glycosidic linkage

• Glycogenolysis is started by phosphorylase enzyme. It is the key regulatory
enzyme for breakdown of glycogen into glucose-1-phosphate. This enzyme
splits alpha-1 → 4-glycosidic linkages in glycogen chains. After every cleavage,
glycogen molecule becomes shorter by one glucose residue. The cleavage of
alpha-1 → 4-glycosidic linkages continues till four glucose moieties remain on
either side of glycogen chain. Phosphorylase enzyme cannot split
alpha-1 → 6-g­ lycosidic linkages as in Fig. 15.7.
Phosphorylase enzyme
It is found in liver cells as well as in skeletal muscle fibers.
–– Liver phosphorylase enzyme
–– Liver contains two forms of enzyme:
1. Phospho-phosphorylase enzyme:
It is an active form of enzyme and exists in phosphorylated form.
2. Dephospho-phosphorylase enzyme:
It is an inactive form and exists in dephosphorylated form.
–– Muscle phosphorylase enzyme
Skeletal muscles contain two forms of enzyme:
1 . Phosphorylase a
It is an active form of enzyme.
2. Phosphorylase b
It is an inactive form of enzyme.

Glucagon hormone has no effect on muscle phosphorylase.
Pyridoxal phosphate as coenzyme (four molecules) is essential for muscle

phosphorylase.

15.9  Glycogenolysis 427
∝ – 1 – 4 –Glycosidic bond
Glycogen
1 15
19
10
Pi Glycogen phosphorylase ∝ – 1 → 6 –Bond

4 – Glucose Glycogen + GLUOCSE – 1 – PHOSPHATE
Residues
{ }One glucose
1 residue shorter

DebranEcnhziynmg e 5
9

4

Four glucose 6 – bond

residues on one side of ∝– 1

Glucose – 1 – Phosphate

Phosphogluco Mg++
mutase

Glucose – 6 – P
Glucose – 6 – Phosphatase

Glucose

Fig. 15.7 Glycogenolysis

Cleavage of alpha-1 → 6-glycosidic linkage

• Debranching enzyme called as amylo-1-6-glucosidase splits 1  →  6-glycosidic
linkage between a branch chain and main chain of glycogen. It liberates a residue
of glucose as in Fig. 15.7.

• Phosphorylase and debranching enzymes cleavage glycogen into glucose-1­-­
phosphate residues.

428 15  Metabolism of Carbohydrates

Conversion of glucose-1-P into glucose-6-phosphate

• Glucose-1-p is converted into glucose-6-P by activity of phosphoglucomutase
enzyme as in Fig. 15.7.

Fate of glucose-6-phosphate

• Glycogen is hydrolyzed into glucose-6-P residues in the liver and skeletal tissues.
• In the liver, glucose-6-phosphatase enzyme dephosphorylates glucose-6-P into

free glucose molecules. This enzyme is also found in renal tubules. Glucose from
liver cells enters systemic circulation and results into hyperglycemia.
• In skeletal tissues, glucose-6-phosphatase enzyme is not found. Therefore,
G-6-P is not converted into glucose molecules. Glucose-6-phosphate enters gly-
colysis cycle and is converted into pyruvate and lactate.

Regulation of Glycogenolysis
Role of phosphorylase enzyme

• It is a key enzyme in glycogenolysis.
• Rise in cyclic AMP in cell activates protein kinase enzyme. It activates phos-

phorylase kinase b (inactive form) into phosphorylase kinase a (active form).
• Phosphorylase kinase a in turn activates dephospho-phosphorylase into phospho-­

phosphorylase. It promotes glycogenolysis.

Role of hormones

• Thyroxine, epinephrine, and glucagon stimulate synthesis of cAMP in cytosol
and promote glycogenolysis.

• Glucagon promotes glycogenolysis in the liver only.

Role of calcium ions

• Rate of glycogenolysis in muscles is higher than liver cells during contraction.
• Phosphorylase kinase enzyme in muscles has four subunits. Its beta subunit has

the ability to bind with Ca++ ions and calmodulin (calcium-modulated protein),
and its activity is enhanced. Calcium ions promote glycogenolysis in muscles.

15.10 Glycogenesis

Definition
Glycogenesis is the biochemical process of formation of glycogen from glucose
in body tissues.

15.10 Glycogenesis 429

Site of Storage of Glycogen
Glycogen is a storage form of carbohydrate in human body. It is stored in liver cells
and skeletal tissues.

• In the liver, about 70–100 g of glycogen is stored. This quantity of glycogen
constitutes 4–6% weight of liver (normal weight = 1.8 kg).

• In skeletal tissues, about 240 g of glycogen is stored, and it represents around
0.7% of the weight of skeletal muscles in the body (weight of skeletal
muscles = 35 kg).

• Total quantity of glycogen storage is between 300 and 350 g in an adult person
with 70 kg of weight of the body.

Site of Occurrence

• It takes place mainly in the liver and skeletal tissues. However, glycogenesis can
also occur in all body tissues.

Steps in Glycogenesis
Phosphorylation of glucose

• Glucose is phosphorylated into glucose-6-phosphate. Reaction is catalyzed by
glucokinase enzyme (liver) and hexokinase enzyme (skeletal tissues).

• Reaction occurs in the presence of Mg ions, and one molecule of ATP is con-
sumed in reaction as in Fig. 15.8a, b.

Conversion of G-6-P into G-1-P

• Glucose-6-P is converted into glucose-1-phosphate through action of phospho-
glucomutase enzyme in the presence of Mg++ as in Fig. 15.8a, b.

Formation of uridine-diphosphate-glucose

• Glucose-1-phosphate is transferred to uridine triphosphate (UTP) molecule to
form uridine diphosphate glucose (UDP-G).

• Reaction is catalyzed by UDP-G pyrophosphorylase enzyme.
• A molecule of inorganic pyrophosphate is liberated in the reaction. It is cleav-

aged into inorganic phosphate by pyrophosphatase enzyme as in Fig. 15.8a, b.

Transfer of glucose residue to glycogen primer

• Glucose residue from UDP-G is transferred to glycogen primer. Its presence is
essential for initiation of glycogen synthesis.