Digestion of dietary proteins
Digestion of dietary proteins starts in the stomach.
Secretion of HCl activates formation of pepsin from
pepsinogen which hydrolyses peptide bonds of dietary
proteins to release aromatic amino acids. Similarly, in
the small intestine digestion continues. Zymogens are
activated for the breakdown of peptide bonds to release
amino acids which are absorbed into the small intestine.
Amino acid deamination
Transamination is a process by which amino group is transferred to
the keto acid to yield the keto acid of the original amino acid and
a new amino acid, catalyzed by amino transferases. Glutamate is
oxidatively deaminated in the mitochondrion and release ammonia.
The Urea Cycle
This cyclic pathway takes place in the liver, partly in the
mitochondria and partly in cytosol. There is interrelation between
urea cycle and citric acid cycle. Fumarate of the urea cycle is
taken up by citric acid cycle. Carbamoyl phosphate synthetase I is
the regulatory enzyme.
Metabolic breakdown of individual amino
acids
Catabolism of amino acids gives rise to the intermediate compounds
of citric acid cycle. Alanine, serine, cysteine and asparagine are
converted to oxaloacetate. Glutamine, proline, arginine and
histidine are converted to α-ketoglutarate through glutamate.
Succinyl CoA is a point of entry for non polar amino acids like
methionine, valine and isoleucine. Leucine is degraded to acetyl CoA
and acetoacetate. Tryptophan, lysine, leucine, phenylalanine,
tyrosine and isoleucine donate their carbons to acetyl CoA. Various
disorders of amino acid catabolism are observed due to the defective
enzymes. Tryptophan, tyrosine, glycine and glutamate are the
precursors of some of the biologically important compounds. Biotin,
Tetrahydrofolate or S-Adenosyl methionine is the enzyme cofactors in
catabolism which transfer carbon compounds.
Amino acid biosynthesis
Nonessential amino acids are formed from intermediates of
carbohydrate metabolism. Alanine is formed from pyruvate and
aspartate from oxaloacetate. Asparagine is formed from aspartate.
Glutamate is formed from α ketoglutarate and glutamine from
glutamate. Glutamate is the precursor of proline and arginine.
Cysteine is synthesized from 3 � phosphoglycerate. During the
process, serine is the intermediate compound which gives rise to
glycine.
Threonine is an essential amino acid. It is formed from β-aspartate.
Methionine and lysine also have a common precursor. Valine, Leucine
and isoleucine are formed from pyruvate. Phenyl alanine, tyrosine
and Tryptophan are formed from phosphoenol pyruvate and erythrose �
4 � phosphate through the intermediate compound called chorismate.
Histidine originates from PRPP and ATP.
