Proteins in diet are from animal sources and
vegetable sources. Animal sources like milk, dairy products, meat, fish, liver
and eggs are rich sources of proteins.
Vegetable sources like cereals, pulses, peas,
beans and nuts are rich in protein.
There are no enzymes in mouth to degrade the
HCl : HCl secreted by the gastric mucosa
destabilizes the secondary structures of the proteins such that it can be
easily acted upon by the enzymes.
The proteolytic enzymes present in the gastric
juice are pepsin, rennin, gastricin and gelatinase.
:It is a potent
proteolytic enzyme and is present in the gastric juices. It issecreted in the
inactive zymogen form called as pepsinogen, which has a molecular weight of
42,500 daltons. In the acidic medium, pepsinogen is cleaved to pepsin and the
reaction is favoured autocatalytically. Pepsin having a molecular weight of
34,500 daltons is an endopeptidase. An endopeptidase is an enzyme that acts on
the peptide linkages in the interior of the protein.
Pepsin acts on protein to convert it to
proteoses and peptones, which are low molecular weight peptides.
Proteins-------------> Proteoses + Peptones
It has a broader specificity and acts on peptide
linkages constituted by the carboxyl group of an aromatic / hydrophobic amino
acid or amino group of a dicarboxylic acid.
It hydrolyzes the soluble casein in milk , which
along with calcium forms insoluble paracaesinate.
The optimum pH for pepsin is 1.6 – 2.5
Rennin is present in infants only and it is
secreted by the gastric mucosa as pro –rennin. It is converted to active rennin
by HCl. It also converts casein in milk to insoluble calcium paracaesinate.
The chief enzymes of the pancreatic juice that
acts on proteins are a) trypsin
chymotrypsin c) carboxy peptidase d) elastases
and e) collagenases
Trypsin, a proteinase is secreted in the
inactive zymogen form called trypsinogen. It is activated by enterokinase and
also autocatalytically in the presence of calcium.
It is an endopeptidase that is specific for
peptide linkages formed by carboxyl groups of basic amino acids, namely
arginine, lysine. The hydrolytic products are polypetides, proteoses, peptones,
di and tri peptides. It cannot hydrolyze peptide linkages which involves
It activates proelastase to elastase,
chymotrypsinogen to chymotrypsin, fibrinogen to fibrin. The optimum pH for
trypsin is 8 – 9.
It is activated by trypsin and also autocatalytically. It hydrolyses peptide
linkages with carboxyl group of aromatic amino acids like tryptophan, tyrosine
and phenyl alanine.
The optimum pH for chymotrypsin is 7 - 8
Two types of carboxy peptidases, carboxy
peptidase A and B are known. Carboxy peptidase A is a metallo enzyme that
contains zinc. Both are exopeptidases. Carboxy peptidase A is specific for
aromatic amino acids at the C terminal end, while carboxy peptidase B is
specific for basic amino acids at the C terminal end.
The optimum pH for both of them lies between
The proteolytic enzymes present in the
intestinal juice are enterokinase, amino peptidase, prolidase and di and tri
Enterokinase is an enzyme that activates trypsin
in the presence of calcium. Aminopeptidases are capable of removing one amino
acid from the N terminal end of the peptide. They cannot hydrolyze the linkages
of di-peptides or if the N terminal amino acid is proline. Prolidases are
enzymes that hydrolyze linkages involving proline.
Thus by the concerted action of all the above
enzymes, proteins are broken down to di and tri-peptides. Di and tri peptidases
present in the intestinal mucosal cells or inside the absorptive cells cleave
them to amino acids.
Amino acids and small peptides that are absorbed
will reach liver through the portal circulation.
Naturally occurring L-amino acids are absorbed
actively, while D-amino acids are absorbed passively. Absorption of amino acids
are similar to those of carbohydrates and they need a carrier and sodium ions.
Amino acids are absorbed via glutathione cycle.
The steps involved in glutathione cycle are
combines with amino acids to form γ-glutamyl amino acid and cysteinyl glycine.
amino acid is transported and hydrolyzed to oxo proline and L.amino acid, which
glycine is cleaved to cysteine and glycine.
is converted back to glutamate.
cysteine and glycine combine together and form glutathione.
phenol or cyanide decreases the absorption of amino acids.
acids compete with one another for absorption. High concentrations of one amino
acid reduces the absorption of the others.
is needed to absorb the amino acids via glutathione cycle.