Digestion of proteins.
Protein digestion
Digestion in Stomach
Stimulated by food acetylcholine, histamine and gastrin
are released onto the cells of the stomach
The combination of acetylcholine, histamine and gastrin
cause the release of the gastric juice.
Mucin - is always secreted in the stomach
HCl - pH 0.8-2.5 (secreted by parietal cells)
Pepsinogen (a zymogen, secreted by the chief cells)
Hydrochloric acid:
 Creates optimal pH for
pepsin
 Denaturates proteins
 Kills most bacteria and
other foreign cells
Pepsinogen (MW=40,000) is activated by the enzyme pepsin
present already in the stomach and the stomach acid.
Pepsinogen cleaved off to become the enzyme pepsin
(MW=33,000) and a peptide fragment to be degraded.
Pepsin partially digests proteins by cleaving the peptide bond
formed by aromatic amino acids: Phe, Tyr, Trp
Digestion in the Duodenum
Stimulated by food secretin and cholecystokinin regulate
the secretion of bicarbonate and zymogens trypsinogen,
chymotrypsinogen, proelastase and procarboxypeptidase
by pancreas into the duodenum
Bicarbonate changes the pH to about 7
The intestinal cells
secrete an enzyme
called
enteropeptidase
that acts on
trypsinogen cleaving
it into trypsin
Trypsin converts chymotrypsinogen into chymotrypsin,
procarboxypeptidase into carboxypeptidase and
proelastase into elastase, and trypsinogen into more
trypsin.
Trypsin which cleaves peptide bonds between basic
amino acids Lys and Arg
Chymotrypsin cleaves the bonds between aromatic amino
acids Phe, Tyr and Trp
Carboxypeptidase which cleaves one amino acid at a
time from the carboxyl side
Aminopeptidase is secreted by the small intestine and
cleaves off the N-terminal amino acids one at a time
Most proteins are completely digested to free amino acids
Amino acids and sometimes short oligopeptides are
absorbed by the secondary active transport
Amino acids are transported via the blood to the cells of
the body.
The ways of entry and using of amino
acids in tissue
The sources of amino acids:
1) absorption in the intestine;
2) protein decomposition;
3) synthesis from the carbohydrates and lipids.
Using of amino acids:
1) for protein synthesis;
2) for synthesis of other nitrogen containing
compounds (creatine, purines, choline, pyrimidine);
3) as the source of energy;
4) for the gluconeogenesis.