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 (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.