Digestion of proteins is initiated by:

Enzymes catalyze the digestion of proteins There are two main classes of proteolytic digestive enzymes (proteases): Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments. - Pepsin in the gastric juice catalyzes hydrolysis of peptide bonds adjacent to amino acids with bulky side-chains (aromatic and branched-chain amino acids and methionine). - Trypsin, Chymotrypsin, and elastase are secreted into the small intestine by the pancreas. - Trypsin catalyzes hydrolysis of lysine and arginine esters. - Chymotrypsin catalyzes hydrolysis esters of aromatic amino acids. - Elastase catalyzes hydrolysis esters of small neutral aliphatic amino acids. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of peptides. - Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal. - Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptidases and tripeptidases in the brush border of intestinal mucosal cells catalyze the hydrolysis of di-and tripeptides, which are not substrates for amino-and carboxypeptidases. Key Concept: Pepsin in the gastric juice catalyses hydrolysis of peptide bonds adjacent to amino acids with bulky side chains. It is the stomach where digestion of proteins is initiated with the help of pepsin.