Mechanical digestion of protein starts in the mouth with the break down of protein into smaller pieces aided by the biting and crushing action of teeth. Protein then travels to the stomach via the esophagus where chemical digestion starts. Cells lining the stomach secretes hydrochloric acid (HCL) and pepsinogen, a proenzyme into the stomach. Cells that secrete HCl are called parietal cells, and those that secrete pepsinogen are called chief cells.
The very acidic condition of the stomach (pH = 1.5 – 3.5) causes pepsinogen to be converted to pepsin, the enzyme responsible for breaking down protein in the stomach to amino acids and small peptides. Pepsin, which has an optimal pH of 2, breaks protein chains from the carboxy end of sites where one of the amino acid is aromatic (tyrosine, tryptophan, or phenylalanine).
After digestion in the stomach, the soupy mush called chyme is delivered to the duodenum which has a pH of about 6. This high pH inactivates pepsin preventing it from digesting the walls of the duodenum. The presence of food in the small intestine causes hormones such as secretin, cholecystokinin (CCK) and enterogastrone to stimulate the pancreas to release enzymes into the duodenum. Proteolytic enzymes are released in their proenzyme form and are converted to their active form in the duodenum. For example trypsinogen is converted by enterokinase to trypsin. Trypsin then activates other proteolytic proenzymes including chymotrypsinogen, procarboxypeptidase, and proelastase to produce chymotrypsin, carboxypeptidase, and elastase respectively.
These enzymes breakdown small peptides into tripeptides, dipeptides and amino acids. Trypsin cleaves the C-terminal side of bonds where there are lysine and arginine residues. Chymotrypsin functions in a similar way as pepsin, targeting aromatic sites of amino acids. Carboxypeptidase and elastase cleaves and releases C-terminal amino acids. Carboxypeptidase may be classified as carboxypeptidase-a which which targets aromatic and aliphatic amino acids and and carboxypeptidase-b which targets the basic amino acids, arginine and lysine.
Amino acids, dipeptides and tripeptides get transported across the villi of the small intestine and into cells lining the villi called enterocytes. Brush border enzymes (enzymes lining the villi) such as amino peptidases, and dipeptidases further breakdown small peptides. Aminopeptidases cleaves amino acids from the N-terminus of peptides, and dipeptidase breaks down dipeptides to two individual amino acids.
Some dipeptides and tripeptides reach the inside of the enterocytes. These are broken down to individual amino acids by enteropeptidases. Amino acids are then transported out of the enterocytes to the capillaries and eventually to the hepatic portal vein which takes proteins to the liver for protein synthesis.