Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.

Explanation

Digestion of Proteins : Enzymes hydrolysing proteins are called proteases or peptidases.
Their action is as follows :
A. Proteolytic enzymes present in gastric juice :
(i) Pepsin. It acts on proteins and forms peptones. It also hydrolyses soluble casein, a milk protein, into paracasein and whey protein.
(ii) Rennin. It is present in calf gastric juice and hydrolyses casein to paracasein leading to milk coagulation.
B. Proteolytic enzymes present in the pancreatic juice :
(i) Trypsin hydrolyzes basic protein into peptides. It also hydrolyzes fibrinogen to fibrin in case of blood sucking animals. It also activates other basic proteases.
(ii) Chymotrypsin is milk coagulating enzyme and hydrolyses casein to paracasein which then coagulates into calcium paracaseinate.
(iii) Carboxypeptidase breaks down the terminal peptide bond and forms amino acids and small peptides.
C. Proteolytic enzymes present in the intestinal juice :
(i) Enteropeptidase (Enterokinase) activates trypsinogen into active trypsin.
(ii) Aminopeptidase hydrolyses the terminal peptide bond of the peptide chain to release the last amino acids.
(iii) Dipeptidase also acts on terminal peptide bond.