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

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Digestion of proteins : 

(i) Protein digestion does not take place in the mouth. 

(ii) Gastric juices of the stomach contains the proteolytic proenzymes, pepsinogen and prorennin which are activated by the hydrochloric acid present in the gastric juice. Pepsin acts on proteins and converts them into proteoses and peptones. Rennin hydrolyses milk protein caesin into paracaesin. 

Proteins \(\overset{pepsin}{\rightarrow}\) Proteoses, peptones and large peptides. 

Caesin \(\overset{Rennin}{\rightarrow}\) Paracaesin 

Paracaesin + Ca→ Calcium paracaesinate 

Calcium paracaesinate \(\overset{pepsin}{\rightarrow}\) Peptones 

(iii) In small intestine, digestion of protein takes places with the help of enzymes of pancreatic juice and intestinal juices. 

(a) Pancreatic juices contains trypsin chymotrypsin and carboxypeptidase. Trypsin acts on proteins, proteoses and peptones and converts them into smaller peptides or dipeptides.

Caryboxypeptidase acts on the terminal peptide bond and release the last amino acid and progressively shorten the peptide chain. 

(b) Intestinal juice contains enterokinase, aminopeptidase and dipeptidases.

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