Question

Describe the process of digestion of proteins in human beings.

Answer 1

Digestion of Proteins in Humans:

1. Protein digestion occurs in the stomach and duodenum.
2. Three main enzymes are responsible for protein digestion: pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas.
3. These enzymes break down food proteins into polypeptides that are then broken down by various exopeptidases and polypeptidases into amino acids.
4. Then finally amino acids are absorbed into the bloodstream through the small intestine.

Digestion of Proteins in the stomach:

1. Gastric juice secreted by gastric glands of the stomach contains HCl and two proenzymes- pepsinogen and prorenin.
2. HCl maintains a pH of 2 - 3.7 and kills bacteria and other pathogens.
3. Gastric juice mixes with food until it becomes a semi-solid mass called Chyme.
4. Proenzymes become activated as below:

$\mathrm{Pepsinogen}\stackrel{\mathrm{HCL}}{\to }\mathrm{Pepsin}$

$\mathrm{Prorennin}\stackrel{\mathrm{HCL}}{\to }\mathrm{Rennin}$

$\mathrm{Casein}\stackrel{\mathrm{Rennin}}{\to }\mathrm{Paracasein}$

$\mathrm{Paracasein}+\mathrm{Ca}\to \mathrm{Calcium}\mathrm{paracasinate}\left(\mathrm{curd}\right)$

$\mathrm{Calcium}\mathrm{paracasinate}\stackrel{\mathrm{Pepsin}}{\to }\mathrm{Peptones}$

$\mathrm{Proteins}\stackrel{\mathrm{Pepsin}}{\to }\mathrm{Peptones}$

Digestion of Proteins in the small intestine:

1. Pancreatic juice contains the proenzymes trypsinogen, chymotrypsinogen, and procarboxypeptidase.

$\mathrm{Trypsinogen}\stackrel{\mathrm{Enterokinase}}{\to }\mathrm{Trypsin}$

$\mathrm{Chymotrypsinogen}\stackrel{\mathrm{Trypsin}}{\to }\mathrm{Chymotrypsin}$

$\mathrm{Prokarboxypeptidase}\stackrel{\mathrm{Trypsin}}{\to }\mathrm{Carboxypeptidase}$

$\mathrm{Peptones}\stackrel{\mathrm{Trypsin}}{\to }\mathrm{Peptides}$

$\mathrm{Peptones}\stackrel{\mathrm{Chymotrypsin}}{\to }\mathrm{Peptides}$

$\mathrm{Peptides}\stackrel{\mathrm{Carboxypeptidase}}{\to }\mathrm{Small}\mathrm{peptides}+\mathrm{Amino}\mathrm{acids}$

2. Intestinal juice contains enterokinase, aminopeptidase, and dipeptidase. The actions of these enzymes are summarized below:

$\mathrm{Peptides}\stackrel{\mathrm{Aminopeptidase}}{\to }\mathrm{Small}\mathrm{peptides}+\mathrm{Amino}\mathrm{acids}$

$\mathrm{Peptides}\stackrel{\mathrm{Dipeptidase}}{\to }\mathrm{Amino}\mathrm{acids}$