Digestion of proteins in human body

  • Proteins of the ingested food are broken down into amino acids by proteases (peptidases).
  • The proteases are secreted in inactive pro-enzymes to protect the mucous membranes of the gut from hydrolysis.
  • Protein digestion starts in the stomach and is completed in the small intestine because saliva contains no protease.

I. Stomach:

  • Food undergoes both mechanical and chemical changes in the stomach.

a. Mechanical change:

  • Wall of the stomach undergoes periodic muscular contractions.
  • This churns and breaks the food mechanically and mixes thoroughly with the gastric juice for proper action of the enzymes.
  • The cardiac and pyloric sphincters remain closed during the churning of food.

b. Chemical change:

  • The gastric juice contains mucus, HCl and two inactive proteases: pepsinogen (propepsin) and prorennin.

1. Hydrochloric acid: It serves many functions:

  • It disinfects the food by killing bacteria.
  • It stops the action of salivary enzymes in the due course of time.
  • It activates pepsinogen to pepsin and prorennin to rennin.
  • It produces an appropriate pH (acidic) for protein digestion.
  • It softens the food and breaks down the cementing material between the cells of the food.
  • It denatures many food proteins, increasing exposure of their peptide bonds to pepsin which facilitates the action of pepsin on them.

2. Pepsinogen:

  • It is first changed by HCl into an active enzyme pepsin which then itself continues to activate additional pepsinogen, the process called autocatalytic activation.
  • Pepsin hydrolyses the proteins partially to proteoses and peptones in an acidic medium (pH 2).
  • It can digest collagen of the white fibers of connective tissues but no keratin of hair, horns and nails.

Pepsinogen + HCl ———-> Pepsin

Proteins ———-> Proteoses and Peptones or peptides (by pepsin)

3. Prorennin:

  • It is first hydrolyzed by HCl to an active enzyme rennin.
  • Rennin hydrolyses the soluble milk protein casein into paracasein and whey protein.
  • Paracasein is spontaneously precipitated in the presence of calcium and insoluble calcium paracaseinate, forming solid curd, or coagulated milk.
  • The coagulated milk stays in the stomach for a longer period for proper action of pepsin.
  • Pepsin converts this calcium paracaseinate also into peptones.
  • Rennet tablets containing rennin obtained from calf are commercially used for curdling milk.

Prorennin + HCl ———-> Rennin

Casein ———-> Paracasein + Whey protein (by rennin)

Paracasein + Calcium ———-> Calcium Paracaseinate

Calcium paracaseinate ———-> peptones (by pepsin)

  • As a person matures, rennin production decreases greatly and stops altogether.

II. Small Intestine:

  • In the small intestine, food receives three alkaline secretions: bile (pH 8) from the liver, pancreatic juice (pH 8.8) and intestinal juice (pH 8.3) from the intestinal glands.
  • Their alkalinity stops the action of pepsin.

a. Bile: It contains no enzymes and hence has no action on proteins.

b. Pancreatic juice:

  • It contains 3 inactive proteases: trypsinogen, chymotrypsinogen and Procarboxypeptidases.
  • Being inactive, the proteases don’t digest the pancreas itself during their secretion and storage.

i. Trypsinogen:

  • It is activated to an enzyme trypsin by a non-digestive enzyme enterokinase or enteropeptidase present in the intestinal juice.
  • Once formed, trypsin activates additional trypsinogen by autocatalysis.
  • Trypsin breaks more proteins, especially basic proteins, into proteoses and peptones (peptides).
  • It has a limited action on collagen and no action on casein and keratin.
  • It coagulates the blood by hydrolyzing its fibrinogen into fibrin in predatory animals.

Trypsinogen ———-> Trypsin (by enterokinase)

Proteins ———-> Peptides (by trypsiin)

ii. Chymotrypsinogen:

  • It is activated to an active enzyme chymotrypsin by tryspsin.
  • It also splits the proteins into peptides.
  • It also coagulates milk in an alkaline medium by hydrolyzing milk protein casein to paracasein and whey proteins.
  • Paracasein coagulates in the presence of calcium, forming calcium paracaseinate.

Chymotrypsinogen ———-> Chymotrypsin (activation by trypsin)

Proteins ———-> Peptides (by chymotrypsin)

iii. Procarboxypeptidases:

  • These are activated to enzymes carboxypeptidases by trypsin.
  • These enzymes separate individual amino acids adjacent to the free terminal carboxyl group of the peptides, reducing them into dipeptides. i. e. they produce amino acids and dipeptides.

Procarboxypeptidases ———-> Carboxypeptidases (by trypsin)

Peptides ———-> Dipeptides + Amino acids (by carboxypeptidase)

III. Intestinal juice:

  • It contains two digestive proteases: aminopeptidases and dipeptidases, and a non-digestive protease enterokinase (enteropeptidase).

a. Enteropeptidase: It activates the pancreatic trypsinogen into trypsin as above mentioned.

b. Aimnopeptidase:

  • These separate individual amino acids adjacent to the free terminal amino group of the peptides, reducing them into dipeptides.
  • In other words, they produce amino acids and dipeptides like in carboxipeptidases of pancreatic juice.

Peptides ———-> Dipeptides + Amino acids (by aminopeptidase)

  1. Dipeptidases: These hydrolyze the dipeptides into amino acids.

Dipeptides ———-> Amino acids (by dipeptidase)

Digestion of proteins in human body