Protein digestion occur in stomach and duodenum with three main enzymes.

What is protein:-

Protein are the natural polymers that are ranked first amongst the chemical substance essential for growth and maintenance of life.

Source of protein:-

The good source of protein in our diet are ;

  • Cereals
  • Pulses
  • Oil seed and nuts
  • Fish, meat and eggs
  • Milk and its products
  • Leafy vegetables,
  • Fruit
  • Soyabean
  • Groundnut
  • Peas and beans

Amino acid are help to made of protein. amino acid are bond with peptide bond and formation of protein.

Daily requirement of protein :-

For good growth of body need protein in daily diet. Daily protein need is near about 70-100 gm.

Growing children, pregnant women and convalescent person need more protein in the diet. Men should draw the 15% of total caloric requirements from proteins.

Digestion of proteins:-

Proteins of the ingested food are broken down to amino acid by peptidases. It is secreted as inactive pro-enzymes to protect the mucous membrane of the gut from hydrolysis. Mechanical Protein digestion beings in the mouth and continue in the stomach and small intestine. But chemical protein digestion being in the stomach and completed in the small intestine. The steps of protein digestion are following:

1. Protein digestion in stomach:-

Food undergoes both mechanical and chemical changes in the stomach.

(a) Mechanical Change:-

Wall of the stomach undergoes periodic muscular contraction. This churns and break the food mechanically and mixes it thoroughly with the gastric juice for proper action of enzymes. The cardiac and pyloric sphincter remain closed during churning of food.

(b) Chemical Change:-

In the stomach, the food meets the gastric juice secreted by the gastric gland. The gastric juice contains mucus, hydrochloric acid and two inactive proteases : pepsinogen and pro-rennin.

Hydrochloric acid :-

  1. It disinfects the food by killing bacteria.
  2. They stops the action of salivary enzyme in due to course of time.
  3. It activate pepsinogen to pepsin and pro-rennin to rennin.
  4. They produce an appropriate pH (acidic) for protein digestion.
  5. Hydrochloric acid denature many food protein, increasing exposure of their peptide bonds to pepsin. This facilitate the action of pepsin on them.


It is first changed by hydrochloric acid into an active enzyme pepsin. Then the pepsin itself continues to active additional pepsinogen. Pepsin hydrolyses the proteins partially to proteoses and peptones in an acid medium. It can digest collagen of the white fibers of connective tissue but not keratin of hair, horns and nails.

Pepsinogen + HCL = Pepsin


It is hydrolysed by hydrochloric acid 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 as insoluble calcium paracaseinate, forming solid curd, or coagulated milk.

The curd stay in the stomach for a longer period for proper action of pepsin. Pepsin converts calcium paracaseinate also into peptone. Rennin are help to obtained rennet tablet. which obtained from calf are commercially used for curling milk.

2. Protein digestion in small intestine:-

In the small intestine, food receives three alkaline secretions :

  • Bile (pH 8) from the liver
  • Pancreatic juice (pH 8.8) from the pancreas.
  • Intestinal juice ( pH 8.3) from the intestinal gland.

Their alkalinity stops the action of pepsin.

(a) Bile:-

Bile contains no enzyme, and has no action on proteins.

(b) Pancreatic juice :-

It contains 3 inactive proteases ; Trypsinogen, chymotrypsinogen and procarboxypeptidase. Being inactive, the proteases do not digest the pancreas itself during their secretion and storage.


Its activation to an enzyme trypsin is started by a non-digestive enzyme enterokinase. That enzyme present in the intestinal juice. Once formed, trypsin activate additional trypsinogen.

This is another instance of auto catalytic. Trypsin break more protein, especially basic proteins, into proteoses and peptones. 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.

Proteins ++ Trypsin =peptidos


It is activated to an enzyme chymotrypsin by trypsin. They also splits the proteins into peptide. It also clots milk in an alkaline medium by hydrolyzing milk protein casein to para-casein and whey proteins. Para-casein coagulates in the presence of calcium, forming calcium para-caseinate (curd).


These are activated to enzyme carboxypeptidase by trypsin. These enzyme separate individual amino acids adjacent to the free terminal carboxyl group of the peptide, reducing them to dipeptides. They, thus, produce amino acids and dipeptides.

(c) Intestinal Juice :-

It contains two digestive proteases ; amino-peptidases and dipeptides, and non digestive protease enterokinase.


It activate the pancreatic Trypsinogen to trypsin as already mentioned.


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


These hydrolyses the dipeptides into amino acids.