Digestion is a hydrolytic process in which complex food materials are mechanically and enzymatically broken down into simpler soluble forms. Proteins are polymers of amino acid residues which are joined end to end by peptide bonds. Proteolytic enzymes break down the peptide bonds. Protein digestion begins in the stomach and is completed in the small intestine. Absent the bolus enters the stomach, hormone called gastrin is secreted in the mucous membrane of pyloric stomach. It gets distributed to the wall of the stomach and stimulates the gastric glands to release the gastric juices. 2 to 3 litres of gastric juice is secreted daily in a man.

Gastric juice: the gastric juice consists of the following: water 90%, hydrochloric acid 0.4% prorennin and pepsinogen, gastric lipase. In the stomach protein and fat are partly digested. Prorennin and pepsinogen are protein digesting proenzymes secreted in inactive form.

Protein Digestion in Stomach:

Pepsinogen is activated byHCl to become active pepsin that starts hydrolysis of protein . Pepsin changes proteins into proteoses, petones and polypeptides. The various steps of chemical changes are summerised follows:


Pepsinogen – Activator / HCI – Pepsin (active)

Protein – Pepsin / +H2O – Proteoses

Proteoses – Pepsin / +H2O – Peptones

Peptones – Pepsin / +H2O – Polypeptides



In young mammals milk protein caseinogens is digested by the help of an enzyme called rennin. It is secreted from the funds absent prorennin which changes into active rennin in the presence of HCl. Rennin curdle milk protein caseinogens which is hydrolysed into casein. Rennin converts casein into solid calcium paracaseinate in presence of Ca++ so that pepsin can reduce it to proteoses absent peptones. The chemical events that take place are summerised absent below:

Proennin – HCl – Rennin

Caseinogen – Rennin – Casein


Casein – Rennin / HCI – Paracasein

Paracasein – Ca++ / (pH 5-6) – Calcium paracaseinate (curd)

Calcium paracaseinate – Pepsin – Proteoses and peptones

Inside the stomach due to hydrolis of protein and grinding action of its muscular wall the food is converted to a paste like pulp called chime that passes into small intestine through pylorus.


Digestion in intestine:

Being stimulated by acidic chime the mucous cells of duodenum secretes cholecystokinin and secretin hormones into blood which induce the gall bladder and pancreas respectively to release their contents interest duodenum through their ducts. Bile effects alkaline medium in the intestine for the ferments to work to their optimum level.

Parcreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidase, pancreatic amylase, lipase, maltase, sucrose, lactese, etc. of these trypsinogen, chymotrypsinogen and procarboxy peptidase are inactive enzymes (proenzymes) for protein digestion. Brunner’s glands, crypts of Leiberkuhn and mucous cells of intestine secret the intestinal juice which have enterokinase, peptidase and many other enzymes.

Enterokinase, procarboxypeptidase and peptidase help in protein digestion. Enterokinase converted trypsinogen into active trypsin. Trypsin activates chymotrypsinogen and procarboxypeptidase into chymotrypsin and carboxypeptidase respectively.typsin also actson proteoses and peptones and changes them to peptides. Now chymotrypsin reduces peptones and proteoses into peptides and carboxypeptidase hydrolyses peptides into smaller peptides and amino acids. Residual peptides still present are changed into amino acids by the enzyme peptidase present in the succus entericus. Variuos reactions occur are as follows:


Trypsinogen – Enterokinase – Trypsin

Proteoses and peptones – trypsin – Peptides.

Chymotrypsinogen – Trypsin – Chymotrypsin

Procarboxypeptidase – Trypsin – Carboxypeptidase


Proteoses >

Peptones > Chymotrypsin – Peptides + Amino acids / pH 8-9

Polypeptides >

Peptides – Peptidases / pH 8-9- Amino acids

Amino acids are the final products of protein digestion.