The nutritional proteins are formed of long chains of amino acids bound to¬gether by peptide linkages. Food proteins are ultimately broken down into amino acids by gastro-in-testinal enzymes. The enzymes for protein digestion are called proteasesor or pepti-dases.
There are two types of peptidases, those are
i) Endopeptidases
ii) Exopeptidases.
For protein digestion, most of proteases are secreted in inactive forms called pro-enzymes. If they were synthesized in active forms, they would have hydrolysed cellular and extracellular proteins of the organism itself. Inactive pro-enzymes are activated at the sites of their actions by specific proteases or by optimal pH changes.
Protein digestion in stomach
Protein Digestion
As saliva has no proteolytic enzyme, protein digestion does not take place in the mouth. Actually the protein digestion starts in the stomach.
The gastric juice of most vertebrates contains Hydrochloric acid (HC1) and pepsinogen. Pepsinogen is converted into active pepsin in the presence of a strongly acidic pH (about 1-2) in the stomach. Pepsin is a very potent proteolytic enzyme and hydrolyses many proteins into smaller molecules of proteoses and peptones. It is a non-specific endopeptidase and acts on a variety of proteins. As the food passes to the duodenum, pepsin action is stopped by the alkalinity of intestinal contents.
Pepsin also hydrolyses soluble casein which is a milk protein, into paracasein and whey protein. Paracasein is then precipitated spontaneously as calcium paracaseinate to form the solid curd. An another milk coagulating protease, called rennin, is also found in the gastric juice of human beings throughout infancy and in calf as protein digestion. However, in adults, gastric juice is devoid of rennin. Rennin is secreted in an inactive form, prorennin,which is transformed into active rennin in the acidic medium. Rennin converts casein into paracasein, leading to milk coagulation.
Protein digestion in intestine
Most protein digestion occurs mainly in the upper small intestine. Pancreatic and intestinal juices hold a number of proteases.
Protein digestion by pancreatic proteases
When proteins leave the stomach, they are mainly in the form of proteoses, peptones, and large polypeptides. At once upon entering the small intestine, the partially breakdown products are attached by the major proteolytic pancreatic enzymes. The majority of the pancreatic proteases are secreted in inactive forms like trypsinogen, chymotrypsinogen and procarboxypeptidase. In the occurrence of enterokinase inactive trypsinogen is converted into active trypsin. After that, trypsin activates chymotrypsinogen and procarboxypeptidase into chymotrypsin and carboxypeptidase in that order. This enables simultaneous action of all pancreatic proteases for a very quick digestion of proteins.
Protein digestion by Trypsin
Trypsin acts best at an alkaline pH, provided in the small intestine by the hydrogencarbonates of pancreatic and intestinal juices and bile. It hydrolyses proteins into peptides, but has no action on keratins and milk protein-casein. In predator animals which drink the blood of their prey, trypsin hydrolyses fibrinogen of blood into fibrin, leading to blood coagulation.
Protein digestion by Chymotrypsin
An important milk- coagulating enzyme that acts in an alkaline medium and hydrolyses casein into paracasein is Chymotrypsin. Paracasein is then coagulated into calcium paracaseinate, Chymotrypsin is specific for the bond on the carboxyl side of tyrosine, phenylalanine or tryptophan.
Protein digestion by Intestinal juice
Last digestion of proteins in the intestinal lumen is achieved by the epithelial cells that line the villi of the small intestine. The intestinal juice contains three important proteolytic enzymes, those are
i) Enteropeptidase
ii) Aminopcptidases
iii) Dipeptidases.
The main action of enterokinases to change the inactive proteolytic proenzyme of the pancreatic juice, trypsinogeq into active trypsin. Aminopeptidase hydrolyse polypeptides to amino acids and simpler peptides by removing the amino acid containing the free amino group from the polypeptide thus progressively cut down the polypeptide chain into dipeptides. When polypeptide has reached the dipeptide stage dipeptide is finally hydrolysed by the activity of dipeptidase. And that is all about the complete protein digestion.