DIGESTION, ABSORPTION AND UTILIZATION OF PROTEINS :
Proteins taken in the diet are digested to amino acids in the stomach and small intestine. Gastric juice contains enzymes pepsin which digests protein in acid medium. It hydrolyses proteins to polypeptides.
Dietary Protein -> Pepsin -> Polypeptides
In the small intestine, pancreatic and intestinal juices contain proteolytic enzymes. Pancreatic juice contains trypsin, chymotrypsin and carboxyl peptidase.
They hydrolyse large protein molecule to smaller
Proteins -> Trypsin and Chymotrypsin -> Peptides + Amino acids
Peptides -> Carboxy peptidase/Amino Peptidase -> Aminoacids
Intestinal juices contains polypeptidases & dipeptidases which hydrolyse polypeptide & dipeptide to individual amino acids. There are several peptidases acting on different proteins. When undigested protein enter large intestines, bacteria causes nitrification of proteins leading to foul smelling flatus.
Proteins are mainly absorbed in the form of amino acids. Amino acids are absorbed by active transport mechanism in the intestinal cells. Sometimes whole protein may be absorbed by the mechanism of pinocytosis.
Absorbed amino acids pass into the portal blood and reach liver where they are converted to proteins. Other amino acids are transported through general circulation and are utilized for protein synthesis in the tissues.
Utilization of proteins in the body :
The amino acids from digested proteins are absorbed rapidly into the blood and passed onto different tissues to meet their needs.
Some non-essential amino acids are synthesized in the liver and also released into the circulation. The amino acids released by hydrolysis of tissue proteins are also added to the amino acid pool in the body.
The protein metabolism of mammals is in a dynamic state and the synthesis and breakdown of tissue protein takes place constantly.
The unwanted amino acids are oxidized in the liver to
yield energy and urea.