Short notes on the digestion of proteins in man
The diet of human beings consists of different types of proteins. As a result of digestion, these protein ultimately get broken down into their simplest units namely amino acids. The digestion of the protein begins in the stomach and is completed in the small intestine. Proteins remain unaltered in the buccal cavity as the saliva does not have any protein digesting enzymes.
In the stomach, the food undergoes both mechanical and chemical digestion. During the periodic muscular contractions of the stomach, the food is churned and is mixed thoroughly with the gastric juice to form soft mass called bolus. The gastric juice in the stomach is secreted by the gastric glands.
This juice contains mucous hydrochloric acid and two proteases viz,, pepsinogen and prorennin. Hydrochloric acid does not digest the proteins but it activates the inactive enzymes pepsinogen and prorennin to pepsin and rennin. Hydrochloric acid also helps in maintaining the proper acidic pH for the action of the protein digesting enzymes.
The enzyme pepsin hydrolyses the large molecules of proteins into peptones and other polypeptides. Pepsin also can digest collagen. The enzyme rennin hydrolyses the milk protein casein into paracasein. Paracasein in the presence of calcium gets converted into calcium paracaseinate. Pepsin converts this also into peptones.
Further digestion of proteins takes place in the small intestine into which also flows the pancreatic juice. The semidigested proteins are acted upon by the pancreatic juice which consists of three protein digesting enzymes namely trypsin, chymotrypsin and carboxypeptidase. These enzymes are actually in their inactive stage but they become activated under the influence of the intestinal juice.
The enzyme trypsin breaks basic proteins into peptides. Chymotrypsin splits proteins into polypeptides and it also hydrolyses milk proteins. The car- boxypeptidases break the polypeptide chain into a number of dipeptides. The intestinal juice also contains two digestive proteases viz., amino peptidases and dipeptidases besides a non digestive protease called enterokinase.
Enterokinase activates the proteases in the pancreatic juice. The amino peptidases break up polypeptide chains to dipeptides and free aminoacids. The dipeptidases break up dipeptides into individual ammino acids. The amino acids are finally absorbed by the villi present in the inner wall of the small intestine.