Biochemistry

The proteins necessary for the body as a plastic material from which cells of all organs, tissues and systems are built come from food. However, dietary proteins cannot be used without prior breakdown in the body, since they have a complex structure and species specificity. Proteins breakdown (hydrolysis) into amino acids that are devoid of species and tissue specificity occurs in the gastrointestinal tract. 8.2 DIGESTION OF PROTEINS IN THE DIGESTIVE SYSTEM Protein digestion begins in the stomach under the influence of gastric juice. It includes hydrochloric acid, which is produced by the parietal cells of the gastric mucosa. Hydrochloric acid has a denaturing effect on the protein, which facilitates its subsequent breakdown. In addition, it promotes the conversion of the pepsinogen proenzyme secreted by the main cells of the gastric mucosa into the active proteolytic enzyme pepsin. The optimal concentration of hydrogen ions for pepsin is 1.5-2.5, which corresponds to the acidity of the gastric juice during digestion. Pepsin loses its activity with an increase in pH of the medium to 6.0 (in the intestine). Pepsin refers to one-component enzymes, i.e., protein enzymes. About 2 g of pepsin are produced in the stomach per day. The catalytic activity of the pepsin of the stomach is very high. It catalyzes a breakdown of peptide bonds in a protein molecule formed by amino groups of aromatic and dicarboxylic amino acids. As a result of the action of pepsin, polypeptides of various sizes (albumoses and peptones) and individual free amino acids are formed. In addition to pepsin, gastric juice contains the proteolytic enzyme gastricsin, the optimal pH of which is in the range of 3.5-4.5. Gastricsin comes into effect in the last stages of digestion of food in the stomach. In the stomach of infants found rennet - chymosin. The optimum action of this enzyme corresponds to pH 3.5-4.0. Under the influence of chymosin in the presence of calcium salts, the caseinogen of milk during hydrolysis is converted to casein and the milk is curdled. Albumins and globulins of animal and vegetable origin are more easily digested in the stomach; proteins of connective tissue (collagen and elastin) are poorly digested and keratin and protamines are not digested at all. Polypeptides and undigested proteins formed in the stomach enter into the initial part of the small intestine - duodenum, and then into its underlying sections. They are exposed to a large group of enzymes produced by the pancreas and the mucous membrane of the small intestine. 159