Biochemistry

Pancreatic juice enters the duodenum and mixes with intestinal juice. This mixture contains proteolytic enzymes that break down proteins into amino acids. These include trypsin, chymotrypsin, carboxypeptidases, aminopeptidases and a large group of tri- and dipeptidases. Trypsin is in the pancreatic juice in an inactive form, in the form of a trypsinogen proenzyme. Its activation occurs under the influence of the intestinal juice enzyme - enterokinase. The activation process requires Ca 2+ ions. The conversion of trypsinogen to trypsin is accomplished by cleaving a small peptide from the N-terminus of the enzyme's peptide chain. Trypsin hydrolyzes both undigested proteins and high molecular weight peptides, acting mainly on peptide bonds between arginine and lysine. The optimum pH for trypsin is 7.0-8.0. Trypsin produces a relatively shallow hydrolysis of the protein, forming polypeptides and a small amount of free amino acids. Trypsin activity can be reduced by a number of inhibitors. These include basic peptides with a molecular weight of 9000 Da. They are found in the pancreas, blood, lungs, and soy beans. A mucoprotein contained in raw eggs reduces the activity of trypsin. Chymotrypsin is the second proteolytic enzyme of the pancreas. It is also secreted in an inactive form, in the form of chymotrypsinogen. Under the influence of trypsin, chymotrypsinogen changes into the active enzyme - chymotrypsin. The action of chymotrypsin is similar to the action of trypsin. The optimum pH for both enzymes is approximately the same; chymotrypsin acts on proteins and polypeptides that contain aromatic amino acids (tyrosine, phenylalanine, tryptophan), as well as on peptide bonds that are not exposed to trypsin (methionine, leucine). Peptides formed as a result of the action on proteins of pepsin, trypsin and chymotrypsin in the lower parts of the small intestine undergo further break down. This process is carried out by carboxypeptidases, aminopeptidases. These enzymes belong to metalloenzymes. They are activated by divalent ions Mg 2+ , Mn n , Co 2+ , which plays an important role in the formation of the enzyme - substrate complex. The mechanism of action of amino and carboxypeptidases consists in removal of terminal amino acids from the peptides having a free amine or carboxyl group. Small peptides that remain undigested, consisting of three to four amino acid residues, are hydrolyzed by specific di- and triaminopeptidases. Thus, as a result of the sequential action of proteolytic enzymes on proteins in the intestine, free amino acids are formed, which are absorbed into the blood through the intestinal wall. 160