Biochemistry

The casein content in milk is 78-85% of all milk proteins. Casein is in the form of colloidal particles or micelles (filaments), which are composed of smaller particles (submicelles). Casein is sensitive to calcium ions. In their presence, it precipitates. Casein is a phosphoprotein, as it contains phosphoric acid. He attaches to himself calcium, magnesium, sodium, minerals. Casein contains 4 fractions that differ in the content of amino acids, phosphoric acid, and sensitivity to calcium ions and rennet. Casein in milk is in the form of a calcium-phosphate-casein complex. It consists of calcium caseinate, colloidal calcium phosphate, citric acid, magnesium, potassium and sodium. After precipitation of casein, whey proteins are detected at the isoelectric point. They include f-lactaglobulin, a-lactoalbumin, serum albumin, immunoglobulins and the proteose peptide fraction. 2.9 PHYSIOLOGICAL ROLE OF PEPTIDES Until the middle of the twentieth century, it was believed that peptides are not an independent class of organic compounds, but are products of incomplete hydrolysis of proteins that are formed during the digestion of food, in the process or during storage of food products. And only after V. Du Vigno (1953) determined the sequence of amino acid residues of the two hormones of the posterior pituitary gland - oxytocin and vasopressin - and reproduced their synthesis chemically, a new point of view on the physiological role and significance of this group of compounds appeared. Currently, a large number of peptides have been discovered that have an individual amino acid sequence and are not even found in hydrolysates of natural proteins. Peptides have a low molecular weight, a wide range of amino acid residues (they include, for example, D-amino acids) and structural features (cyclic, branched). The names of the peptides are formed from the names of amino acid residues by listing them sequentially, starting with the NH2-terminal residue, with the addition of the suffix -yl, except for the C-terminal amino acid, the name of which remains unchanged. In nature, there are two types ofpeptides, one of them is synthesized and plays a physiological role in the life of the body, the other is formed by chemical or enzymatic hydrolysis of the body's proteins or outside it. Peptides formed during in vitro hydrolysis are widely used to analyze the amino acid sequence of proteins. Using peptides, the amino acid sequence of the enzyme lysozyme, pancreatic hormone insulin, cobra venom neurotoxin, aspartate aminotransferase, pepsinogen, lactogenic hormone and other biologically active compounds of the body was deciphered. 73