Biochemistry

By biological value (nutritional value) amino acids are divided into non- essential and essential. Essential amino acids cannot be synthesized by the body from other compounds. A human receives them only with food. There are eight such amino acids: from unsubstituted aliphatic ones - valine, leucine, isoleucine', from aliphatic substituted ones - threonine, lysine, methionine', aromatic - phenylalanine', from heterocyclic - tryptophan. Two amino acids - arginine and histidine are essential for the child's body, while in adults they are partially synthesized, but in insufficient quantities. Higher plants may contain all the amino acids necessary for protein synthesis. In the human body, only 10 of the 20 amino acids that are most often found in proteins are synthesized, so they are non-essential amino acids. Proteins also have modified amino acids. Chemical modification usually occurs after the initial amino acid is incorporated into the protein. One example of an important modification is the oxidation of two-SH groups of cysteine residues to form a cystine amino acid containing a disulfide bond. H 2 N-CH-COOH + H,N-CH-COOH -2H I I CH2 -SH HS-H 2 C +2H Cysteine Cysteine H,N-CH-COOH h 2 n - ch - cooh ;==: I I CH 2 -S --------- S-H 2 C Disulfide bond Cystine Stereoisomerism of amino acids Stereoisomers are compounds with the same sequence of atoms, but with their different arrangement in space. In all proteinogenic amino acids, with the exception of glycine, the a-carbon atom has four different substituents (chiral carbon atom). H2N-C — COOH H Glycine (a-carbon atom is not chiral) CH 3 la H 2 N-C-C00H I H Alanine (a-carbon atom is chiral) 42