Biochemistry

Table 2.4 - Amino acid composition of proteins, % by weight of protein Aminoacid Milk casein Egg albumin Collagen Pepsin Corn zein Human hemoglobin Alanine 3,2 6,7 9,5 - 9,8 9,0 Glycine 2,0 3,1 27,2 6,4 0 4,2 Valine 7,2 7,1 3,4 7,1 1,9 10,3 Leucine 9,2 9,2 5,6 10,4 25,0 14,0 Isoleucine 6,1 7,0 - 10,8 - 0 Proline 10,6 3,6 15,1 5,0 9,0 4,8 Phenylalanine 5,0 7,7 2,5 6,4 7,6 7,3 Tyrosine 6,3 3,7 1,0 8,5 5,9 2,9 Tryptophan 1,2 1,2 0 2,4 0,2 1,9 Serine 6,3 8,2 3,4 12,2 1,0 4,4 Threonine 4,9 ■ 4,0 2,3 9,6 0 5,2 Cysteine + Cystine 0,3 1,9 0 2,1 0,9 1,0 Methionine 2,8 5,2 0,8 1,7 2,4 1,2 Arginine 4,1 5,7 8,6 1,0 1,6 3,3 Histidine 3,1 2,4 0,7 0,9 0,8 8,8 Lysine 8,2 6,3 4,5 0,9 0 9,6 Aspartic acid 7,1 9,3 6,3 16,0 1,8 9,6 Glutamic acid 22,4 16,5 11,3 11,9 31,3 6,6 Quantitative amino acid analysis is used to characterize any protein preparation. Each protein has a well-defined amino acid composition. In most proteins, aspartic and glutamic acids, leucine and lysine account for up to 50% of the total number of amino acids. And the content of amino acids such as tryptophan, cysteine and histidine does not exceed 3%. Some amino acids are completely absent in a number of proteins (for example, com zein does not contain lysine, threonine and glycine). Therefore, proteins can be complete and inferior in amino acid composition. So, milk casein and egg albumin are complete proteins: they contain all proteinogenic amino acids, including essential proteins. On the contrary, corn collagen and zein are partial proteins, since they lack a number of amino acids, including essential. 2.3 BIOLOGICAL ROLE OF SEPARATE AMINO ACIDS Glycine or glycocol is a precursor to the purine ring of blood heme and forms the so-called paired compounds, with bile acids, for example, cholic acid, it forms glycocholic acid, with benzoic acid - hippuric acid. Glycolic acid takes part in the process of lipid absorption, and toxic benzoic acid is excreted in the form of hippuric acid. Alanine. As a side chain, it is part of cyclic amino acids (phenylalanine, tyrosine, histidine, tryptophan). The biological role is also determined by the presence of its derivatives: serine, cysteine, cystine, threonine, methionine. 46