Chemistry and Biology of Proteins - F. Haurowitz 1953

Hydrolytic Cleavage of Proteins
Amino Acid Composition of Proteins

The Analytical Methods mentioned above were used to determine the Amino Acid Composition of various Proteins. Typically, the results are expressed as the percentage of each amino acid present in the protein. This method of calculation is not entirely precise because a protein molecule contains not intact amino acid molecules (H₂N—R—COOH), but rather amino acid residues (—HN—R—СО—). It would be preferable to state the exact number of molecules of each amino acid per protein molecule. However, this is only possible when the Molecular Weight of the protein has been accurately determined. If each amino acid is designated by the first three letters of its name, the composition of crystalline ß-lactoglobulin can be represented as follows: гли₈, ала₂₉, вал₂₁, лей₅₀, изолей₂₇, про₁₅, фен₉, арг₇, гис₄, лиз₃₃, асп₃₆, глут₅₆, сер₂₀, тре₂₁ циc(SH)₄, циc(SS/2)₈, мет₉, тир_9, три₄, аммиак₃₂ [72]. Similarly, the formula for Pepsin can be expressed as: гли₂₉, ала₂₉, вал₂₁ лей₂₇, изолей₂₈, про₁₅, фен₁₉, арг₂, гис₂, лиз₂, асп₄₁ глут₂₈, сер₄₀, тре₂₈, цис(SH)₂, циc(SS/2)₄, мет₄, тирю, три₄, аммиак₃₂, (—РО₃Н₂)₁ [125].

Since the molecular weights of proteins vary over a wide range, formulas of this type are not entirely suitable for comparing the proportions of different Amino Acids within a protein molecule. It should be noted, however, that it is precisely these ratios that determine the physical and Chemical properties of proteins, as the latter depend primarily on The Structure of the R side chains in the peptide groups.

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Depending on the structure of their side chains, Amino acids can be divided into four groups: 1) amino acids with nonpolar side chains (Glycine, Alanine, valine, leucine, isoleucine, Proline, phenylalanine); 2) amino acids with side chains that dissociate as bases (Arginine, Histidine, Lysine, hydroxylysine); 3) amino acids with side chains that dissociate as acids (aspartic and glutamic acids, provided their carboxyl groups are free); 4) amino acids with polar, non-dissociating side chains (Cysteine, cystine, Methionine, Serine, Threonine, hydroxyproline, Tyrosine, Tryptophan, asparagine, glutamine).

These four groups of Amino acids are presented in Table 1. To allow comparison of The amino acid compositions of different proteins, all values given in Table 1 are normalized to a protein unit with a molecular weight of 100,000.