Protein Chemistry - Part 1 - General Protein Chemistry - Ashmarin I. P. 1968

Chemical composition of proteins
Amino acid composition of proteins

Although the Amino Acid Composition of more than 50 Proteins has been studied to date, the results obtained do not allow for any broad generalizations. However, a comparison of these findings suggests that valine, leucine, and isoleucine are the most widely distributed Amino Acids in proteins, with isoleucine content being lower than that of leucine. Phenylalanine, Proline, Tyrosine, aspartic acid, glutamic acid, and cystine are also typically present in proteins, whereas Tryptophan, Methionine, Lysine, Histidine, Arginine, Glycine, and Alanine occur less frequently. Nevertheless, these general statements do not always apply to individual specific proteins that possess distinct structural features.

For example, protamines are characterized by a very high arginine content (up to 80%) and the complete absence of leucine and glutamic acid. Histones are notable for a high content of arginine and lysine, an almost complete absence of tryptophan and cystine, and a very low content of other cyclic amino acids such as tyrosine and phenylalanine. Serum albumin contains equal amounts of leucine and isoleucine; Silk Fibroin is extremely rich in glycine (50%), alanine, and Serine (approximately 25% each). A very high glycine content (up to 30%) has also been found in Collagen; moreover, Collagen is the only protein that contains hydroxyproline and hydroxylysine.

It is precisely these variations in the composition of different proteins that preclude the establishment of any universal regularities governing the quantitative ratios of amino acids within proteins.