Discovery and Occurrence

There are a number of ways of classifiying amino acids. Since their side chains are the deciding factors for intra- and intermolecular interactions in proteins, and hence, for protein properties, amino acids can be classified as  

Alanine was isolated from silk fibroin by Weyl in 1888. It is present in most proteins and is particularly enriched in silk fibroin (35%). Gelatin and zein contain about 9% alanine, while its content in other proteins is 2-7%. Alanine is considered nonessential for humans. 

Aspartic Acid was isolated from legumes by Ritt-hausen in 1868. It occurs in all animal proteins, primarily in albumins at a concentration of 6-10%. Alfalfa and com proteins are rich in aspartic acid (14.9% and 12.3%, respectively) while its content in wheat is low (3.8%). Aspartic acid is nonessential. 

Cystine was isolated from bladder calculi by Wolaston in 1810 and from horns by Moerner in 1899. Its content is high in keratins (9%). Cystine is very important since the peptide chains of many proteins are connected by two cysteine residues, i. e. by disulfide bonds. A certain conformation may be fixed within a single peptide chain by disulfide bonds. Most proteins contain 1-2% cystine. Although it is itself nonessential, cystine can partly replace methionine which is an essential amino acid. 

Glutamine was first isolated from sugar beet juice by Schulze and Bosshard in 1883. Its occurrence in protein (edestin) was confirmed by Damodaran in 1932. Glutamine is readily converted into pyrrolidone carboxylic acid, which is stable between pH 2.2 and 4.0, but is readily cleaved to glutamic acid at other pH s  

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See also in sourсe #XX -- [ , ]

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