Tompa P
Institute of Enzymology, Hungarian Academy of Sciences, Budapest.
J Mol Evol. 1988;27(2):147-53. doi: 10.1007/BF02138375.
Of the 20 protein amino acids, 16 have a methylene group at the beta position, and a further three bear a methine group. No aromatic, carboxamido, carboxylic carbon, or hetero atoms are attached directly to the alpha carbon, but they are separated by this methylene or occasionally by a longer n-alkylene spacer group. Therefore, the structure of the protein amino acids should rather be formulated as H2N-CH((CH2)n-R')-COOH instead of the generally accepted H2N-CH(R)-COOH. The appearance of and the role played by the spacer group are discussed in an evolutionary context. It is suggested that the spacer group appeared as a result of prebiotic selection, based on the relative abundance, racemization rate, and suitability for thermal polymerization of the protein amino acids and their homologs with various spacer group lengths. At the biotic level of evolution the requirements for ribosomal polymerization, as well as the abilities of polypeptides to maintain a stable and flexible three-dimensional structure and to bind ligands are considered and are proposed to have been responsible for the possible exclusion of longer spacer groups. It is concluded that the general role of the spacer group is to ensure the uniformity of the constant regions H2N-CH(-)-COOH and the individuality of the R' contact groups by spatially separating them.
在20种蛋白质氨基酸中，16种在β位有一个亚甲基，另外三种带有一个次甲基。没有芳香族、羧酰胺基、羧基碳或杂原子直接连接在α碳上，而是通过这个亚甲基或偶尔通过更长的正亚烷基间隔基团将它们隔开。因此，蛋白质氨基酸的结构应更确切地表示为H2N-CH((CH2)n-R')-COOH，而不是普遍接受的H2N-CH(R)-COOH。在进化背景下讨论了间隔基团的出现及其作用。有人认为，间隔基团的出现是益生元选择的结果，这是基于蛋白质氨基酸及其具有不同间隔基团长度的同系物的相对丰度、消旋化速率以及热聚合的适宜性。在生物进化水平上，考虑了核糖体聚合的要求，以及多肽维持稳定和灵活三维结构以及结合配体的能力，并认为这些因素可能导致更长间隔基团被排除。得出的结论是，间隔基团的总体作用是通过在空间上分离它们来确保恒定区H2N-CH(-)-COOH的一致性和R'接触基团的独特性。
