氨基酸的分子结构与遗传密码简并及二维集合分类

根据氨基酸遗传密码子的简并程度，可将64个遗传密码子分为高简并度类（3，4，6度简并组）和低简并度类（1，2度简并组）两大类。高简并度类有9个氨基酸，其分子量比较小，等电点的分布比较集中。低简并度类有11个氨基酸，其分子结构比较复杂，参考Taylor对氨基酸特性的分类图，本文提出以分子量（M）及等电点（P）作为氨基酸的化学特性坐标，作出其二维集合MP分类图，MP分类图可以反映出氨基酸的各种属性，如分子量的大小，简并度的高低，极性与非极性、带电荷或不带电荷，疏水性与亲水性，以及氨基酸残基的种类等。根据氨基酸的分类分析，可以认为：高简并度氨基酸多数是脂烃类和羟脂烃类的氨基酸，分子量比较小，分子结构比较简单，大部分为疏子性，主要组成跨膜结构或蛋白质的结构域，可能是出现较早的氨基酸；而低简并度的氨基酸，分子结构比较复杂，分子量比较大，多数是和蛋白质功能有密切联系的基团，可能是进化出现较晚的结构。

MOLECULAR STRUCTURE RELATED TO GENETIC CODE DEGENERACY AND TWO DIMENSION CLASSIFICATION OF AMINO ACIDS

According to the degree of degeneracy of genetic codes, 64 genetic codons can be subdivided into two groups, the high degenerate group (3,4,6 triplets degeneracy) and the low degenerate group (single and 2 triplets degeneracy). There are 9 amino acids which belong to the high degenerate group (G, A, S, P, V, T, L, I, R) and 11 amino acids to the low degenerate group (C, N, D, Q, K, E, M, H, F, Y, W). Amino acids of the high degenerate group have relatively simple molecular structure, rather small molecular weights and comparatively concentrated distribution of isoelectric points. While in the low degenerate group, molecular structure is more complex, with relatively large molecular weights, and the distribution of their isoelectric points is more dispersed. Based on the two dimension distribution of molecular weights (M) and isoelectric points (P) of amino acids, a set of classification graph (Venn's diagram) of amino acids can be obtained. The MP classification graph can demonstrate many chemical properties of amino acids, such as: size of molecular weights, degree of degeneracy, polar or non-polar, charged or non-charged, hydrophobic or hydrophilic, and the functional groups of the residues. It is suggested that the amino acids of high degenerate group are mostly small and simple, and constitute the transmembranic structure or the structural domains of protein molecules. So, amino acids of high degenerate group might appear in the early evolution stage. On the other hand, the amino acids of low degenerate group are rather large and complex, and ultimately correlate to the functional domains of protein molecules, then, the amino acids of low degenerate group might appear more lately during evolution.