WHAT AMINO-ACID PROPERTIES AFFECT PROTEIN EVOLUTION

We studied 10 protein-coding mitochondrial genes from 19 mammalian spe cies to evaluate the effects of 10 amino acid properties on the evolut ion of the genetic code, the amino acid composition of proteins, and t he pattern of nonsynonymous substitutions. The 10 amino acid propertie s studied are the chemical composition of the side chain, two polarity measures, hydropathy, isoelectric point, volume, aromaticity, aliphat icity, hydrogenation, and hydroxythiolation. The genetic code appears to have evolved toward minimizing polarity and hydropathy but not the other seven properties. This can be explained by our finding that the presumably primitive amino acids differed much only in polarity and hy dropathy, but little in the other properties. Only the chemical compos ition (C) and isoelectric point (IE) appear to have affected the amino acid composition of the proteins studied, that is, these proteins ten d to have more amino acids with typical C and IE values, so that nonsy nonymous mutations tend to result in small differences in C and IE, Al l properties, except for hydroxythiolation, affect the rate of nonsyno nymous substitution, with the observed amino acid changes having only small differences in these properties, relative to the spectrum of all possible nonsynonymous mutations.