The constraints between amino acids influence the unequal distribution
of codons and protein sequence evolution
Abstract
4 nucleotides(A, U, C, G) constitute 64 codons at free combination but
64 codons are unequally assigned to 21 items (20 amino acids plus one
stop). About 500 amino acids are known but only 20 ones are selected to
make up the proteins. However, the relationships between amino acid and
codon and between 20 amino acids have been unclear. In this paper, we
studied on the relationships between 20 amino acids in 33 species and
found there were three constraints between 20 amino acids, such as the
relatively stable mean carbon and hydrogen(C:H) ratios(0.50), similarity
interactions between the constituent ratios of amino acids, and the
frequency of amino acids according with Poisson distribution under a
certain conditions. We demonstrated that the unequal distribution of 64
codons and the choice of amino acids in molecular evolution would be
constrained to remain stable C:H ratios. The constituent ratios and
frequency of 20 amino acids in a species or a protein are two
determinants of protein sequence evolution, so this findings showed the
constraints between 20 amino acids played an important role in protein
sequence evolution.