Analysis of amino acid substitution during divergent evolution: the 400 by 400 dipeptide substitution matrix

Most formal methods for analyzing the divergent evolution of protein sequences assume a Markov model where position i in a polypeptide chain undergoes amino acid substitution independently from position i + 1. The large number of aligned homologous sequence pairs available from the exhaustive matching of the protein sequence database makes it possible to examine this assumption empirically. We have constructed a 400 by 400 matrix that reports empirical probabilities for the interconversion of all pairs of dipeptides in proteins undergoing divergent evolution. Comparison of these probabilities with those expected if substitution at adjacent positions in a protein sequence were independent reveals interesting patterns that arise through the breakdown of this assumption. Several of these are useful in extracting conformational information from patterns of conservation and variation in homologous protein sequences.