Genetic code redundancy and its differential influence on the evolution of protein interiors versus exteriors

The distinctive amino acid compositions of protein exteriors and interiors were compared to the composition bias imposed by genetic code redundancy. It transpired that the synonym allocation is biased more in favour of those residues which are preferred in interiors, and this leads to an average interior residue being more probable and less mutable compared to an exterior residue. The general implications for protein evolution are discussed in association with the known evolutionary behavior of particular protein families. It is suggested that some proteins may have their structural history "fossilised" in their interiors and that the "amino acid" code is in reality a "protein" code.