A stability pattern of protein hydrophobic mutations that reflects evolutionary structural optimization

We have determined the effect of mutations involving isoleucine and valine (i.e., mutations I -> V and V -> I) on the stability of Escherichia coli thioredoxin. Despite the similarity in chemical structure (V and I differ only in a methyl group), we find that many environments are optimized to a significant extent for either V or I. We find, furthermore, that a plot of effect of hydrophobic mutations on stability versus packing density shows a strikingly simple pattern that clearly reflects evolutionary structural optimization. The existence of such patterns suggests the possibility of rationalizing (and perhaps even predicting) mutation effects on protein stability on the basis of evolutionary models. By evolutionary model'' we specifically refer in this context to a model for mutation effects on stability in which certain physical features of the mutated residue environments are evaluated from an assumption regarding how such environments have been selected during protein evolution (as opposed to a purely physical model'' in which those features would be derived from some kind of energetics analysis of the protein structural characteristics). To illustrate this novel approach and provide general guidelines for its application, we develop here a simple evolutionary model that successfully explains the effect of the I <-> V mutations on thioredoxin stability.