Evolution and adaptation of proteins : towards non-natural activities

Cyrochrome P450 BM3 is a bacterial cytochrome which has been the subject of the most extensive study to date in terms of the application of directed evolution techniques. These studies, carried out mainly by F. Arnold (Caltech, USA) and presented in this lecture, illustrate the possibility of using high-throughput mutagenesis to transform an enzyme capable of oxidizing a natural substrate (in this case, a long-chain fatty acid) into an enzyme capable of oxidizing a very different, non-natural substrate (propane, a short-chain alkane). This work sheds light both on the paths that protein evolution and adaptation processes must take, and, perhaps, on the paths taken by the natural evolution of living organisms. The more mutated and unstable a protein is, the less able it is to evolve. It is therefore the most stable variants, i.e. those with the least destabilizing mutations, that will have the greatest chance of evolving to improve an activity. The others will disappear. One mechanism by which natural selection favors the ability to evolve is to stabilize proteins undergoing adaptive evolution, or to find compensations for the effects of destabilizing mutations. Furthermore, a protein's ability to evolve is linked to its capacity for accessory activities and substrate freedom.