The evolution of catalytic function in the HIV-1 protease.

The evolution of species is a complex phenomenon based on the optimization of a multidimensional function referred to as fitness. At the level of biomolecular evolution, the fitness function can be reduced to include physiochemical properties relevant to the biological function of a particular molecule. In this work, questions involving the physical-chemical mechanisms underlying the evolution of HIV-1 protease are addressed through molecular simulation and subsequent analysis of thermodynamic properties related to the activity of the enzyme. Specifically, the impact of 40 single amino acid mutations on the binding affinity toward the matrix/capsid (MA/CA) substrate and corresponding transition state intermediate has been characterized using a molecular mechanics Poisson-Boltzmann surface area approach. We demonstrate that this approach is capable of extracting statistically significant information relevant to experimentally determined catalytic activity. Further, no correlation was observed between the effect of mutations on substrate and transition state binding, suggesting independent evolutionary pathways toward optimizing substrate specificity and catalytic activity. In addition, a detailed analysis of calculated binding affinity data suggests that ground-state destabilization (reduced binding affinity for the substrate) could be a contributing factor in the evolutionary optimization of HIV-1 protease. A numerical model is developed to demonstrate that ground-state destabilization is a valid mechanism for activity optimization given the high concentrations of substrate experienced by the functional enzyme in vivo.