Free energy perturbation study on a Trp-binding mutant (Ser88-->Cys) of the trp-repressor

The Ser88-->Cys mutant of the trp-repressor showed a lower affinity for the corepressor than the wild-type repressor [delta delta G = 1.7 +/- 0.3 kcal/mol, Chou and Matthews (1989) J. Biol. Chem., 264, 18314-18319]. A molecular dynamics/free energy cycle perturbation study was performed to understand the origin of the decreased affinity. A value (delta delta G = 1.58 +/- 0.28 kcal/mol) comparable with the experimental value was obtained by the simulation. Free energy component analysis revealed that destabilization of the van der Waals interaction between Ser88 and Trp109 (corepressor) mainly contributed to the decreased affinity of the mutant. The rotational transition of the hydroxyl (sulfhydryl) group of Ser88 (Cys88) during the simulations affected the contributions of Arg84 and water to the free energy change in the aporepressor and those of Arg84 and Trp109 to that in the holorepressor. However, the contributions from different residues compensated each other, and the total free energy changes were almost invariable in the various simulations.