Kinetic studies of protein farnesyltransferase mutants establish active substrate conformation

The zinc metalloenzyme protein farnesyltransferase (FTase) catalyzes the transfer of a 15-carbon farnesyl moiety from farnesyl diphosphate (FPP) to a cysteine residue near the C-terminus of a protein substrate. Several crystal structures of inactive FTase.FPP.peptide complexes indicate that K164alpha interacts with the alpha-phosphate and that H248beta and Y300beta form hydrogen bonds with the beta-phosphate of FPP [Strickland, C. L., et al. (1998) Biochemistry 37, 16601-16611]. Mutations K164Aalpha, H248Abeta, and Y300Fbeta were prepared and analyzed by single turnover kinetics and ligand binding studies. These mutations do not significantly affect the enzyme affinity for FPP but do decrease the farnesylation rate constant by 30-, 10-, and 500-fold, respectively. These mutations have little effect on the pH and magnesium dependence of the farnesylation rate constant, demonstrating that the side chains of K164alpha, Y300beta, and H248beta do not function either as general acid-base catalysts or as magnesium ligands. Mutation of H248beta and Y300beta, but not K164alpha, decreases the farnesylation rate constant using farnesyl monophosphate (FMP). These data suggest that, contrary to the conclusions derived from analysis of the static crystal structures, the transition state for farnesylation is stabilized by interactions between the alpha-phosphate of the isoprenoid substrate and the side chains of Y300beta and H248beta. These results suggest an active substrate conformation for FTase wherein the C1 carbon of the FPP substrate moves toward the zinc-bound thiolate of the protein substrate to react, resulting in a rearrangement of the diphosphate group relative to its ground state position in the binding pocket.