Farnesyl Protein Transferase:  Identification of K164α and Y300β as Catalytic Residues by Mutagenesis and Kinetic Studies

Farnesyl protein transferase (FPT) is an alpha/beta heterodimeric zinc enzyme that catalyzes posttranslational farnesylation of many key cellular regulatory proteins, including oncogenic Ras. On the basis of the recently reported crystal structure of FPT complexed with a CVIM peptide and alpha-hydroxyfarnesylphosphonic acid, site-directed mutagenesis of the FPT active site was performed so key residues that are responsible for substrate binding and catalysis could be identified. Eight single mutants, including K164N alpha, Y166F alpha, Y166A alpha, Y200F alpha, H201A alpha, H248A beta, Y300F beta, and Y361F beta, and a double mutant, H248A beta/Y300F beta, were prepared. Steady-state kinetic analysis along with structural evidence indicated that residues Y200 alpha, H201 alpha, H248 beta, and Y361 beta are mainly involved in substrate binding. In addition, biochemical results confirm structural observations which show that residue Y166 alpha plays a key role in stabilizing the active site conformation of several FPT residues through cation-pi interactions. Two mutants, K164N alpha and Y300F beta, have moderately decreased catalytic constants (kcat). Pre-steady-state kinetic analysis of these mutants from rapid quench experiments showed that the chemical step rate constant was reduced by 41- and 30-fold, respectively. The product-releasing rate for each dropped approximately 10-fold. In pH-dependent kinetic studies, Y300F beta was observed to have both acidic and basic pKa values shifted 1 log unit from those of the wild-type enzyme, consistent with a possible role for Y300 beta as an acid-base catalyst. K164N alpha had a pKa shift from 6.0 to 5.3, which suggests it may function as a general acid. On the basis of these results along with structural evidence, a possible FPT reaction mechanism is proposed with both Y300 beta and K164 alpha playing key catalytic roles in enhancing the reactivity of the farnesyl diphosphate leaving group.