Sterol C24-methyltransferase: Mechanistic studies of the C-methylation reaction with 24-fluorocycloartenol

The mechanism of the C-methylation reaction was studied with the allylic substrate analog 24-fluorocycloartenol 10 assayed with soybean sterol C24-methyltransferase (SMT). 10 is an effective competitive inhibitor (Ki = 32 μM) of the SMT, and the electron-withdrawing α-fluorine substituent was shown to suppress the rate of the C-methylation reaction by one order of magnitude relative to the natural cycloartenol substrate, kcat = 0.02 min−1 versus 0.6 min−1; alternately 10 can prevent the critical hydride shift of H24 to C25 to afford time-dependent inactivation of SMT (kinact = 0.32 min−1).