Insights into the Mechanistic Dichotomy of the Protein Farnesyltransferase Peptide Substrates CVIM and CVLS

Protein farnesyltransferase (FTase) catalyzes farnesylation
of
a variety of peptide substrates. 3H α-secondary kinetic
isotope effect (α-SKIE) measurements of two peptide substrates,
CVIM and CVLS, are significantly different and have been proposed
to reflect a rate-limiting SN2-like transition state with
dissociative characteristics for CVIM, while, due to the absence of
an isotope effect, CVLS was proposed to have a rate-limiting peptide
conformational change. Potential of mean force quantum mechanical/molecular
mechanical studies coupled with umbrella sampling techniques were
performed to further probe this mechanistic dichotomy. We observe
the experimentally proposed transition state (TS) for CVIM but find
that CVLS has a symmetric SN2 TS, which is also consistent
with the absence of a 3H α-SKIE. These calculations
demonstrate facile substrate-dependent alterations in the transition
state structure catalyzed by FTase.