Lipases are widely used enzymes that
catalyze hydrolysis and alcoholysis
of fatty acid esters. At high concentrations of small alcohols such
as methanol or ethanol, many lipases are inhibited by the substrate.
The molecular basis of the inhibition of Candida antarctica lipase B (CALB) by methanol was investigated by unbiased molecular
dynamics (MD) simulations, and the substrate binding kinetics was
analyzed by Markov state models (MSMs). The modeled fluxes of productive
methanol binding at concentrations between 50 mM and 5.5 M were in
good agreement with the experimental activity profile of CALB, with
a peak at 300 mM. The kinetic and structural analysis uncovered the
molecular basis of CALB inhibition. Beyond 300 mM, the kinetic bottleneck
results from crowding of methanol in the substrate access channel,
which is caused by the gradual formation of methanol patches close
to Leu140 (helix α5), Leu278, and Ile285 (helix α10) at
a distance of 4–5 Å from the active site. Our findings
demonstrate the usefulness of unbiased MD simulations to study enzyme–substrate
interactions at realistic substrate concentrations and the feasibility
of scale-bridging by an MSM analysis to derive kinetic information.