TIM is an important enzyme in the glycolytic pathway. Despite numerous
experimental and theoretical studies, the precise catalytic mechanism
for TIM is still not clear.
Three mechanisms proposed for the triosephosphate isomerase (TIM)
catalyzed reactions were
studied with the combined QM/MM approach using
B3LYP/6-31+G(d,p) as the QM method. The two pathways that involve an
enediol species were found to give similar values for the barriers and
the calculated rates are in satisfactory agreement with experiment.
By contrast, the mechanism that involves intramolecular proton transfer
in the enediolate was found to be energetically unfavorable due to
electrostatic interactions with His 95 , a conserved residue in TIM from
different organisms. A perturbation analysis was used to determine
the residues that make the major contribution to catalysis.
Proton transfer rate constants including tunneling
NMR chemical shifts and hydrogen bonding interactions