QUANTUM-MECHANICAL AND MOLECULAR MECHANICAL STUDIES ON A MODEL FOR THE DIHYDROXYACETONE PHOSPHATE GLYCERALDEHYDE PHOSPHATE ISOMERIZATION CATALYZED BY TRIOSEPHOSPHATE ISOMERASE (TIM)

Ab initio (SCF + MP2) and molecular mechanical calculations were carried out on a model for the reaction catalyzed by triosephosphate isomerase [TIM]. This is the 1st time that ab initio SCF [self-consistent field], correlation energy and environmental effect calculations were carried out on all the chemical steps of an enzymatic reaction, along with molecular mechanical simulation of some steps. The quantum mechanical calculations show how the TIM-catalyzed reaction, one of the most efficient known, can have as its rate-limiting step product dissociation in that the effect of the enzyme is to make the chemical steps very rapid. The enzyme does this by stabilizing the enzyme-intermediate complex so that it becomes of approximately equal stability to the enzyme-substrate complex. This lowers the barrier between these species to the range of 10-15 kcal/mol. The molecular mechanical calculations were used to generate refined coordinates for the enzyme-substrate and enzyme-intermediate complexes, which were essential in evaluating environmental effects on the quantum mechanical energies. They were also used to suggest the effect of genetic mutation of the key active site histidine residue in TIM to a glutamine. The calculations suggest the chemical steps in this mutant TIM will be less effective than in the normal enzyme, for reasons which had not been suggested heretofore.