CHEMICAL-REACTION MECHANISMS IN-VACUO, IN SOLUTION AND IN ENZYME FIELDS - ISOMERIZATION CATALYZED BY TRIOSE PHOSPHATE ISOMERASE (TIM)

The first step of the reversible isomerization of dihydroxyacetone pho sphate (DHAP) to glyceraldehyde-3-phosphate catalyzed by triose phosph ate isomerase has been studied in the past by us [J. Mol. Biol., 191 ( 1986) 23], making use of a very simple model of the enzyme field and n ot including the phosphate group. The effect of this group on the reac tion energetics in vacuo, in solution (using the polarizable continuum model) and in the enzyme field is considered at the 4-31G/SCF level. The reliability of the phosphate description has been checked by compu ting reference points at the 6-31+G level; also, the effect of electro n correlation has been taken into account including correlation correc tions at the MP2 level. The enzyme active site around the reacting par tners, modeled with AMBER4, has been included in the calculations at t he 4-31G level as a rigid charge cloud made up of either the five key residues (Lys 13, His 95, Ser 96, Glu 97 and Arg 98) or the 37 residue s within 10 Angstrom of Glu 165 and the substrate. The enediolate of D HAP (ENEP)... acetic acid (the latter as a model of protonated Glu 165 ) adduct turns out to be almost isoenergetic with the DHAP ... acetate complex in either environment compared to an energy gap of 33 kcal mo l(-1) in vacuo. The reaction barrier located by constructing a potenti al energy surface in the presence of the enzyme field is only 20 kcal mol(-1), compared to a calculated barrier of 40 kcal mol(-1) in vacuo and 28 kcal mol(-1) in solution.