On transition structures for hydride transfer step: A theoretical study of the reaction catalyzed by dihydrofolate reductase enzyme

A theoretical study is presented of the catalytic mechanism of dihydrofolate reductase (DHFR) enzyme based upon the characterization of the transition structure (TS) for the hydride transfer step. Analytical gradients at AM1 and PM3 semiempirical levels have been used to characterize the saddle point of index one (SPi-1) on global energy hypersurface for the hydride transfer in the active site of DHFR enzyme. The geometry, stereochemistry, electronic structure, and transition vector (TV) components associated to SPi-1 are qualitatively computational level independent. The TV amplitudes show primary and secondary isotope effects to be strongly coupled. The geometrical arrangement of the TS results in optimal frontier orbital interaction. Comparison of the TS geometry with the X-ray coordinates shows that the TS can be fitted without any stress at the active site. By comparison of the TS for the hydride transfer step in models of related enzymes, the geometries and the components of TV for TSs in hydride transfer steps are transferable and invariant. The results of this study suggest that the primary function of the enzyme is to constrain the reactants in the endo configuration, situating the system in a neighborhood of a SPi-1. This fact is in accordance with Pauling's postulate for enzyme catalysis. (C) 1996 Academic Press, Inc.