AB-INITIO AND DENSITY-FUNCTIONAL THEORY STUDIES OF THE CATALYTIC MECHANISM FOR ESTER HYDROLYSIS IN SERINE HYDROLASES

We present results from ab initio and density functional theory studie s of the mechanism for serine hydrolase catalyzed ester hydrolysis. A model system containing both the catalytic triad and the oxyanion hole was studied. The catalytic triad was represented by formate anion, im idazole, and methanol. The oxyanion hole was represented by two water molecules. Methyl formate was used as the substrate. In the acylation step, our computations show that the cooperation of the Asp group and oxyanion hydrogen bonds is capable of lowering the activation barrier by about 15 kcal/mol. The transition state leading to the first tetrah edral intermediate in the acylation step is rate limiting with an acti vation barrier (Delta E-0) of 13.4 kcal/mol. The activation barrier in the deacylation step is smaller. The double-proton-transfer mechanism is energetically unfavorable by about 2 kcal/mol. The bonds between t he Asp group and the His group, and the hydrogen bonds in the oxyanion hole, increase in strength going from the Michaelis complex toward th e transition state and the tetrahedral intermediate. In the acylation step, the tetrahedral intermediate is a very shallow minimum on the en ergy surface and is not viable when molecular vibrations are included. (C) 1998 John Wiley & Sons, Inc.