An inhomogeneous self-consistent reaction field theory of protein core effects. Towards a quantum scheme for describing enzyme reactions

Abstract

A quantum chemical framework is described where the electronic properties of model subsystems inside or at the surface of globular proteins (enzymes) can be calculated. Protein and solvent surroundings are incorporated in our effective Schrödinger equation. The theory is a generalization of the SCRF of protein core effects [O. Tapia, F. Sussman, and E. Poulain, J. Theor. Biol. 71, 49 (1978)]. A test has been carried out within the CNDO–INDO approximate scheme on the proton relay system of liver alcohol dehydrogenase. The results have elicited the fundamental role played by the protein core potential and the polarization potential in stabilizing ion-pair structures against canonical H-bonded ones. The polarization field introduces a nonlinear dependency on the model system wave function via the field created by its charge density. This fact is instrumental for a proper description of a highly polarized subsystem coupled to a polarizable surrounding medium.