Identification of a Protein-Promoting Vibration in the Reaction Catalyzed by Horse Liver Alcohol Dehydrogenase

In this article we present computational studies of horse liver alcohol dehydrogenase (HLADH). The computations identify a rate-promoting vibration that is symmetrically coupled to the reaction coordinate. In HLADH a bulky amino acid (Val203) is positioned at the face of the nicotinamide adenine dinucleotide (NAD(+)) cofactor distal to alcohol substrate to restrict the separation of reactants and control the stereochemistry. Molecular dynamics simulations were performed on the dimeric HLADH, including the NAD cofactor, the substrate, and the crystallographic waters, for three different configurations, reactants, products, and transition state. From the spectral density for the substrate-NAD relative motion, and that for the NAD-Val203 relative motion, we find that the two motions are in resonance. By computing the associated spectrum, we find that the reaction coordinate is coupled with the substrate-NAD motion, and from the fact that the coupling vanishes at or near the transition state (demonstrated by the disappearance of strong features in the spectral density), we conclude that the substrate-NAD motion plays the role of a promoting vibration symmetrically coupled to the reaction coordinate.