Combining electronic structure methods with the calculation of hydrogen vibrational wavefunctions: Application to hydride transfer in liver alcohol dehydrogenase

This paper presents an application of a computational approach combining el ectronic structure methods with the calculation of hydrogen vibrational wav efunctions. This application is directed at elucidating the nature of the n uclear quantum mechanical effects in the oxidation of benzyl alcohol cataly zed by liver alcohol dehydrogenase (LADH). The hydride transfer from the be nzyl alcohol substrate to the NAD(+) cofactor is described by a 148-atom mo del of the active site. The hydride potential energy curves and the associa ted hydrogen vibrational wavefunctions are calculated for structures along minimum energy paths and straight-line reaction paths obtained from electro nic structure calculations at the semiempirical PM3 and ab initio RHF/3-21G levels. The results indicate that, for these levels of theory, the hydride transfer is adiabatic and hydrogen tunneling does not play a critical role along the minimum energy path. In contrast, nonadiabatic effects and hydro gen tunneling are shown to be important along the more relevant straight-li ne reaction paths. The secondary hydrogens were found to be significantly c oupled to the transferring hydride near the transition state. In addition, the puckering of the NAD(+) ring was found to be a dominant contribution to the reaction coordinate near the transition state. Further from the transi tion state, the reaction coordinate is a mixture of many heavy-atom modes, including the donor-acceptor distance and the distance between the substrat e and the neighboring zinc and serine residue. These results imply that hyd rogen tunneling in LADH is strongly impacted by the puckering of the NAD(+) ring (which modulates the asymmetry of the hydride potential energy curve) and the distance between the donor and acceptor carbons (which modulates t he barrier of the hydride potential energy curve).