SEMIEMPIRICAL AM1 AND PM3 STUDIES OF THE ENZYMATIC MECHANISM OF HORSELIVER ALCOHOL-DEHYDROGENASE

Semiempirical (AM1 and PM3) calculations on active site models have be en performed to study the mechanism of horse liver alcohol dehydrogena se (HLADH). The active site model used in the calculations consists of a Zn(II) ion coordinated by derivatives of Cys 46, Cys 174, His 67 an d an alkoxide/aldehyde, and also by derivatives of Ser 48 and NAD(+)/N ADH. The theoretical calculations show drastic differences in ground s tate energy levels for model systems incorporating negatively charged cysteine residues compared with active site models based on neutral cy steine residues. The lower enzymatic activity of HLADH towards isoprop anol can be rationalized using the active site models presented in thi s study, A negative charge on hydrogen being transferred in the transi tion state can be calculated, pointing to a hydride transfer mechanism . Probability calculations suggest that hydrogen tunnelling may occur. However, to draw definite conclusions one should take into account th e dynamics of the, enzyme system. In agreement with literature data, w ater most probably does not act as a fifth ligand of zinc in the terna ry complex. It is not clear from the calculations whether water is inv olved in the proton relay mechanism or not.