together with one or two water, hydroxide, aldehyde, alcohol and alkoxide ligands. The parametrization is tailored for the active site of alcohol dehydrogenase and is obtained entirely from quantum chemical computations. The force-field reproduces excellently the geometry of quantum chemically optimized zinc complexes as well as the crystallographic geometry of the active site of alcohol dehydrogenase and small organic structures.
The parametrization is used in molecular dynamics simulations and molecular mechanical energy minimisations of alcohol dehydrogenase with a four- or five-coordinate catalytic zinc ion. The active-site zinc ion seems to prefer four-coordination over five-coordination by at least 36 kJ/mole. The only stable binding site of a fifth ligand at the active-site zinc ion is opposite to the normal substrate site, in a narrow cavity behind the zinc ion. Only molecules of the size water or smaller may occupy this site.
There are large fluctuations in the geometry of the zinc coordination
sphere. A four-coordinate water molecule alternates frequently (every 7
ps) between the substrate site and the fifth binding site and even two
five-coordinate water molecules may interchange ligation sites without
prior dissociation. Ligand exchange at the zinc ion probably proceeds by
a dissociative mechanism. The results show that it is essential to allow
for bond stretching degrees of freedom in molecular dynamics simulations
to get a correct description of the dynamics of the metal coordination
sphere; bond length constraints may restrict the accessible part of the
phase space and therefore lead to qualitatively erroneous results.