How are the ready and unready states of nickel-iron hydrogenase activated by H2? A density functional theory study

We have explored possible mechanisms for the formation of the catalytically active Ni(a)-S state of the enzyme, nickel iron hydrogenase, from the Ni*(r) (ready) or Ni*(u) (unready) state, by reaction with H(2), using density functional theory calculations with the BP86 functional in conjunction with a DZVP basis set. We find that for the reaction of the ready state, which is taken to have an -OH bridge, the rate determining step is the cleavage of H(2) at the Ni(3+) centre with a barrier of approximately 15 kcal mol(-1). We take the unready state to have a -OOH bridge, and find that reaction with H(2) to form the Ni(r)-S state can proceed by two possible routes. One such path has a number of steps involving electron transfer, which is consistent with experiment, as is the calculated barrier of approximately 19 kcal mol(-1). The alternative pathway, with a lower barrier, may not be rate determining. Overall, our predictions give barriers in line with experiment, and allow details of the mechanism to be explored which are inaccessible from experiment.