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 Nia-S state of the enzyme, nickel iron hydrogenase, from the Nir* (ready) or Niu* (unready) state, by reaction with H2, 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 H2 at the Ni3+ centre with a barrier of ∼15 kcal mol-1. We take the unready state to have a -OOH bridge, and find that reaction with H2 to form the Nir-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 ∼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. © the Owner Societies 2006.