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.
Jayapal
Prabha
P
School of Chemistry, University of Manchester, Manchester, UK M13 9PL.
Sundararajan
Mahesh
M
Hillier
Ian H
IH
Burton
Neil A
NA