How does the ([NiFe]) hydrogenase enzyme work?

•There are several discrepancies in the current hydrogenase reaction model.•We concluded that in H2-oxidizing reaction, there are 2 autocatalytic reaction steps.•First autocatalytic step is in the activation process, it is simply an electron transfer.•Second autocatalytic step is in the active cycle.•Second autocatalytic step is probably accompanied by a small conformational change.

Previous experimental results and theoretical considerations led to the conclusion that there are two autocatalytic reaction steps in the H2-oxidizing reaction of hydrogenase. One autocatalytic reaction occurs when the enzyme is mainly in inactive forms (Ni-A and/or Ni-B). This finding is supported by the special patterns of H2 oxidation in a thin-layer reaction chamber, and by the special, long lag phase observed. The autocatalytic step takes place between two enzyme forms, one of which also interacts directly with the terminal electron acceptor, i.e. the autocatalyst is an enzyme form in which [FeS]distal is reduced. During the autocatalytic interaction, an electron is passed from the autocatalyst to the other enzyme form. Inactive hydrogenase is used as a substrate in this autocatalytic step for another, activated hydrogenase molecule.The second autocatalytic step is located in the enzyme cycle. Its presence is supported by the facts that the Ni-R form is exclusively diamagnetic and that the steady-state concentration of the product of the H2-oxidizing reaction (i.e. reduced benzyl viologen) displays a strong enzyme concentration dependence. This enzyme concentration dependence does not depend on the “active-inactive” states of the enzyme and remains valid in consecutive H2-oxidizing - proton-reducing reaction series performed on the same sample. This autocatalytic step involves an enzyme–enzyme interaction; both interacting enzyme forms should participate in the catalytic cycle of the enzyme. Since the reduction of all [FeS] clusters would also be possible in a non-autocatalytic reaction, we hypothesize a small conformational change in the enzyme, catalyzed by the autocatalyst, which removes a block in the electron flow in either the [NiFe]/[FeS]proximal or the [FeS]proximal/[FeS]distal reaction step, or removes a blockade of the penetration of gaseous H2 from the surface to the [NiFe] cluster. As far as we are aware, this is the first experimental observation of such a phenomenon on an enzyme. The interaction of two protein forms and the conformation change as a result of the interaction between the two enzyme forms resembles those in prion reactions.