[Seminar] How does the hydrogenase enzyme work?

Seminar

Title: How does the hydrogenase enzyme work?

Speaker: Csaba Bagyinka

Affiliation: Institute of Biophysics, Biological Research Center, Szeged, Hungary

Abstract
In the lecture we will demonstrate that current hydrogenase working models are inconsistent and they can only be improved if one or two autocatalytic steps are incorporated into the enzyme cycle. We present experimental evidences that T. roseopersicina hydrogenase catalyzed oxidation of H2 includes at least one autocatalytic step. This finding is based on the special patterns of H2-oxidation in a thin-layer reaction chamber, on the autocatalytic oscillations in the fast absorption kinetics of the reduced methyl viologen-initiated reaction of hydrogenase, and on the special, long lag phase observed. The special patterns involve reaction fronts that start spontaneously and at random times at different points in the reaction chamber; blue spheres of reduced benzyl viologen are seen expanding at constant speed and amplitude. 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. Since the steady-state concentration of the product of the H2 oxidizing reaction (reduced benzyl viologen) displays a strong enzyme concentration dependence we conclude that the autocatalytic step occurs inside the catalytic enzyme-cycle and not in the enzyme activation process. Consequently, both interacting enzyme forms should participate in the catalytic cycle of the enzyme. The interaction of the two enzyme forms should result in a conformational change in the enzyme–substrate form. Because the reduction of all [FeS] clusters would 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 [NiFe]/[FeS]proximal or the [FeS]proximal/[FeS]distal reaction step, or removes a block of the penetration of gaseous hydrogen from the surface to the [NiFe] cluster. Such properties of the H2-oxidizing reaction have also been demonstrated in the case of Desulfovibrio baculatus hydrogenase indicating that autocatalytic behavior is not restricted only to the hydrogenase from T. roseopersicina. 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 to that of prion reactions.