Biological hydrogen activation is cooperative: One activates the dinuclear center and a second is oxidized

Molecular hydrogen is activated by two classes of enzymes, Fe-hydrogenases and NiFe-hydrogenases, which have similar sulfur-bridged dinuclear metal active sites with biologically unusual ligands CO and CN - . While detailed structural knowledge on the biocatalyst is available, the mode and the locus of binding of the substrate, H 2 , has not yet been established. A kinetic study of the hydrogen oxidation reaction by Fe-hydrogenase from Desulfovibrio vulgaris (Hildenborough) reveals a biphasic activation mechanism from as isolated, resting enzyme, via an intermediate state of relatively low activity, to maximally active enzyme. H 2 itself is the causative agent for the two subsequent enzyme activation processes. Kinetic model analysis suggests that the steady-state assumption of Michaelis–Menten kinetics does not apply to this hydrogenase. It appears that activation by hydrogen is cooperative: a catalytic H 2 with high binding affinity is turned over at high rate only after a second, regulatory H 2 with low affinity has been bound to the active site. The proposed novel paradigm of two hydrogen molecules binding simultaneously to the active site may hold for hydrogenases in general.