(Invited) Investigating the Hydrogenase Mechanism By Protein Film Electrochemistry

Abstract

With turnover frequencies up to 106 s-1, hydrogenases are the fastest molecular hydrogen catalyst. They are classified according to the metal content of their active, distinguishing the NiFe and FeFe as the two major classes. The two classes display an outstanding organometallic cofactor, with cyanido- and carbonyl- groups coordinating the metals. Their mechanism includes the stabilization of a metal-hydride state as the most reduced intermediate of the catalytic cycle. Hydrogenase (Hase) have driven mechanistic studies for decades, including extensive works by protein film electrochemistry targetting aspects as diverse as intramolecular diffusion of gases, long distance electron transfer, or susceptibility to inhibitors. This communication will focus on two other questions, distinguishing steady state functioning and in-activation processes (see figure). The catlaytic bias is the ability of an enzyme to turnover faster one direction than another. Amongst hydrogenases, the catalytic bias spans full range, from extremely biased to essentially neutral enzymes. Several works have brought clues to understand the molecular basis of the catalytic bias for both Hase classes, pointing out to the intramolecular electron transfer chain as the main effector. Our recent results on the study of the hydrogen dependent CO2 reductase reveal the role of the affinity for hydrogen in the bias. The susceptibility to overpotential corresponds to in-activation processes triggered by redox conditions that are far from equilibrium, and plausibly encountered in the life cycle of bioelectrochemical devices. One particular phenomenon is the reductive inactivation of FeFe-Hases, undergoing both in solution and on the electrode, that remains largely to understand. Our works on the enzyme HydA1 from microalgae suggest that this inactivation can be reversed under maintained reductive conditions, unlike discussed in the literature. Figure 1