Structural insight on the mechanism of an electron-bifurcating [FeFe] hydrogenase.

Electron-bifurcation is a fundamental energy conservation mechanism in nature in which two electrons from an intermediate potential electron donor are split so that one is sent along a high potential pathway to a high potential acceptor and the other is sent along a low potential pathway to a low potential acceptor. This process allows endergonic reactions to be driven by exergonic ones and is an alternative, less recognised, mechanism of energy coupling to the well-known chemiosmotic principle. The electron-bifurcating [FeFe] hydrogenase from <i>Thermotoga maritima</i> (HydABC) requires both NADH and ferredoxin to reduce protons generating hydrogen. The mechanism of electron-bifurcation in HydABC remains enigmatic in spite of intense research efforts over the last few years. Structural information may provide the basis for a better understanding of spectroscopic and functional information. Here, we present a 2.3 Å electron cryo-microscopy structure of HydABC. The structure shows a heterododecamer composed of two independent 'halves' each made of two strongly interacting HydABC heterotrimers connected via a [4Fe-4S] cluster. A central electron transfer pathway connects the active sites for NADH oxidation and for proton reduction. We identified two conformations of a flexible iron-sulfur cluster domain: a 'closed bridge' and an 'open bridge' conformation, where a Zn²⁺ site may act as a 'hinge' allowing domain movement. Based on these structural revelations, we propose a possible mechanism of electron-bifurcation in HydABC where the flavin mononucleotide serves a dual role as both the electron bifurcation center and as the NAD⁺ reduction/NADH oxidation site.