Structural and spectroscopic insight on the mechanism of the electron-bifurcating [FeFe] hydrogenase from Thermotoga maritima

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Electron bifurcation is an important mechanism of energy conservation in nature alongside substrate level phosphorylation and chemiosmotic coupling. However, compared to the latter two mechanisms, electron bifurcation is still poorly understood. The electron bifurcating/confurcating [FeFe] hydrogenase from Thermotoga maritima (HydABC) simultaneously oxidises both NADH and ferredoxin and reduces protons generating hydrogen. How this enzyme operates is completely unknown, and the sites of ferredoxin binding and electron bifurcation/confurcation are under debate. Here, I will present the 2.3 Å structure of HydABC solved by cryoelectron microscopy as an important step towards understanding its mechanism. The entire structure is a heterododecamer, Hyd(ABC)4, composed of two independent Hyd(ABC)2 ‘halves’ each made of two strongly interacting HydABC trimers electrically connected via a histidine-ligated [4Fe-4S] cluster in HydA, forming a potential bus-bar system. A flexible helix at the N-terminus of HydB may interact with ferredoxin via a « fly-casting » mechanism. Using symmetry expansion additional conformations of the C-terminal domain (CTD) of HydB were identified: a “closed bridge” conformation with the CTD of HydB bridging to an adjacent trimer, and an “open bridge” conformation with the CTD of HydB pointing away from the complex. This « bridge » structure is highly conserved in known bifurcating [FeFe] hydrogenases and is likely to be crucial for the mechanism. Furthermore, a monomeric zinc site was identified connecting the CTD of HydB to the rest of HydB, which may act as a « hinge » regulating the flexibility in this region. Together with previous spectroscopic studies, these results point to a novel mechanism of electron-bifurcation and will allow further targeted studies of the mechanism of this complicated and fascinating enzyme.

https://www.cec.mpg.de/de/forschung/anorganische-spektroskopie/dr-james-birrell