Crystal structure of the O₂-tolerant membrane-bound hydrogenase 1 from Escherichia coli in complex with its cognate cytochrome b

We report the 3.3 Å resolution structure of dimeric membrane-bound O₂-tolerant hydrogenase 1 from Escherichia coli in a 2:1 complex with its physiological partner, cytochrome b. From the short distance between distal [Fe₄S₄] clusters, we predict rapid transfer of H₂-derived electrons between hydrogenase heterodimers. Thus, under low O₂ levels, a functional active site in one heterodimer can reductively reactivate its O₂-exposed counterpart in the other. Hydrogenase 1 is maximally expressed during fermentation, when electron acceptors are scarce. These conditions are achieved in the lower part of the host's intestinal tract when E. coli is soon to be excreted and undergo an anaerobic-to-aerobic metabolic transition. The apparent paradox of having an O₂-tolerant hydrogenase expressed under anoxia makes sense if the enzyme functions to keep intracellular O₂ levels low by reducing it to water, protecting O₂-sensitive enzymes during the transition. Cytochrome b's main role may be anchoring the hydrogenase to the membrane.