Structure of the membrane-bound formate hydrogenlyase complex from Escherichia coli

The prototypical hydrogen-producing enzyme, the membrane-bound formate hydrogen lyase (FHL) complex from Escherichia coli , links formate oxidation at a molybdopterin-containing formate dehydrogenase to proton reduction at a [NiFe] hydrogenase. It is of intense interest due to its ability to efficiently produce H 2 during fermentation, its reversibility, allowing H 2 -dependent CO 2 reduction, and its evolutionary link to respiratory complex I. FHL has been studied for over a century, but its atomic structure remains unknown. Here we report cryo-EM structures of FHL in its aerobically- and anaerobically-isolated forms at resolutions reaching 2.6 Å. This includes well-resolved density for conserved loops linking the soluble and membrane arms believed to be essential in coupling enzymatic turnover to ion translocation across the membrane in the complex I superfamily. We describe an unpredicted metal-binding site near the interface of FdhF and HycF subunits that may play a role in preventing reverse activity in vivo , and evaluate possible structural determinants of the bias toward hydrogen production over its oxidation.