FIG 8.

Ech of M. barkeri. Ech consists of 6 subunits (EchABCDEF), of which EchA and EchB are membrane bound and EchA is the likely location for H⁺ translocation (shown in red). The remaining 4 subunits are located in the cytoplasm and facilitate the transfer of electrons between Fd and H₂/H⁺ by way of 3 cubane Fe-S clusters ([4Fe4S]). EchE contains the bimetallic [NiFe] active site for H₂ formation/oxidation. Bidirectional arrows indicate Ech is a reversible enzyme, and brown lines trace the putative pathway of electron transport. In one direction, the exothermic oxidation of Fd_red allows for internal H₂ production and translocation of H₂ to the outside of the cell.