Figure 1.

(A) Schematic of the inner membrane respiratory chain of Shewanella grown anaerobically with fumarate as the terminal electron acceptor. Electrons generated during catabolism are donated to the MQ-7 pool, which is reoxidized by CymA. Based on primary sequence analysis,⁴⁶ homology to NrfH for which a crystal structure is available,³⁸ and biochemical analysis of both CymA and other NapC/NirT superfamily members, CymA is known to contain a single N-terminal transmembrane α-helix with a single globular “head” domain facing the periplasm. CymA transfers the electrons to the periplasmic enzyme flavocytochrome c₃ (Fcc₃), which reduces fumarate to succinate. The site of action of the competitive inhibitor, 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), is indicated in magenta. (B) Schematic of CymA containing inner-membrane architecture on an electrode surface in the presence of Fcc₃ and a chemical reducing agent dithionite (S₂O₄^2–). In both panels, chemical reactions and ET steps are shown with black and green arrows, respectively.