A functional description of CymA, an electron-transfer hub supporting anaerobic respiratory flexibility in Shewanella

CymA (tetrahaem cytochrome c) is a member of the NapC/NirT family of quinol dehydrogenases. Essential for the anaerobic respiratory flexibility of shewanellae, CymA transfers electrons from menaquinol to various dedicated systems for the reduction of terminal electron acceptors including fumarate and insoluble minerals of Fe(III). Spectroscopic characterization of CymA from Shewanella oneidensis strain MR-1 identifies three low-spin His/His co-ordinated c-haems and a single high-spin c-haem with His/H2O co-ordination lying adjacent to the quinol-binding site. At pH 7, binding of the menaquinol analogue, 2-heptyl-4-hydroxyquinoline-N-oxide, does not alter the mid-point potentials of the high-spin (approximately −240 mV) and low-spin (approximately −110, −190 and −265 mV) haems that appear biased to transfer electrons from the high- to low-spin centres following quinol oxidation. CymA is reduced with menadiol (Em=−80 mV) in the presence of NADH (Em=−320 mV) and an NADH–menadione (2-methyl-1,4-naphthoquinone) oxidoreductase, but not by menadiol alone. In cytoplasmic membranes reduction of CymA may then require the thermodynamic driving force from NADH, formate or H2 oxidation as the redox poise of the menaquinol pool in isolation is insufficient. Spectroscopic studies suggest that CymA requires a non-haem co-factor for quinol oxidation and that the reduced enzyme forms a 1:1 complex with its redox partner Fcc3 (flavocytochrome c3 fumarate reductase). The implications for CymA supporting the respiratory flexibility of shewanellae are discussed.

INSIGHTS INTO THE MECHANISMS OF QUINOL OXIDATION IN CYTOCHROME bc1 IN LIGHT OF THE STRUCTURE OF THE BACTERIAL COMPLEX

The ten years since the first publication of the structure from bovine heart mitochondria in 1997 have significantly broadened our structural knowledge of cytochrome bc1 and b6f complexes from various organisms under a variety of conditions providing unprecedented mechanistic insights into the function of these essential enzymes. Still many questions remain. The bifurcated electron transfer at the quinol oxidation ( Q P ) site is one of the most difficult and unresolved problems. Intertwined with it, the proton translocation pathway and the quinol oxidation chemistry have remained focuses of intense research. Structural studies of mitochondrial bc1 complexes have not only provided an atomic view of the bc1 complex, defining many critical, functionally important features such as the locations of the Q P and Q N sites, but have also offered a number of important clues to mechanistic issues leading to the formulation of the "conformational switch of ISP" hypothesis. Intensive biochemical, genetic, and biophysical studies coupled with structural investigations have provided strong support for this hypothesis. The recent structure determination of the bc1 from the photosynthetic bacterium R. sphaeroides promises further insight.