PNNL

Abstract

CymA 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 characterisation of CymA from Shewanella oneidensis MR-1 identifies three low-spin His/His coordinated c-hemes and a single high-spin c-heme with His/H2O coordination 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 (ca. -240 mV) and low-spin (ca. -110, -190 and -265 mV) hemes 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 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 nonheme cofactor for quinol oxidation and that the reduced enzyme forms a 1:1 complex with its redox partner Fcc3. The implications for CymA supporting the respiratory flexibility of shewanellae are discussed.