The Auxiliary NADH Dehydrogenase Plays a Crucial Role in Redox Homeostasis of Nicotinamide Cofactors in the Absence of the Periplasmic Oxidation System in Gluconobacter oxydans NBRC3293

has the unique property of a glucose oxidation system in the periplasmic space, where glucose is oxidized incompletely to ketogluconic acids in a nicotinamide cofactor-independent manner. Elimination of the gene for membrane-bound glucose dehydrogenase, the first enzyme for the periplasmic glucose oxidation system, induces a metabolic change whereby glucose is oxidized in the cytoplasm to acetic acid. strain NBRC3293 possesses two molecular species of type II NADH dehydrogenase (NDH), the primary and auxiliary NDHs that oxidize NAD(P)H by reducing ubiquinone in the cell membrane. The substrate specificities of the two NDHs are different from each other: primary NDH (p-NDH) oxidizes NADH specifically but auxiliary NDH (a-NDH) oxidizes both NADH and NADPH. We constructed NBRC3293 derivatives defective in the gene for a-NDH, in the gene, and in both. Our Δ derivative yielded higher cell biomass on glucose, as reported previously, but grew at a lower rate than the wild-type strain. The Δ derivative showed growth behavior on glucose similar to that of the wild type. The Δ Δ double mutant showed greatly delayed growth on glucose, but its cell biomass was similar to that of the Δ strain. The double mutant accumulated intracellular levels of NAD(P)H and thus shifted the redox balance to reduction. Accumulated NAD(P)H levels might repress growth on glucose by limiting oxidative metabolisms in the cytoplasm. We suggest that a-NDH plays a crucial role in redox homeostasis of nicotinamide cofactors in the absence of the periplasmic oxidation system in Nicotinamide cofactors NAD and NADP mediate redox reactions in metabolism. , a member of the acetic acid bacteria, oxidizes glucose incompletely in the periplasmic space-outside the cell. This incomplete oxidation of glucose is independent of nicotinamide cofactors. However, if the periplasmic oxidation of glucose is abolished, the cells oxidize glucose in the cytoplasm by reducing nicotinamide cofactors. Reduced forms of nicotinamide cofactors are reoxidized by NADH dehydrogenase (NDH) on the cell membrane. We found that two kinds of NDH in have different substrate specificities: the primary enzyme is NADH specific, and the auxiliary one oxidizes both NADH and NADPH. Inactivation of the latter enzyme in cells in which we had induced cytoplasmic glucose oxidation resulted in elevated intracellular levels of NAD(P)H, limiting cell growth on glucose. We suggest that the auxiliary enzyme is important if grows independently