Acinetobacter calcoaceticus can incompletely oxidize aldose sugars to the corresponding aldonic acids. This reaction can serve as an auxiliary energy source for the organism. An increase in biomass yields is observed in acetate-limited chemostat cultures grown in the presence of, for example, xylose. However, experimental and theoretical discrepancies exist with respect to the magnitude of the yield enhancement as a result of xylose addition. We previously observed increases in cell yields that were unexpectedly high. In contrast, other data were in agreement with the theoretical predictions. In this paper, evidence is presented indicating that this discrepancy is likely to be due to errors in the methodology used for our previous investigation, in particular with respect to the determination of biomass concentrations.

Quinoprotein glucose dehydrogenase (GDH; EC 1.1.99.17) was partially purified from cell-free extracts of Acinetobacter calcoaceticus LMD79.41. The enzyme oxidized monosaccharides (d-glucose, d-allose, 2-deoxy-d-glucose, d-galactose, d-mannose, d-xylose, d-ribose and l-arabinose) as well as disaccharides (d-lactose, d-maltose and d-cellobiose). Intact cells of A. calcoaceticus LMD79.41 also oxidized these monosaccharides, but not the disaccharides. The difference in substrate specificity can not be explained by impermeability of the outer membrane for disaccharides, since right-side-out membrane vesicles did not oxidize disaccharides either. Destruction of the cytoplasmic membrane strongly affected the catalytic properties of GDH. Not only did the affinity towards some monosaccharides change substantially, but disaccharides also became good substrates upon solubilization of the enzyme. Thus, at least in A. calcoaceticus LMD79.41, the oxidation of disaccharides by GDH can be considered as an in vitro ‘artefact’ caused by the removal of the enzyme from its natural environment.

Evidence is presented that in Acinetobacter calcoaceticus oxidation of glucose to gluconate by the periplasmic quinoprotein glucose dehydrogenase (EC 1 . 1 .99.17) leads to energy conservation. Membrane vesicles prepared from cells grown in carbon-limited chemostat culture exhibited (1) a high rate of glucose-dependent oxygen consumption and gluconate production, (2) glucosemediated cytochrome reduction, (3) uncoupler sensitive, glucose-dependent generation of a membrane potential and (4) glucose-driven accumulation of amino acids. Furthermore, oxidation of glucose to gluconate by whole cells was associated with ATP synthesis. These results confirm and extend previous observations that periplasmic glucose oxidation can act as a driving force for energy-requiring processes. It is therefore concluded that the incomplete oxidation of glucose by bacteria may serve as an auxiliary energy-generating system.