PATHWAYS FOR UTILIZATION OF CARBON RESERVES IN DESULFOVIBRIO-GIGAS UNDER FERMENTATIVE AND RESPIRATORY CONDITIONS

The sulfate-reducing bacterium Desulfovibrio gigas accumulates large a mounts of polyglucose as an endogenous carbon and energy reserve, In t he absence of exogenous substrates, the intracellular polysaccharide w as utilized, and energy was conserved in the process (H. Santos, P. Fa releira, A. V. Xavier, L. Chen, M.-Y. Liu, and J. LeGall, Biochem. Bio phys. Res. Commun. 195:551-557, 1993), When an external electron accep tor was not provided, degradation of polyglucose by fell suspensions o f D. gigas yielded acetate, glycerol, hydrogen, and ethanol, A detaile d investigation of the metabolic pathways involved in the formation of these end products was carried out, based on measurements of the acti vities of glycolytic enzymes in cell extracts, by either spectrophotom etric or nuclear magnetic resonance (NMR) assays. All of the enzyme ac tivities associated with the glycogen cleavage and the Embden-Meyerhof pathway were determined as well as those involved in the formation of glycerol from dihydroxgacetone phosphate (glycerol-3-phosphate dehydr ogenase and glycerol phosphatase) and the enzymes that catalyze the re actions leading to the production of ethanol (pyruvate decarboxylase a nd ethanol dehydrogenase). The key enzymes of the Entner-Doudoroff pat hway were not detected, The methylglyoxal bypass was identified as a s econd glycolytic branch operating simultaneously with the Embden-Meyer hof pathway, The relative contribution of these two pathways for polyg lucose degradation was 2:3, C-13-labeling experiments with cell extrac ts using isotopically enriched glucose and C-13-NMR analysis supported the proposed pathways. The information on the metabolic pathways invo lved in polyglucose catabolism combined with analyses of the end produ cts formed from polyglucose under fermentative conditions provided som e insight into the role of NADH in D. gigas, In the presence of electr on accepters, NADH resulting from polyglucose degradation was utilized for the reduction of sulfate, thiosulfate, or nitrite, leading to the formation of acetate as the only carbon end product besides CO2, Evid ence supporting the role of NADH as a source of reducing equivalents f or the production of hydrogen is also presented.