Function of a glutathione-dependent formaldehyde dehydrogenase in Rhodobacter sphaeroides formaldehyde oxidation and assimilation

Despite its reactivity with many biological molecules, formaldehyde can be commonly encountered by virtually all cells. The widespread existence of glutathione-dependent formaldehyde dehydrogenases (GSH-FDH) in procaryotes and eucaryotes suggests this enzyme plays a central and universal role in biological formaldehyde oxidation. This work sought to determine the role of GSH-FDH in the facultative phototrophic bacterium Rhodobacter sphaeroides. Growth phenotypes of wild type and mutant cells, changes in enzyme specific activities, and the pattern of C-13-labeled compounds detected by NMR spectroscopy cumulatively suggest that R. sphaeroides GSH-FDH can play a critical role in formaldehyde metabolism under both photosynthetic and aerobic respiratory conditions. In photosynthetic cells, the data indicate that GSH-FDH generates reducing power, in the form of NADH, and one-carbon skeletons that are oxidized to carbon dioxide for subsequent assimilation by the Calvin-Benson-Bassham cycle. For example, use of methanol as a sole photosynthetic carbon source increases the specific activities of GSH-FDH, an NAD-dependent formate dehydrogenase, and the key Calvin-Benson-Bassham cycle enzyme, ribulose-1,5-bisphosphate carboxylase. This role of GSH-FDH is also supported by the pattern of [C-13]formaldehyde oxidation products that accumulate in photosynthetic cells and the inability of defined GSH-FDH or Calvin cycle mutants to use methanol as a sole carbon source. Our data also suggest that GSH-FDH acts in formaldehyde dissimilation when aerobic respiratory cultures cometabolize methanol and succinate.