Function of a Glutathione-Dependent Formaldehyde Dehydrogenase in Rhodobacter sphaeroides Formaldehyde Oxidation and Assimilation

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Abstract
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 13C-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 [13C]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.