Abstract
Flavin-based electron bifurcation is a long hidden mechanism of energetic coupling present mainly in anaerobic bacteria and archaea that suffer from energy limitations in their environment. Electron bifurcation saves precious cellular ATP and enables lithotrophic life of acetate-forming (acetogenic) bacteria that grow on H2 + CO2 by the only pathway that combines CO2 fixation with ATP synthesis, the Wood–Ljungdahl pathway. The energy barrier for the endergonic reduction of NADP+, an electron carrier in the Wood–Ljungdahl pathway, with NADH as reductant is overcome by an electron-bifurcating, ferredoxin-dependent transhydrogenase (Nfn) but many acetogens lack nfn genes. We have purified a ferredoxin-dependent NADH:NADP+ oxidoreductase from Sporomusa ovata, characterized the enzyme biochemically and identified the encoding genes. These studies led to the identification of a novel, Sporomusa type Nfn (Stn), built from existing modules of enzymes such as the soluble [Fe–Fe] hydrogenase, that is widespread in acetogens and other anaerobic bacteria.
Highlights
Flavin-based electron bifurcation is a long hidden mechanism of energetic coupling present mainly in anaerobic bacteria and archaea that suffer from energy limitations in their environment
Some acetogens lack nfn genes and must have a different enzyme to reduce N ADP+. To find this missing link, we have studied the metabolism of the acetogen Sporomusa ovata and will provide evidence for a novel type of NADH-dependent reduced ferredoxin:NADP+ oxidoreductase built from existing redox modules
Almost all proteins of the methyl- and the carbonyl branch were found to be encoded in one gene cluster (SOV_1c07560- SOV_1c07730, Supplementary Fig. 1), but the genes encoding a SeCys-containing formate dehydrogenase (Fdh), phosphotransacetylase (Pta) and acetate kinase (Ack) are located somewhere else in the genome (SOV_1c07830 + SOV_1c07840, SOV_1c07460 and SOV_1c10930)
Summary
Flavin-based electron bifurcation is a long hidden mechanism of energetic coupling present mainly in anaerobic bacteria and archaea that suffer from energy limitations in their environment. We have purified a ferredoxin-dependent NADH:NADP+ oxidoreductase from Sporomusa ovata, characterized the enzyme biochemically and identified the encoding genes These studies led to the identification of a novel, Sporomusa type Nfn (Stn), built from existing modules of enzymes such as the soluble [Fe–Fe] hydrogenase, that is widespread in acetogens and other anaerobic bacteria. Studied so far lack a membrane-bound transhydrogenase; they use electron bifurcation to overcome the energetic barrier by an NADH-dependent reduced ferredoxin:NADP+ oxidoreductase (known as Nfn in acetogens)[19]. Some acetogens lack nfn genes and must have a different enzyme to reduce N ADP+ To find this missing link, we have studied the metabolism of the acetogen Sporomusa ovata and will provide evidence for a novel type of NADH-dependent reduced ferredoxin:NADP+ oxidoreductase built from existing redox modules
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