A natural fusion of flavodiiron, rubredoxin, and rubredoxin oxidoreductase domains is a self-sufficient water-forming oxidase of Trichomonas vaginalis

New Electron-Transfer Chain to a Flavodiiron Protein in Fusobacterium nucleatum Couples Butyryl-CoA Oxidation to O2 Reduction

Fusobacterium nucleatum, a Gram-negative obligate anaerobe, is common to the oral microbiota, but the species is known to infect other sites of the body where it is associated with a range of pathologies. At present, little is known about the mechanisms by which F. nucleatum mitigates against oxidative and nitrosative stress. Inspection of the F. nucleatum subsp. polymorphum ATCC 10953 genome reveals that it encodes a flavodiiron protein (FDP; FNP2073) that is known in other organisms to reduce NO to N2O and/or O2 to H2O. FNP2073 is dicistronic with a gene encoding a multicomponent enzyme termed BCR for butyryl-CoA reductase. BCR is composed of a butyryl-CoA dehydrogenase domain (BCD), the C-terminal domain of the α-subunit of the electron-transfer flavoprotein (Etfα), and a rubredoxin domain. We show that BCR and the FDP form an α4β4 heterotetramic complex and use butyryl-CoA to selectively reduce O2 to H2O. The FAD associated with the Etfα domain (α-FAD) forms red anionic semiquinone (FAD•-), whereas the FAD present in the BCD domain (δ-FAD) forms the blue-neutral semiquinone (FADH•), indicating that both cofactors participate in one-electron transfers. This was confirmed in stopped-flow studies where the reduction of oxidized BCR with an excess of butyryl-CoA resulted in rapid (<1.6 ms) interflavin electron transfer evidenced by the formation of the FAD•-. Analysis of bacterial genomes revealed that the dicistron is present in obligate anaerobic gut bacteria considered to be beneficial by virtue of their ability to produce butyrate. Thus, BCR-FDP may help to maintain anaerobiosis in the colon.

Characterization of the iron-sulfur clusters in the nitrogenase-like reductase CfbC/D required for coenzyme F430 biosynthesis

Coenzyme F430 is a nickel-containing tetrapyrrole, serving as the prosthetic group of methyl-coenzyme M reductase in methanogenic and methanotrophic archaea. During coenzyme F430 biosynthesis, the tetrapyrrole macrocycle is reduced by the nitrogenase-like CfbC/D system consisting of the reductase component CfbC and the catalytic component CfbD. Both components are homodimeric proteins, each carrying a [4Fe-4S] cluster. Here, the ligands of the [4Fe-4S] clusters of CfbC2 and CfbD2 were identified revealing an all cysteine ligation of both clusters. Moreover, the midpoint potentials of the [4Fe-4S] clusters were determined to be -256 mV for CfbC2 and -407 mV for CfbD2. These midpoint potentials indicate that the consecutive thermodynamically unfavorable 6 individual "up-hill" electron transfers to the organic moiety of the Ni2+-sirohydrochlorin a,c-diamide substrate require an intricate interplay of ATP-binding, hydrolysis, protein complex formation and release to drive product formation, which is a common theme in nitrogenase-like systems.

The flavodiiron protein from Syntrophomonas wolfei has five domains and acts both as an NADH:O2 or an NADH:H2 O2 oxidoreductase

Flavodiiron proteins (FDPs) are a family of enzymes with a significant role in O2 /H2 O2 and/or NO detoxification through the reduction of these species to H2 O or N2 O, respectively. All FDPs contain a minimal catalytic unit of two identical subunits, each one having a metallo-β-lactamase-like domain harboring the catalytic diiron site, and a flavodoxin-like domain. However, more complex and diverse arrangements in terms of domains are found in this family, of which the class H enzymes are among the most complex. One of such FDPs is encoded in the genome of the anaerobic bacterium Syntrophomonas wolfei subsp. wolfei str. Goettingen G311. Besides the core domains, this protein is predicted to have three additional ones after the flavodoxin core domain: two short-chain rubredoxins and a NAD(P)H:rubredoxin oxidoreductase-like domain. This enzyme, FDP_H, was produced and characterized and the presence of the predicted cofactors was investigated by a set of biochemical and spectroscopic methodologies. Syntrophomonas wolfei FDP_H exhibited a remarkable O2 reduction activity with a kcat  = 52.0 ± 1.2 s-1 and a negligible NO reduction activity (~ 100 times lower than with O2 ), with NADH as an electron donor, that is, it is an oxygen-selective FDP. In addition, this enzyme showed the highest turnover value for H2 O2 reduction (kcat  = 19.1 ± 2.2 s-1 ) ever observed among FDPs. Kinetic studies of site-directed mutants of iron-binding cysteines at the two rubredoxin domains demonstrated the essential role of these centers since their absence leads to a significant decrease or even abolishment of O2 and H2 O2 reduction activities.