F420H2DH family

The H⁺-translocating F₄₂₀H₂ Dehydrogenase (F₄₂₀H₂DH) Family (TC# 3.D.9) is a member of the Na⁺ transporting Mrp superfamily. A single F₄₂₀H₂ dehydrogenase (also referred to as F₄₂₀H₂:quinol oxidoreductase) from the methanogenic archaeon, Methanosarcina mazei Gö1, has been shown to be a redox driven proton pump. The F₄₂₀H₂DH of M. mazei has a molecular size of about 120 kDa and contains Fe-S clusters and FAD. A similar five-subunit enzyme has been isolated from Methanolobus tindarius. The sulfate-reducing Archaeoglobus fulgidus (and several other archaea) also have this enzyme.^[1]

Function

Reduction of 2-hydroxyphenazine by F₄₂₀H₂DH is accompanied by the translocation of 1 H+ per 2 electrons transferred.

Transport Reaction

The overall vectorial reaction catalyzed by F₄₂₀H₂DH is

Reduced donor (2e⁻) + H⁺ (in) ⇌ oxidized acceptor (2e⁻) + H⁺ (out)

Role in Methanogenesis

Methanomassiliicoccus luminyensis has been isolated from the human gut and requires H₂ and methanol or methylamines to produce methane. The organism lacks cytochromes, indicating that it cannot couple membrane-bound electron transfer reactions with the extrusion of protons or sodium ions using other known methanogenic pathways. Furthermore, M. luminyensis contains a soluble MvhAGD/HdrABC complex, as found in obligate hydrogenotrophic methanogens, but the energy conserving methyltransferase (MtrA-H) is absent.^[2][3] Evidence has been presented that M. luminyensis uses two types of heterodisulfide reductases (HdrABC and HdrD) in an energy conserving process.^[4] RT-qPCR studies revealed that genes coding for both heterodisulfide reductases were expressed at high levels.^[4] Other genes with high transcript abundance were fpoA as part of the operon encoding the 'headless' F₄₂₀H₂ dehydrogenase and atpB as part of the operon encoding the A₁A_o ATP synthase. High activities of the soluble heterodisulfide reductase HdrABC and the hydrogenase MvhADG were found in the cytoplasm. Also, heterologously produced HdrD could reduce CoM-S-S-CoB using reduced methylviologen as electron donor. It is proposed that membrane-bound electron transfer is based on the conversion of two molecules of methanol and the concurrent formation of two molecules of the heterodisulfide CoM-S-S-CoB.^[4] First, the HdrABC/MvhADG complex catalyzes the H₂-dependent reduction of CoM-S-S-CoB and the formation of reduced ferredoxin. Second, reduced ferredoxin is oxidized by the 'headless' F₄₂₀H₂ dehydrogenase, thereby translocating up to 4 H⁺ across the membrane, and electrons are channeled to HdrD for the reduction of the second heterodisulfide.^[4]

Homology

The gene cluster encoding the F₄₂₀H₂DH includes 12 genes, fpoABCDHIJKLMNO. Several of the subunits are related to those of the mitochondrial 'complex I' NDH family members (TC# 3.D.1). Thus, the gene products, FpoA, H, J, K, L, M and N, are highly hydrophobic and are homologous to subunits that form the membrane integral module of NDH-1. FpoB, C, D and I have their counterparts in the amphipathic membrane-associated module of NDH-1. However, homologues of the hydrophilic subunits of the NADH-oxidizing complex are absent.^[5]

See also

Transporter Classification Database

References

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