EC Number   |
General Information |
Reference |
|---|
  1.8.7.3 | evolution |
conservation of Hdr and Fmd structures suggests that the complex of both is common among hydrogenotrophic methanogens |
765833 |
| 1.8.7.3 | evolution |
HdrA homologues are found in many other microorganisms, i.e. anaerobic methanotrophic archaea, sulfate-reducing bacteria and archaea, sulfur-oxidizing bacteria, acetogenic bacteria, knallgas bacteria, and metal-reducing bacteria |
765831 |
| 1.8.7.3 | evolution |
HdrA homologues are found in many other microorganisms, i.e. anaerobic methanotrophic archaea, sulfate-reducing bacteria and archaea, sulfur-oxidizing bacteria, acetogenic bacteria, knallgas bacteria, and metal-reducing bacteria. The fifth cysteine Cys197' of Methanothermococcus thermolithotrophicus HdrA is exchanged for a selenocysteine in HdrA from Methanocaldococcus jannaschii |
-, 765831 |
| 1.8.7.3 | evolution |
the energy-conserving system in Methanomassiliicoccus luminyensis is unique, and the enzymes involved in this process are not found in this combination in members of the other methanogenic orders. The composition of the enzymes involved in ion translocation across the cytoplasmic membrane is different from all other methanogenic archaea |
764654 |
| 1.8.7.3 | metabolism |
a methyl-CoM reductase catalyzes the reductive resolution of methyl-CoM to form methane and a disulfide CoM conjugate of coenzyme B (CoM-S-S-CoB). This is the penultimate conserved step in all pathways of methanogenesis leaving only the reduction of CoM-S-S-CoB and recycling of the key methanogen cofactors CoM and CoB.. The reduction of CoM-S-S-CoB is often coupled to the oxidation of H2, an exergonic reaction with a significant negative free-energy change. Methanogens conserve the free energy by coupling this exergonic reaction to the hydrogen-dependent reduction of ferredoxin, which on its own would be an endergonic reaction. These reactions are coupled through flavin-based electron bifurcation that maximizes the energy efficiency in hydrogenotrophic methanogenesis. Heterodisulfide reductase (Hdr) is the valve that closes the cycle of methanogenesis, allowing energy to be conserved and providing an energetic advantage to the cell. Hdr catalyzes flavin-based electron bifurcation that results in the exergonic reduction of heterodisulfide (CoMS-S-CoB) coupled to the endergonic reduction of ferredoxin. In the proposed mechanism of bifurcation, the bifurcation-site flavin receives two electrons from a single donor, and then bifurcate one electron to reduce the heterodisulfide and one electron to reduce ferredoxin. Another round of bifurcation results in the complete reduction of CoM-S-S-CoB to HS-CoB and HS-CoM and 2 equivalents of reduced ferredoxin. Hdr receives two electrons from the oxidation of H2 through MvhAGD [NiFe]-hydrogenase EC 1.8.98.4 |
-, 764472 |
| 1.8.7.3 | metabolism |
in methanogenic archaea, the carbon dioxide (CO2) fixation and methane-forming steps are linked through the heterodisulfide reductase (HdrABC)-[NiFe]-hydrogenase (MvhAGD) complex that uses flavin-based electron bifurcation to reduce ferredoxin and the heterodisulfide of coenzymes M and B |
-, 765831 |
| 1.8.7.3 | metabolism |
reduction of the disulfide of coenzyme M and coenzyme B (CoMS-SCoB) by heterodisulfide reductases (HdrED and HdrABC) is the final step in all methanogenic pathways. Flavin-based electron bifurcation (FBEB) by soluble HdrABC homologues play additional roles in driving essential endergonic reactions at the expense of the exergonic reduction of CoMS-SCoM. In the first step of the CO2 reduction pathway, HdrABC complexed with hydrogenase (EC 1.12.1.2) or formate dehydrogenase generates reduces ferredoxin (Fdx2-) for the endergonic reduction of CO2 coupled to the exergonic reduction of CoMS-SCoB dependent on FBEB of electrons from H2 or formate, respectively. Roles for HdrABC:hydrogenase complexes are also proposed for pathways wherein the methyl group of methanol is reduced to methane with electrons from H2. The HdrABC complexes catalyze FBEB-dependent oxidation of H2 for the endergonic reduction of Fdx driven by the exergonic reduction of CoMS-SCoB. The Fdx2- supplies electrons for reduction of the methyl group to methane. In H2- independent pathways, threefourths of the methyl groups are oxidized producing Fdx2- and reduced coenzyme F420 (F420H2). The F420H2 donates electrons for reduction of the remaining methyl groups to methane requiring transfer of electrons from Fdx2- to F420. HdrA1B1C1 is proposed to catalyze FBEB-dependent oxidation of Fdx2- for the endergonic reduction of F420 driven by the exergonic reduction of CoMS-SCoB, see for EC 1.8.98.4. In H2- independent acetotrophic pathways (EC 1.8.98.5), the methyl group of acetate is reduced to methane with electrons derived from oxidation of the carbonyl group mediated by Fdx. Electron transport involves a membrane-bound complex (Rnf) that oxidizes Fdx2- and generates a NaC gradient driving ATP synthesis. It is postulated that F420 is reduced by Rnf requiring HdrA2B2C2 catalyzing FBEB-dependent oxidation of F420H2 for the endergonic reduction of Fdx driven by the exergonic reduction of CoMS-SCoB (EC 1.8.98.4). The Fdx2- is recycled by Rnf and HdrA2B2C2 thereby conserving energy. The HdrA2B2C2 is also proposed to play a role in Fe(III)-dependent reverse methanogenesis. A flavin-based electron confurcating (FBEC) HdrABC complex is proposed for nitrate-dependent reverse methanogenesis in which the oxidation of CoM-SH/CoB-SH and Fdx2- is coupled to reduction of F420. The F420H2 donates electrons to a membrane complex that generates a proton gradient driving ATP synthesis |
-, 764753 |
| 1.8.7.3 | metabolism |
the first reaction of the methanogenic pathway from carbon dioxide (CO2) is the reduction and condensation of CO2 to formyl-methanofuran, catalyzed by formyl-methanofuran dehydrogenase (Fmd, EC 1.12.7.2). Strongly reducing electrons for this reaction are generated by heterodisulfide reductase (Hdr, EC 1.8.7.3) in complex with hydrogenase or formate dehydrogenase (Fdh) using a flavin-based electron-bifurcation mechanism |
765833 |
| 1.8.7.3 | metabolism |
the process of methanogenesis in Methanomassiliicoccus luminyensis involves the transfer of methyl group from methanol or methylamines to 2-mercaptoethanesulfonate (HSCoM). The resulting methyl-S-CoM is reduced to methane by the methyl-CoM reductase which uses 7-mercaptoheptanoylthreonine phosphate (HS-CoB) as a reductant and forms the heterodisulfide (CoM-S-SCoB). The membrane-bound electron transport is based on the headless Fpo complex, which accepts electrons from Fdred and channels these electrons to the heterodisulfide reductase HdrD. And HdrD reduces the final electron acceptor the heterodisulfide (CoB-S-S-CoM) |
764654 |
| 1.8.7.3 | more |
enzymological and structural characterizations of Fdh-Hdr-Fmd complexes from Methanospirillum hungatei. The complexes catalyze the reaction using electrons from formate and the reduced form of the electron carrier F420. Conformational changes in HdrA mediate electron bifurcation, and polyferredoxin FmdF directly transfers electrons to the CO2 reduction site, as evidenced by methanofuran-dependent flavin-based electron bifurcation even without free ferredoxin, a diffusible electron carrier between Hdr and Fmd. Conformational changes within the HdrA subunit provide a conformationally gated pathway for electrons to and from the bifurcating flavin adenine dinucleotide (FAD). The dimeric Fdh- Hdr-Fmd structure reveals that FdhAB and HdrABC are connected via MvhD and that a polyferredoxin FmdF bridges HdrABC and FmdABCDG. Tertiary and quartenary enzyme complex structures and structure-function analysis, detailed overview |
765833 |
