EC Number   |
General Information |
Reference |
|---|
   1.8.98.4 | metabolism |
generalized model for electron and proton flux during hydrogen interspecies electron transfer (HIT) and direct interspecies electron transfer (DIET) with growth on ethanol as an example, overview. Electron and protons are transported by different mechanisms during DIET. Electron transfer is direct, through e-pili and other electrical contacts. Protons move by diffusion creating a positive proton pressure outside the cell. A mechanism for proton translocation into the cell is required for charge balance in the cytoplasm when cytoplasmic electron acceptors (EA) are reduced and to prevent acidification of the external space between cells. The transcriptome reflects faster growth during HIT and possible greater importance of membrane and outer-surface proteins during DIET. Model for electron and proton flux for carbon dioxide reduction to methane in Methanosarcina barkeri during DIET-based growth, overview |
-, 764762 |
| 1.8.98.4 | 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 or formate dehydrogenase generates reduced 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. 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, three-fourths 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. When switched from growth with methanol to growth with acetate, Methanosarcina acetivorans upregulates an electron bifurcating heterodisulfide reductase (HdrA2B2C2) that oxidizes F420H2 and reduces Fdx driven by reduction of CoMS-SCoB. A role has been proposed for sHdrA2B2C2 dependent on reduction of NAD-like coenzyme F420 (F420) by the Rnf complex analogous to Fdx-dependent reduction of NADC by homologous Rnf complexes from the domain Bacteria. In this way, Fdx reduced by HdrA2B2C2 is re-oxidized by Rnf thereby supplementing the translocation of Na+. HdrA2B2C2 is involved in reverse methanogenesis |
764753 |
| 1.8.98.4 | more |
physiological studies of direct interspecies electron transfer (DIET) require defined co-cultures. Geobacter metallireducens is an environmentally relevant pure culture model for electron-donating partners for DIET because Geobacter species function as the electron-donating partner in important methanogenic environments such as anaerobic digesters and terrestrial wetlands. Studies with defined co-cultures in which Geobacter metallireducens is the electron-donating partner for DIET have suggested that c-type cytochromes and electrically conductive pili [e-pili] facilitate electron transport from Geobacter metallireducens to the electron accepting partner. However, Methanosarcina barkeri, a methanogen shown to participate in DIET, does not possess outer-surface c-type cytochromes or e-pili |
-, 764762 |
| 1.8.98.4 | physiological function |
direct interspecies electron transfer (DIET) is important in diverse methanogenic environments. In DIET, electrically conductive pili [e-pili] and associated electron transport proteins deliver electrons to cytoplasmic electron acceptors. Protons have to be translocated into the cytoplasm for charge balance. The transcriptome of Methanosarcina barkeri grown via DIET in co-culture with Geobacter metallireducens compared with its transcriptome when grown via H2 interspecies transfer (HIT) with Pelobacter carbinolicus shows that transcripts for the F420H2 dehydrogenase (Fpo) and the heterodisulfide reductase, HdrABC, are more abundant during growth on DIET. Electrons delivered to methanophenazine in the cell membrane are transferred to Fpo (cf. EC 1.8.98.1). The external proton gradient necessary to drive the otherwise thermodynamically unfavorable reverse electron transport for Fpo-catalyzed F420 reduction is derived from protons released from Geobacter metallireducens metabolism. Reduced F420 is a direct electron donor in the carbon dioxide reduction pathway and also serves as the electron donor for the proposed HdrABC-catalyzed electron bifurcation reaction in which reduced ferredoxin (also required for carbon dioxide reduction) is generated with simultaneous reduction of CoM-S-S-CoB |
-, 764762 |
| 1.8.98.4 | physiological function |
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, three-fourths 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. When switched from growth with methanol to growth with acetate, Methanosarcina acetivorans upregulates an electron bifurcating heterodisulfide reductase (HdrA2B2C2) that oxidizes F420H2 and reduces Fdx driven by reduction of CoMS-SCoB. A role has been proposed for HdrA2B2C2 dependent on reduction of NAD-like coenzyme F420 (F420) by the Rnf complex analogous to Fdx-dependent reduction of NADC by homologous Rnf complexes from the domain Bacteria |
764753 |
