|Gene:||mbhF||Accession Number: G-12664 (MetaCyc)|
Species: Pyrococcus furiosus
Component of: Mbh hydrogenase (extended summary available)
Subunit of: Mbh hydrogenase
Species: Pyrococcus furiosus
Hydrogenases are found in a variety of microorganisms and catalyze the reversible reaction of proton reduction to H2 formation. This enables some organisms to use H2 as a source of reducing equivalents under both aerobic and anaerobic conditions, while in other organisms the enzyme reduces protons to H2, thereby releasing reducing equivalents obtained from the anaerobic degradation of organic substrates.
[NiFe] hydrogenases are well characterized enzymes that have play a vital role in H2 metabolism [Hedderich04]. A subset of [NiFe] hydrogenases are multisubunit membrane-bound enzyme complexes. They consist of at least four hydrophilic and two integral membrane subunits which form the core of the complex [Hedderich04]. They have closely related counterparts in energy-conserving NADH:quinone oxidoreductase (complex I), which is present in the inner mitochondrial membrane and in the cytoplasmic membrane of numerous microorganisms. Complex I catalyzes electron transfer from NADH to quinone or menaquinone and couples this to the translocation of H+ or sodium ions across a membrane.
The reaction catalyzed by the hydrogenases is significantly different from the reaction catalyzed by complex I. They can utilize electrons derived from reduced ferredoxins or polyferredoxins to catalyze the reduction of H+ [Hedderich05]. This exergonic reaction is coupled to energy conservation via electron-transport phosphorylation. In a reaction driven by reverse electron transport, other family members of this hydrogenase can provide the cell with reduced ferredoxin using H2 as the electron donor [Hedderich05].
About this Enzyme
The Pyrococcus furiosus genome contains the mbhA-N operon which encodes a [NiFe] hydrogenase-like membrane bound complex with 14 subunits [Silva00]. Six of the deduced proteins share homology with subunits found in ech hydrogenase and Escherichia coli hydrogenase 3. The mbh operon encodes an additional 8 small proteins predicted to form membrane spanning helices. The subunit linked to H2 production in this enzyme is homologous to the catalytic subunit of the well characterized cytoplasmic [NiFe] hydrogenase of Desulfovibrio gigas [Calusinska10].
The bioenergetics of H2 production catalyzed by mbh hydrogenase were studied using inverted membrane vesicles from Pyrococcus furiosus [Sapra03]. These studies revealed that the enzyme favors H2 evolution over uptake and requires reduced ferredoxin for activity. H2 production is coupled to ATP synthesis, suggesting that the enzyme is chemiosmotically linked to an ATP synthase [Sapra03]. Thus, mbh hydrogenase functions as a redox driven ion pump that can generate a proton motive force from H+ reduction coupled to electrons derived from ferredoxin oxidation [Vignais04]. A similar mechanism has been observed for the ech hydrogenase of Caldanaerobacter subterraneus tengcongensis.
This raises the question, what supplies the reaction with reduced ferredoxin? There are two sites for the generation of reduced ferredoxin in the glycolytic pathway. Tungsten containing glyceraldehyde-3-phosphate ferredoxin oxidoreductase catalyzes the oxidation of D-glyceraldehyde 3-phosphate to 3-phospho-D-glycerate and pyruvate:ferredoxin oxidoreductase catalyzes the oxidation of pyruvate to acetyl-CoA resulting in the production of reduced ferredoxin.
Enzymatic reaction of: hydrogenase
Synonyms: mbh hydrogenase, ech hydrogenase
EC Number: 22.214.171.124
The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the direction in which it was curated.
This reaction is reversible.
In Pathways: hydrogen production III
The activity of the enzyme was detected and purified from the membrane fraction of Pyrococcus furiosus. A TBLASTN search with the large subunit of E.coli hydrogenase 2 revealed three ORFs in the P.furiosus genome with a high similarity score. Two of the 3 sets of genes belonged to the gene cluster encoding the soluble sulfhydrogenase and hydrogenase II. The third detected ORF encodes a protein very similar to the large subunits of some membrane bound hydrogenases. A total of 14 contiguous genes were detected in this putative operon and been named mbh.
Calusinska10: Calusinska M, Happe T, Joris B, Wilmotte A (2010). "The surprising diversity of clostridial hydrogenases: a comparative genomic perspective." Microbiology 156(Pt 6);1575-88. PMID: 20395274
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