<p>Formylmethanofuran dehydrogenases (<db_xref db="EC" dbkey="1.2.99.5"/>) is found in methanogenic and sulphate-reducing archaea. The enzyme contains molybdenum or tungsten, a molybdopterin guanine dinuceotide cofactor (MGD) and iron-sulphur clusters [<cite idref="PUB00008259"/>]. It catalyses the reversible reduction of CO₂ and methanofuran via N-carboxymethanofuran (carbamate) to N-formylmethanofuran, the first and second steps in methanogenesis from CO₂ [<cite idref="PUB00015859"/>, <cite idref="PUB00042893"/>]. This reaction is important for the reduction of CO₂ to methane, in autotrophic CO₂ fixation, and in CO₂ formation from reduced C₁ units [<cite idref="PUB00016174"/>]. The synthesis of formylmethanofuran is crucial for the energy metabolism of archaea. Methanogenic archaea derives the energy for autrophic growth from the reduction of CO₂ with molecular hydrogen as the electron donor [<cite idref="PUB00042895"/>]. The process of methanogenesis consists of a series of reduction reactions at which the one-carbon unit derived from CO₂ is bound to C₁ carriers.</p><p>There are two isoenzymes of formylmethanofuran dehydrogenase: a tungsten-containing isoenzyme (Fwd) and a molybdenum-containing isoenzyme (Fmd). The tungsten isoenzyme is constitutively transcribed, whereas transcription of the molybdenum operon is induced by molybdate [<cite idref="PUB00000170"/>]. The archaea <taxon tax_id="187420">Methanobacterium thermoautotrophicum</taxon> contains a 4-subunit (FwdA, FwdB, FwdC, FwdD) tungsten formylmethanofuran dehydrogenase and a 3-subunit (FmdA, FmdB, FmdC) molybdenum formylmethanofuran dehydrogenase [<cite idref="PUB00016174"/>]. </p><p>This entry represents subunit B (FmdB and FwdB) of formylmethanofuran dehydrogenases. The other subunits are subunit A (<db_xref db="INTERPRO" dbkey="IPR012027"/>), subunit C (<db_xref db="INTERPRO" dbkey="IPR012023"/>), subunit D (<db_xref db="INTERPRO" dbkey="IPR012040"/>), subunit E (<db_xref db="INTERPRO" dbkey="IPR012204"/>) and subunit F (<db_xref db="INTERPRO" dbkey="IPR016459"/>). Some organisms also encode a fusion of the C and D subunits(<db_xref db="INTERPRO" dbkey="IPR012048"/>).</p> Formylmethanofuran dehydrogenase, subunit B