Chemically demanding reductive conversions in biology, such as the reduction of dinitrogen to ammonia or the Birch-type reduction of aromatic compounds, depend on Fe/S-cluster-containing ATPases. These reductions are typically catalyzed by two-component systems, in which an Fe/S-cluster-containing ATPase energizes an electron to reduce a metal site on the acceptor protein that drives the reductive reaction. Here, we show a two-component system featuring a double-cubane [Fe 8 S 9 ]-cluster [{Fe

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... S 4 (SCys) 3 } 2 (μ 2 -S)]. The doublecubane-cluster-containing enzyme is capable of reducing small molecules, such as acetylene (C 2 H 2 ), azide (N 3 − ), and hydrazine (N 2 H 4 ). We thus present a class of metalloenzymes akin in fold, metal clusters, and reactivity to nitrogenases. Fe/S-cluster | nitrogenase | acetylene | ATPase | electron transfer Significance Our ability to reduce stable small molecules, such as dinitrogen or carbon dioxide, is as vital as it is demanding and requires energetic electrons and a catalyst. In nature, these requirements are met by two-component enzymes: an electrondonating metallo-ATPase and the principal catalyst, a metalloprotein with a low-potential cofactor. Here, we present a two-component enzyme in which the catalyst houses a doublecubane type [Fe 8 S 9 ]-cluster. Iron-sulfur clusters with so high nuclearity were so far only known from nitrogenase, an enzyme notorious for its capacity to reduce various small molecules. The enzyme not only shares structural features with nitrogenase, but is also able to reduce acetylene, indicating its potential employment for reductive reactions of our choice.