Structure of the F 420 H 2 :quinone oxidoreductase of Archaeoglobus fulgidus

Abstract

The F 420 H 2 :quinone oxidoreductase from the sulfate‐reducing archaeon Archaeoglobus fulgidus is encoded by the fqo gene cluster which comprises 11 genes ( fqo J , K , M , L , N , A , BC , D , H , I , F ). The last gene of the cluster, fqoF , was overexpressed in Escherichia coli . The purified subunit was able to oxidize reduced cofactor F 420 using the electron‐acceptor system methyl viologen plus metronidazole. The specific activity at 78 °C was 64 µmol F 420 H 2 oxidized·min −1 ·(mg protein) −1 . The purified polypeptide contained 10.6 mol non‐heme iron, 7.2 mol acid‐labile sulfur and 0.7 mol FAD per mol protein. With the exception of fqoF , the deduced amino‐acid sequences of all other genes show homologies to distinct subunits of NADH–quinone oxidoreductases from prokaryotes and eukaryotes. Thus, it is concluded that the F 420 H 2 ‐dependent and the NADH‐dependent enzyme are functional equivalents. Both proteins are the initial enzymes of membrane‐bound electron‐transport systems and are involved in energy conservation. In parallel with bacterial complex I, the F 420 H 2 :quinone oxidoreductase may be composed of three subcomplexes. FqoF functions as the input device adjusted to the oxidation of reduced cofactor F 420 H 2 , thereby replacing subunits of the input module of complex I that are not present in A. fulgidus . The subunits FqoB, FqoCD and FqoI may form the membrane‐associated module and transfer electrons to the membrane‐integral module. It is most likely that the last subcomplex is composed of FqoA, FqoH, FqoJ, FqoK, FqoL, FqoM and FqoN. All subunits are highly hydrophobic and are probably involved in the reduction of a special menaquinone with a fully reduced isoprenoid side chain present in the cytoplasmic membrane of A. fulgidus .