F420H2 oxidase (FprA) from Methanobrevibacter arboriphilus, a coenzyme F420-dependent enzyme involved in O2 detoxification

Cell suspensions of Methanobrevibacter arboriphilus catalyzed the reduction of O₂ with H₂ at a maximal specific rate of 0.4 U (mol/min) per mg protein with an apparent K_m for O₂ of 30 M. The reaction was not inhibited by cyanide. The oxidase activity was traced back to a coenzyme F₄₂₀-dependent enzyme that was purified to apparent homogeneity and that catalyzed the oxidation of 2 F₄₂₀H₂ with 1 O₂ to 2 F₄₂₀ and 2 H₂O. The apparent K_m for F₄₂₀ was 30 M and that for O₂ was 2 M with a V_max of 240 U/mg at 37°C and pH 7.6, the pH optimum of the oxidase. The enzyme did not use NADH or NADPH as electron donor or H₂O₂ as electron acceptor and was not inhibited by cyanide. The 45-kDa protein, whose gene was cloned and sequenced, contained 1 FMN per mol and harbored a binuclear iron center as indicated by the sequence motif H–X–E–X–D–X₆₂–H–X₁₈–D–X₆₀–H. Sequence comparisons revealed that the F₄₂₀H₂ oxidase from M. arboriphilus is phylogenetically closely related to FprA from Methanothermobacter marburgensis (71% sequence identity), a 45-kDa flavoprotein of hitherto unknown function, and to A-type flavoproteins from bacteria (30–40%), which all have dioxygen reductase activity. With heterologously produced FprA from M. marburgensis it is shown that this protein is also a highly efficient F₄₂₀H₂ oxidase and that it contains 1 FMN and 2 iron atoms. The presence of F₄₂₀H₂ oxidase in methanogenic archaea may explain why some methanogens, e.g., the Methanobrevibacter species in the termite hindgut, cannot only tolerate but thrive under microoxic conditions.Dedicated to Hans Schlegel on the occasion of his 80th birthday.