The F420H2:heterodisulfide oxidoreductase system from Methanosarcina species

Abstract

F₄₂₀H₂-dependent CoB-S-S-CoM reduction as catalyzed by the F₄₂₀H₂:heterodisulfide oxidoreductase from Methanosarcina strains was observed in a defined system containing purified F₄₂₀H₂ dehydrogenase from Methanosarcina mazei Gö1, 2-hydroxyphenazine and purified heterodisulfide reductase from Methanosarcina thermophila. The process could be divided into two partial reactions: (1) reducing equivalents from F₄₂₀H₂ were transferred to 2-hydroxyphenazine by the F₄₂₀H₂ dehydrogenase with a V _max value of 12 U/mg protein; (2) reduced 2-hydroxyphenazine acted as electron donor for CoB-S-S-CoM reduction as catalyzed by the heterodisulfide reductase. The specific activity was 14–16 U/mg protein at 37°C and 60–70 U/mg protein at 60°C. The partial reactions could be combined in the presence of both enzymes. Under these conditions reduced 2-hydroxyphenazine was rapidly oxidized by the heterodisulfide reductase thereby producing the electron acceptor for the F₄₂₀H₂ dehydrogenase. Above a concentration of 50 μM of 2-hydroxyphenazine, the specific activity of the latter enzyme reached the V _max value. When other phenazines or quinone derivatives were used as electron carriers, the activity of F₄₂₀H₂-dependent CoB-S-S-CoM reduction was much lower than the rate obtained with 2-hydroxyphenazine. Thus, this water-soluble analogue of methanophenazine best mimics the natural electron acceptor methanophenazine in aqueous systems.