The F₄₂₀H₂:heterodisulfide oxidoreductase system from Methanosarcina species

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 Vmax 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 degrees C and 60-70 U/mg protein at 60 degrees 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 microM of 2-hydroxyphenazine, the specific activity of the latter enzyme reached the Vmax 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.