Membrane-bound F420H2-dependent heterodisulfide reduction in Methanococcus voltae

Washed membranes prepared fr om H-2+CO2- Or formate-grown cells of Methanococcus voltae catalyzed the oxidation of coenzyme F420H2 and the reduction of the heterodisulfide (CoB-S-S-CoM) of 2-mercaptoethanesulfonate and 7-mercaptoheptanoylthreonine phosphate, which is the terminal electron acceptor of the methanogenic pathway. The reaction followed a 1:1 stoichiometry according to the equation: F420H2 + COB-S-S-CoM --> F-420 + CoM-SH + CoB-SH. These findings indicate that the reaction depends on a membrane-bound F420H2-oxidizing enzyme and on the heterodisulfide reductase, which remains partly membrane-bound after cell lysis. To elucidate the nature of the F420H2-oxidizing protein, washed membranes were solubilized with detergent, and the enzyme was purified by sucrose density centrifugation, anion-exchange chromatography, and gel filtration. Several lines of evidence indicate that F420H2 oxidation is catalyzed by a membrane-associated F-420-reducing hydrogenase. The purified protein catalyzed the H-2-dependent reduction of methyl viologen and F-420. The apparent molecular mass and the subunit composition (43, 37, and 27 kDa) are almost identical to those of the F-420-reducing hydrogenase that has already been purified from Me. voltae. Moreover, the N-terminus of the 37-kDa subunit is identical to the amino acid sequence deduced from the fruG gene of the operon encoding the selenium-containing F-420-reducing hydrogenase from Mc. voltae. A distinct F420H2 dehydrogenase, which is present in methylotrophic methanogens, was not found in this organism.