H2: heterodisulfide oxidoreductase, a second energy-conserving system in the methanogenic strain G�1

Abstract

Washed everted vesicles of the methanogenic bacterium strain Go1 catalyzed an H2-dependent reduction of the heterodisulfide of HS-CoM (2-mercaptoethanesulfonate) and HS-HTP (7-mercaptoheptanoylthreonine phosphate) (CoM-S-S-HTP). This process was independent of coenzyme F420 and was coupled to proton translocation across the cytoplasmic membrane into the lumen of the everted vesicles. The maximal H+/CoM-S-S-HTP ratio was 2. The tranmembrane electrochemical gradient thereby generated was shown to induce ATP synthesis from ADP+Pi, exhibiting a stoichiometry of 1 ATP synthesized per 2 CoM-S-S-HTP reduced (H+/ATP=4). ATP formation was inhibited by the uncoupler 3,5-di-tert-butyl-4-hydroxy-benzylidene-malononitrile (SF 6847) and by the ATP synthase inhibitor N,N′-dicyclohexylcarbodiimide (DCCD). This energy-conserving system showed a stringent coupling. The addition of HS-CoM and HS-HTP at 1 mM each decreased the heterodisulfide reductase activity to 50% of the control. Membranes from Methanolobus tindarius showed F420H2-dependent but no H2-dependent heterodisulfide oxidoreductase activity. Neither of these activities was detectable in membranes of Methanococcus thermolithotrophicus.