H-2 - HETERODISULFIDE OXIDOREDUCTASE, A 2ND ENERGY-CONSERVING SYSTEM IN THE METHANOGENIC STRAIN GO1

Washed everted vesicles of the methanogenic bacterium strain Go1 catalyzed an H-2-dependent reduction of the heterodisulfide of HS-CoM (2-mercapto-ethanesulfonate) and HS-HTP (7-mercaptoheptanoyl-threonine 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 transmembrane electrochemical gradient thereby generated was shown to induce ATP synthesis from ADP + P(i), 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 F420H-2-dependent but no H-2-dependent heterodisulfide oxidoreductase activity. Neither of these activities was detectable in membranes of Methanococcus thermolithotrophicus.