The final step in methane formation

Abstract

Methyl‐coenzyme M reductase (= component C) from Methanobacterium thermoautotrophicum (strain Marburg) was highly purified via anaerobic fast protein liquid chromatography on columns of Mono Q and Superose 6. The enzyme was found to catalyze the reduction of methylcoenzyme M (CH 3 ‐S‐CoM) with N ‐7‐mercaptoheptanoylthreonine phosphate (H‐S‐HTP = component B) to CH 4 . The mixed disulfide of H‐S‐CoM and H‐S‐HTP (CoM‐S‐S‐HTP) was the other major product formed. The specific activity was up to 75 nmol min −1 mg protein −1 . In the presence of dithiothreitol and of reduced corrinoids or titanium(III) citrate the specific rate of CH 3 ‐S‐CoM reduction to CH 4 with H‐S‐HTP increased to 0.5–2 μmol min −1 mg protein −1 . Under these conditions the CoM‐S‐S‐HTP formed from CH 3 ‐S‐CoM and H‐S‐HTP was completely reduced to H‐S‐CoM and H‐S‐HTP. Methyl‐CoM reductase was specific for H‐S‐HTP as electron donor. Neither N ‐6‐mercaptohexanoylthreonine phosphate (H‐S‐HxoTP) nor N ‐8‐mercaptooctanoylthreonine phosphate (H‐S‐OcoTP) nor any other thiol compound could substitute for H‐S‐HTP. On the contrary, H‐S‐HxoTP (apparent K i =0.1 μM) and H‐S‐OcoTP (apparent K i = 15 μM) were found to be effectives inhibitors of methyl‐CoM reductase, inhibition being non‐competitive with CH 3 ‐S‐CoM and competitive with H‐S‐HTP.