Catalytic properties of the heterodisulfide reductase involved in the final step of methanogenesis

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Abstract
Reduction of the heterodisulfide of coenzyme M (H‐S‐CoM) and 7‐mercaptoheptanoyl(L)threonine phosphate (H‐S‐HTP) is a partial reaction in methanogenesis. The CoM‐S‐S‐HTP reductase mediating this reaction has thus far not been studied. We report here that the enzyme from Methanobacterium thermoautotrophicum and Methanosarcina barkeri catalyzes the reduction of CoM‐S‐S‐HTP with reduced viologen dyes and, in the reverse direction, the oxidation of H‐S‐CoM plus H‐S‐HTP to the heterodisulfide by methylene blue. The CoM‐S‐S‐HTP reductase from M. thermoautotrophicum (strain Marburg) was partially purified (30‐fold) to a specific activity of 10 μmol·min−1·mg protein−1. The enzyme was highly substrate specific: e.g. neither the heterodisulfide derived from 6‐mercaptohexanoylthreonine phosphate nor the homodisulfide of H‐SCoM or of HSHTP was reduced. The D‐enantiomer of CoM‐S‐S‐HTP was, however, converted at 35% of the specific rate of the L‐form. Apparent Km and apparent Vmax values for substrates and products were determined.