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

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 K m and apparent V max values for substrates and products were determined.