Methane formation from methyl-coenzyme M in a system containing methyl-coenzyme M reductase, component B and reduced cobalamin

Abstract

Purified methyl‐CoM reductase of Methanobacterium thermoautotrophicum was found to catalyze the reduction of methyl‐coenzyme M to methane at a specific rate of 100 nmol × min⁻¹× mg protein⁻¹ in a system containing partially purified component B, cob(III)alamin (B‐12a), and, as electron donors, dithiothreitol or SnCl₂. Under these experimental conditions B‐12a was reduced to cob(II)alamin (B‐12r), which is known to disproportionate to cob(I)alamin (B‐12s) and B‐12a to a slight extent. Methane formation from methyl‐CoM was inhibited by propyl iodide, methyl iodide, chloroform and carbon tetrachloride, which were shown to react with B‐12s to the corresponding light‐sensitive alkyl cobalamin. Inhibition by propyl iodide was less effective in light‐exposed samples. From these findings it is concluded that in this assay system B‐12s might serve as electron donor for the enzymatic reduction of methyl‐CoM to methane.