Structural modifications and kinetic studies of the substrates involved in the final step of methane formation in Methanobacterium thermoautotrophicum.

ABSTRACT

The 2-(methylthio)ethanesulfonic acid (CH3-S-CoM) reductase catalyzes the final methane-yielding reaction in fastidiously anaerobic methanogenic archaebacteria. This step involves the reductive demethylation of CH3-S-CoM with reducing equivalents from N-7-(mercaptoheptanoyl)-L-threonine O3-phosphate (HS-HTP) to yield methane and the nonsymmetrical disulfide of 2-mercaptoethanesulfonic acid and HS-HTP. We chemically synthesized modified analogs of CH3-S-CoM (which has two carbons in the ethylene bridge) and of HS-HTP (which has seven carbons in the side chain); analog pairs possessed an overall correct number of side chain carbons (i.e., a total of nine in combination). They were simultaneously added to anaerobic cell extracts of Methanobacterium thermoautotrophicum delta H. The ability of the extracts to reductively demethylate the modified substrates was tested by gas chromatography. We also describe here previously unknown inhibitors of methanogenesis, 6-(methylthio)hexanoyl-L-threonine O3-phosphate (a structural analog of HS-HTP) and sodium bromomethanesulfonic acid (a structural analog of CH3-S-CoM). Both analogs were found to be effective competitive inhibitors with respect to HS-HTP. These substrate analogs were also found to inhibit a recently described photoactivation of homogeneous inactive reductase (K. D. Olson, C. W. McMahon, and R. S. Wolfe, Proc. Natl. Acad. Sci. USA 884099-4103, 1991). In addition, we probed the mechanism of action of a potent inhibitor of the enzyme, 2-bromoethanesulfonic acid, a structural analog of CH3-S-CoM.