Thiamine-dependent Decarboxylation Reactions in Methanogenic Coenzyme M Biosynthesis David E. Graham, The University of Texas at Austin Chemistry and Biochemistry Department. Methanogens produce more than 400 Tg of methane each year, corresponding to 80% of global methane production from all sources. This methane is a valuable energy source, as well as a greenhouse gas. To develop new ways of controlling methanogen- esis, we are elucidating the biosynthetic pathways for Coenzyme M (CoM; 2-mercaptoethane-sulfonate). CoM is the terminal methyl carrier in all methanogens, so treatments that block the biosynthesis of CoM or CoB could be selective inhibitors of methanogenesis. ESI-MS analysis of reaction products identified cysteate in the reaction mixture with cysteate synthase. We have identified a new enzyme in the microorganism Methanosarcina acetivorans that allows this microbe to make CoM by a different pathway from most methanogens. In this scheme, cysteate synthase catalyzes the elimination of phosphate from phosphoserine, and the resulting dehydroalanine is sulfonated to form L-cysteate. Transamination produces sulfopyruvate. Sulfopyruvate decarboxylase (SPyDC) is a thiamine-dependent enzyme that catalyzes the penultimate step in CoM biosynthesis. We developed a new expression system and analytical techniques to study this oxygen-sensitive protein and identify new inhibitors.