Methanosarcina acetivorans requires methanol:coenzyme M methyltransferases for ethane formation from ethanol

Corrinoid-dependent enzymes either catalyze methyltransfer reactions, or they generate substrate radicals using adenosylcobalamin for subsequent rearrangement reactions. The corrinoid-dependent methyltransferases are present in all domains of life and assumed to be exclusive for methyl-groups. In Methanosarcina, however, trace ethane production from ethanol has been shown in vivo, which led to the hypothesis that corrinoid-dependent methanol-specific methyltransferases are promiscuous towards also accepting ethyl-groups. Here, we show that the conversion of ethanol to trace amounts of ethane in Methanosarcina acetivorans involves homologous reactions of the known methanol-to-methane metabolism. The methanol methyltransferase (MtaB) activates ethanol and loads the ethyl-group onto the corrinoid-containing methyl-accepting protein (MtaC). Besides MtaCB, substrate promiscuity in corrinoid:coenzyme M methyltransferase (MtaA) and methyl-coenzyme M reductase (Mcr) are required to grant the microbe the capacity for ethane production. We show that the MtaCB subunits of M. acetivorans can activate ethanol, however, the ethane yields compared to methane are ca. 3 orders of magnitude lower. The ethyl-transfer capability was confirmed for each of the three MtaCB isozyme by quantifying the amount of ethane produced by mtaCB double deletion strains during growth in ethanol-supplemented media and in resting-cell suspensions. Ethane formation requires the cells to be grown on methanol to trigger the expression of the mtaCB genes, and detectable ethane formation starts only after all methanol has been consumed. Demonstrating that corrinoid-dependent methanol-specific methyltransferases process ethyl groups extends the pool of reactions to be considered in metabolic networks and suggests possible routes for biogenic ethane in nature.