Abstract
Corrinoid-dependent methyltransferases catalyze methyl-group transfer reactions in all domains of life. These enzymes are generally considered exclusive for C1-substrates (methyl-groups). However, in Methanosarcina trace ethane production from ethanol has been demonstrated 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 proceeds via the known methanol-to-methane metabolism, involving the methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MtaB) and a corrinoid-containing methyl-accepting protein (MtaC), but via transfer of ethyl groups instead of methyl groups. We demonstrate that all three isozymes of the methanol specific MtaB subunit and the corrinoid protein MtaC of M. acetivorans are promiscuous towards accepting ethanol, granting the microbe capacity of ethane production via promiscuity downstream in Co-methyl-5-hydroxybenzimidazolylcobamide:2-mercaptoethanesulfonate methyltransferase (MtaA) and methyl-coenzyme M reductase (Mcr). We assessed the ethyl-group transfer efficiency of each of the three isozymes and engineered chimeras that combine 2 different MtaA subunits with the 3 isoforms of MtaCB together to increase the ethane production capability of M. acetivorans. Demonstrating that corrinoid-dependent coenzyme M methyltransferases can catalyze transfer of higher alkyl groups extends the pool of reactions to be considered in metabolic networks.