Methylamine-specific methyltransferase paralogs in Methanosarcina are functionally distinct despite frequent gene conversion

摘要

Methylamine-mediated methanogenic metabolism in Methanosarcina acetivorans. a A methylamine-specific methyltransferase catalyzes the first step in the methyl-transfer reactions that leads to generation of methyl-coenzyme M (CH3-CoM). For every four molecules of CH3-CoM generated, one is oxidized to CO2 in a step-wise fashion and the other three are reduced to methane (CH4) with the sulfhydrl group of coenzyme B (CoB) as the electron donor by the enzyme methyl-coenzyme M reductase (MCR). The heterodisulfide (CoM-S-S-CoB) produced by MCR is reduced to regenerate CoM and CoB by heterodisulfide reductase (HDR) using reducing equivalents obtained from the oxidation of CH3-CoM to CO2. b The methyl-transfer reaction from methylamine to CH3-CoM involves two methyltransferases. The first methyltransferase (MT1) is substrate-specific and comprises of a heterodimer of a methyltransferase protein (MtmB) and a corrinoid-binding protein (MtmC). MT1 catalyzes the transfer of the methyl group from methylamine to the corrinoid protein (MtmC). This reaction releases free ammonium ions (NH4+). The second methyltransferase (MT2) catalyzes the methyl-transfer reaction from the corrinoid protein (MtmC) to CoM to generate CH3-CoM. cMethanosarcina acetivorans encodes two copies of the methylamine-specific MT1 operon in its chromosome. The operon in green (mtmC1B1) is found in the vicinity of the methylamine-specific MT2 gene (mtbA1) and a methylated amine-specific permease gene (mttP). The operon in orange (mtmC2B2) is encoded more than 2 Mb away on the chromosome next to genes encoding components of the NH4+ assimilation pathway