MtcB, a member of the MttB superfamily from the human gut acetogenEubacterium limosum, is a cobalamin-dependent carnitine demethylase

The trimethylamine methyltransferase MttB is the first described member of a superfamily comprising thousands of microbial proteins. Most members of the MttB superfamily are encoded by genes that lack the codon for pyrrolysine characteristic of trimethylamine methyltransferases, raising questions about the activities of these proteins. The superfamily member MtcB is found in the human intestinal isolateEubacterium limosumATCC 8486, an acetogen that can grow by demethylation ofl-carnitine. Here, we demonstrate that MtcB catalyzesl-carnitine demethylation. When growing onl-carnitine,E. limosumexcreted the unusual biological product norcarnitine as well as acetate, butyrate, and caproate. Cellular extracts ofE. limosumgrown onl-carnitine, but not lactate, methylated cob-(I)alamin or tetrahydrofolate usingl-carnitine as methyl donor. MtcB, along with the corrinoid protein MtqC and the methylcorrinoid:tetrahydrofolate methyltransferase MtqA, were much more abundant inE. limosumcells grown onl-carnitine than on lactate. Recombinant MtcB methylates either cob(I)alamin or Co(I)-MtqC in the presence ofl-carnitine and, to a much lesser extent, gamma-butyrobetaine. Other quaternary amines were not substrates. Recombinant MtcB, MtqC, and MtqA methylated tetrahydrofolate vial-carnitine, forming a key intermediate in the acetogenic Wood-Ljungdahl pathway. To our knowledge, MtcB methylation of cobalamin or Co(I)-MtqC represents the first described mechanism of biologicall-carnitine demethylation. The conversion ofl-carnitine and its derivative gamma-butyrobetaine to trimethylamine by the gut microbiome has been linked to cardiovascular disease. The activities of MtcB and related proteins inE. limosummight demethylate proatherogenic quaternary amines and contribute to the perceived health benefits of this human gut symbiont.