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

Abstract

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 isolate Eubacterium limosum ATCC 8486, an acetogen that can grow by demethylation of L-carnitine. Here, we demonstrate that MtcB catalyzes L-carnitine demethylation. When growing on L-carnitine, E. limosum excreted the unusual biological product norcarnitine as well as acetate, butyrate, and caproate. Cellular extracts of E. limosum grown on L-carnitine, but not lactate, methylated cob(I)alamin or tetrahydrofolate using L-carnitine as methyl donor. MtcB, along with the corrinoid protein MtqC, and the methyl-corrinoid:tetrahydrofolate methyltransferase MtqA were much more abundant in E. limosum cells grown on L-carnitine than on lactate. Recombinant MtcB methylates either cob(I)alamin or Co(I)-MtqC in the presence of L-carnitine, and to a much lesser extent, γ-butyrobetaine. Other quaternary amines were not substrates. Recombinant MtcB, MtqC, and MtqA methylated tetrahydrofolate via L-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 biological L-carnitine demethylation. The conversion of L-carnitine and its derivative γ-butyrobetaine to trimethylamine by the gut microbiome has been linked to cardiovascular disease. The activities of MtcB and related proteins in E. limosum might demethylate proatherogenic quaternary amines and contribute to the perceived health benefits of this human gut symbiont.

The MttB superfamily member MtyB from the human gut symbiont Eubacterium limosum is a cobalamin-dependent γ-butyrobetaine methyltransferase

“…E. limosum has been previously demonstrated to demethylate several QAs (8,9,46), but not the proatherogenic QA, -butyrobetaine. We found that when E. limosum ATCC 8486 is cultured with butyrobetaine (Fig.…”

Colon Carcinogenesis: The Interplay Between Diet and Gut Microbiota

“…There are evidences implying the role of gut microbiota as a mediator of the association between diet, TMAO, and diseases, with alteration in the composition or structure of intestinal microbiota influencing TMAO production and hence, affecting the risk of colonic lesions ( Koeth et al., 2013 ; Tang et al., 2013 ; Romano et al., 2015 ; Cho et al., 2017 ; Shi et al., 2018 ; Chan et al., 2019 ). Of note, although most of the intestinal microbiota promote TMAO production from its precursors , Eubacterium limosum was shown to be able to metabolize TMA precursors through carnitine demethylation to form a product that cannot be readily converted into TMA, revealing the potential of this gut bacterium in reducing TMAO level in the gut ( Kountz et al., 2020 ).…”

Acetobase: database update and reanalysis of formyltetrahydrofolate synthetase amplicon sequencing data from the anaerobic digesters

“…In these environments, they are not only involved in the carbon cycling via acetate but also extensively involved in the metabolism of ethanol, methanol, formate, butyrate, lactate, vanillate, chlorinated compounds, cellulose, mono-tetra saccharides, methylated amines etc. (Wolin and Miller, 1994;Das and Ljungdahl, 2003;Lovell and Leaphart, 2005;Rey et al, 2010;Hügler and Sievert, 2011;Wen et al, 2015;Lechtenfeld et al, 2018;Yang, 2018;Picking et al, 2019;Kountz et al, 2020).…”