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

The production of trimethylamine (TMA) from quaternary amines such as L-carnitine or gamma-butyrobetaine (4-(trimethylammonio)butanoate) by gut microbial enzymes has been linked to heart disease. This has led to interest in enzymes of the gut microbiome that might ameliorate net TMA production, such as members of the MttB superfamily of proteins, which can demethylate TMA (e.g., MttB) or L-carnitine (e.g., MtcB). Here, we show that the human gut acetogen Eubacterium limosum demethylates gamma-butyrobetaine and produces MtyB, a previously uncharacterized MttB superfamily member catalyzing the demethylation of gamma-butyrobetaine. Proteomic analyses of E. limosum grown on either gamma-butyrobetaine or DL-lactate were employed to identify candidate proteins underlying catabolic demethylation of the growth substrate. Three proteins were significantly elevated in abundance in gamma-butyrobetaine-grown cells: MtyB, MtqC (a corrinoid-binding protein), and MtqA (a corrinoid:tetrahydrofolate methyltransferase). Together, these proteins act as a gamma-butyrobetaine:tetrahydrofolate methyltransferase system, forming a key intermediate of acetogenesis. Recombinant MtyB acts as a gamma-butyrobetaine:MtqC methyltransferase but cannot methylate free cobalamin cofactor. MtyB is very similar to MtcB, the carnitine methyltransferase, but neither was detectable in cells grown on carnitine nor was detectable in cells grown with gamma-butyr-obetaine. Both quaternary amines are substrates for either enzyme, but kinetic analysis revealed that, in comparison to MtcB, MtyB has a lower apparent K-m for gamma-butyrobetaine and higher apparent V-max, providing a rationale for MtyB abundance in gamma-butyrobetaine-grown cells. As TMA is readily produced from gamma-butyrobetaine, organisms with MtyB-like proteins may provide a means to lower levels of TMA and proatherogenic TMA-N-oxide via precursor competition.