The MtsA subunit of the methylthiol:coenzyme M methyltransferase of Methanosarcina barkeri catalyses both half-reactions of corrinoid-dependent dimethylsulfide: coenzyme M methyl transfer.
Methanogenesis from dimethylsulfide requires the intermediate methylation of coenzyme M. This reaction is catalyzed by a methylthiol:coenzyme M methyltransferase composed of two polypeptides, MtsA (a methylcobalamin:coenzyme M methyltransferase) and MtsB (homologous to a class of corrinoid proteins involved in methanogenesis). Recombinant MtsA was purified and found to be a homodimer that bound one zinc atom per polypeptide, but no corrinoid cofactor. MtsA is an active methylcobalamin:coenzyme M methyltransferase, but also methylates cob(I)alamin with dimethylsulfide, yielding equimolar methylcobalamin and methanethiol in an endergonic reaction with a K(eq) of 5 x 10(-)(4). MtsA and cob(I)alamin mediate dimethylsulfide:coenzyme M methyl transfer in the complete absence of MtsB. Dimethylsulfide inhibited methylcobalamin:coenzyme methyl transfer by MtsA. Inhibition by dimethylsulfide was mixed with respect to methylcobalamin, but competitive with coenzyme M. MtbA, a MtsA homolog participating in coenzyme M methylation with methylamines, was not inhibited by dimethylsulfide and did not catalyze detectable dimethylsulfide:cob(I)alamin methyl transfer. These results are most consistent with a model for the native methylthiol:coenzyme M methyltransferase in which MtsA mediates the methylation of corrinoid bound to MtsB with dimethylsulfide and subsequently demethylates MtsB-bound corrinoid with coenzyme M, possibly employing elements of the same methyltransferase active site for both reactions.
Tallant
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TC
Department of Microbiology, Ohio State University, Columbus, Ohio 43210, USA.
Paul
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Krzycki
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JA
D019843:Archaeal Proteins; D001483:Base Sequence; D002384:Catalysis; D017931:DNA Primers; D007700:Kinetics; D017021:Methanosarcina barkeri / Q000201:enzymology*; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry / Q000378:metabolism*; D013440:Sulfides / Q000378:metabolism*; D014805:Vitamin B 12 / Q000378:metabolism*
D019843:Archaeal Proteins; D017931:DNA Primers; D013440:Sulfides; D008780:Methyltransferases; C023894:methylcobalamin-coenzyme M methyltransferase; D014805:Vitamin B 12