The biosynthesis of methylated amino acids in the active site region of methyl-coenzyme M reductase

The global production of the greenhouse gas methane by methanogenic archaea reaches 1 billion tons per annum. The final reaction releasing methane is catalyzed by the enzyme methyl-coenzyme M reductase, The crystal structure of methyl-coenzyme RI reductase from Methanobacterium thermoautotrophicum r evealed the presence of five modified amino acids within the alpha-sub-unit and near the active site region. Four of these modifications were C-, N-, and S-methylations, two of which, 2-(S)-methylglutamine and 5-(S)-methylarg inine, have never been encountered before. We have now confirmed these modi fications by mass spectrometry of chymotryptic peptides, With methyl-coenzy me M reductase purified from cells grown in the presence of L-[methyl-D-3]m ethionine, it was shown that the methyl groups of the modified amino acids are derived from the methyl group of methionine rather than from methyl-coe nzyme RI, an intermediate in methane formation. The D-3 labeling pattern wa s found to be qualitatively and quantitatively the same as in the two methy l groups of the methanogenic coenzyme F-430, which are known to be introduc ed via S-adenosylmethionine. From the results, it is concluded that the met hyl groups of the modified amino acids in methyl-coenzyme RI reductase are biosynthetically introduced by an S-adenosylmethionine-dependent post-trans lational modification. A mechanism for the methylation of glutamine at C-2 and of arginine at C-5 is discussed.