It was expressed as fmol of [H]-methyl transferred from radioactively labelled DNA to protein per mg of total cell extract protein.
This would be followed by methyl transfer from the second SAM, producing MeSiA37.
The major difference between MetH and class B RSMTs is the predicted mechanism of methyl transfer from MeCbl to the substrate.
Integration of the intact methyl group of l-[methyl-H3]methionine into the pyridinol moiety suggests that methyl transfer from methionine is catalyzed by a SAM-dependent methyltransferase, as opposed to a radical SAM enzyme.
However, recent work shows that methyl transfer from SAM to the enzyme does not depend on Ado radical formation and can proceed in the absence of substrate tRNA suggesting that sulfur insertion may not precede methyl transfer.
The reaction proceeds by methyl transfer from 5-methyltetrahydrofolate to MS-bound cob(I alamin to form MeCbl, followed by transfer of the methyl group to homocysteine to form methionine and regeneration of cob(I alamin (Refs 109, 110).
MetH catalyzes methyl transfer from methyltetrahydrofolate (Me-FH4) to homocysteine, in which the cobalamin cofactor cycles between MeCbl and cob(I alamin forms, as it is alternately demethylated by homocysteine and remethylated by Me-FH4.
This is due to the key role of cobalt in the direct conversion of methanol to methane, as it forms the metallocenter of corrinoid compounds that catalyze the methyl transfer from methanol to methyl-S-CoM (Thauer 1998; Fermoso et al. 2008).
These results indicate that the initial step of SN2 methyl transfer from SAM to the sulfane sulfur site at the auxiliary [4Fe 4S] cluster is rate-limiting in comparison to the ensuing more rapid radical-dependent methylthio transfer from cluster to substrate.
PhpK, isolated from Kitasatospora phosalacinea, is a P-methyltransferase that carries out methyl transfer from methylcobalamin to 2-acetylamino-4-hydroxyphosphinylbutanoate (N-acetyldemethylphosphinothricin, NAcDMPT) to form the only known carbon phosphorus carbon linkage to occur in nature, 2-acetylamino-4-hydroxymethylphosphinylbutanoate (N-acetylphosphinothricin, NAcPT).
Cystathionine γ-synthase and -β-lyase likely provide homocysteine as an acceptor for methyl transfer from S-methylmethionine, catalyzed by homocysteine S-methyltransferase [ 55], while Met synthase is essential to recycle homocysteine produced in the S-adenosylmethionine cycle, which appeared highly active according to the high representation of the Met adenosyltransferase activity category.
