7-methylxanthosine synthase () is an enzyme that catalyzes the chemical reaction
This is a methylation reaction in which the nucleoside, xanthosine, is converted to 7-methylxanthosine. The methyl group comes from the cofactor, S-adenosyl methionine (SAM), which becomes S-adenosyl-L-homocysteine (SAH). The enzyme was characterised from Coffea arabica.
This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:xanthosine N7-methyltransferase. Other names in common use include xanthosine methyltransferase, XMT, xanthosine:S-adenosyl-L-methionine methyltransferase, CtCS1, CmXRS1, CaXMT1, and S-adenosyl-L-methionine:xanthosine 7-N-methyltransferase. This reaction is the first methylation step in the biosynthesis of caffeine and theobromine.
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