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Published on April 13, 2007; 10.1104/pp.106.094854

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Received December 15, 2006
Accepted April 5, 2007

The Structure of Two N-methyltransferases from the Caffeine Biosynthetic Pathway

Andrew A. McCarthy * and James G. McCarthy

European Molecular Biology Laboratory, 6 rue Jules Horowitz, BP 181, Grenoble 38042, France; Nestlé Research and Development, 101 Avenue Gustave Eiffel, Notre-Dame D'OE, BP 49416, Tours 37097, France

* Corresponding author; email: andrewmc{at}embl.fr.

Caffeine (1,3,7-trimethylxanthine) is a secondary metabolite produced by certain plant species and an important component of coffee and tea. Here we describe the structures of two S-adenosyl-L-methione (SAM) dependant N-methyltransferases that mediate caffeine biosynthesis in Coffea canephora (robusta), xanthosine methyltransferase (XMT) and 1,7 dimethylxanthine methyltransferase (DXMT). Both were co-crystallized with the demethylated cofactor, S-adenosyl-L-cysteine (SAH), and substrate, either xanthosine (XMT) or theobromine (DXMT). Our structures reveal several elements that appear critical for substrate selectivity. S316 in XMT appears central to the recognition of xanthosine. Likewise, a change from Q161 in XMT to H160 in DXMT is likely to have catalytic consequences. A F266 to I266 change in DXMT is also likely to be crucial for the discrimination between mono and dimethyl transferases in coffee. These key residues are probably functionally important and will guide future studies with implications for the biosynthesis of caffeine and its derivatives in plants. Finally, we propose an enzymatic mechanism whereby XMT could potentially generate 7-methylxanthine from xanthosine.




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