Theobromine synthases

Figure 4 Caffeine biosynthesis. XMT, xanthosine N-methyltransferase (also called 7-methylxanthosine synthase) XN, methyixanthosine nucleotidase MXMT, 7-methylxanthine-N-methyltransferase (also called theobromine synthase) DXMT, dimethyIxanthine-N-methyltransferase (also called caffeine synthase).

The biosynthesis of caffeine begins with the methylation of xanthosine to yield N-methylxanthosine by the enzyme xanthosine N-methyltransferase (XMT) (also called 7-methy-Ixanthosine synthase) (171-173). N-methylxanthosine is converted to N-methylxanthine by methylxanthine nucleosidase, an enzyme that has not been cloned yet (174). N-methylxanthine is converted to theobromine by 7-methylxanthine-N-methyl-transferase (MXMT) (also called theobromine synthase), a second N-methyltransferase (171, 175). Theobromine is converted to caffeine by a final N-methyltransferase, dimethylxanthine-N-methyltransferase (DXMT) (also called caffeine synthase) (171). 

Fig. 8.8 Biosynthetic pathway for tropane and purine alkaloids, (a) Tiopane alkaloid ODC ornithine decarboxylase, PMT putrescine Ai-methyltransferase). (b) Purine alkaloid XMT xanthosine W-methyltransferase/7-methylxanthosine synthase, XN 7-methylxanthosine nucleotidase, MXMT 7-methylxanthine Ai-methyltransferase/theobromine synthase, DXMT dimethyl-xanthine Ai-methyltransferase/caffeine synthase) (Adopted from Ref. [15])...

Fig. 29.1 The biosynthetic pathways of caffeine from xanthosine. The major pathway that consists of four steps is shown with solid arrows. Conversion of 7-methylxanthosine to 7-methylxanthine is catalyzed by nucleosidase activity involved in 7-methylxanthosme synthase or a specific Af-methyl nncleosidase. Minor pathways, shown with dotted arrows, are theoretically possible. They may occur because of the broad substrate specificity of caffeine synthase or other Ai-methyltransferases. Enzymes 7mXS 7-methylxanthosine synthase, CS caffeine synthase, TS theobromine synthase, SAM 5-adenosyl-L-methionine, SAH S-adenosyl-L-homocysteine...

Coffee and tea plants seem to contain a variety of N-methyltransferase enzymes that have varying substrate specificity (168, 169). For example, a caffeine synthase enzyme isolated from tea leaves catalyzes both the N-methylation of N-methylxanthine and theobromine (176). The substrate specificity of the methyltransferases can be changed by site-directed mutagenesis (177), and the crystal structure of two of the N-methyltransferases has been reported recently (178). 

Scheme 4. Biosynthesis of the purine alkaloids caffeine and theobromine. Molecular clones have been isolated for all enzymes shown. Abbreviations CS, caffeine synthase MXN, 7-methyIxanthosine nucleosidase MXS, 7-methyIxanthosine synthase.

Most recently, genetic engineering opened the opportunity to knock-out caffeine-synthase and eliminate both of the last steps in the biosynthesis, methyla-tion of 7-methylxanthine and theobromine respectively. The goal is to cultivate thereby caffeine-free tea and coffee, without affecting their high polyphenol content as current decaffeination processes do. [511, 512]... 

---