Figure 1.

Pathways for the biosynthesis of caffeine. Solid arrows indicate major biosynthesis routes; dotted arrows indicate minor pathways. A XMP → 7-methyl XMP → 7-methylxanthosine pathway is operative in coffee leaves but not in tea leaves. Xanthine is converted to purine alkaloid via a minor route; it is also the entry point in the purine catabolism pathway (based on data reviewed by Ashihara and Crozier [1999]).