Coffee caffeine biosynthesis

While caffeine biosynthesis in coffee and tea has been reasonably well investigated, little information is available about the biosynthetic pathways of methylxanthines in cacao. Published studies34 35 have established the presence of 7-methylxanthine and adenine in cocoa. Since both coffee and tea exhibit similar pathways where theobromine is a direct precursor for caffeine, it is reasonable to assume that a similar mechanism is possible in cacao. 

Schulthess, B.H. and Baumann, T.W., Stimulation of caffeine biosynthesis in suspension cultured coffee cells and the in-situ existence of 7-methylxanthosine, Phytochemistry, 38,1381,1995. 

Ogita, S, Uefuji, H Morimoto, M. and Sano, H. 2004. Application of RNAi to confirm theobromine as the major intermediate for caffeine biosynthesis in coffee plants with potential for construction of decaffeinated varieties. Plant Molecular Biology, 54(6) 931-941. 

The PA caffeine is produced from xanthosine via three distinct N-methylations (Fig.7.5).87 89 Partially purified enzyme extracts from tea (Camellia senensis) and coffee (Coffea arabica) were shown to exhibit all three activities, suggesting either that the A-methyltransferase steps in caffeine biosynthesis are catalyzed by a single enzyme, or by multiple enzymes with similar properties.90 However, a specific A-methyltransferase purified from coffee was active only toward 7-methylxanthine and theobromine91 An A-methyltransferase catalyzing the methylation of methylxanthines and designated caffeine synthase (CS) was purified from tea.92 CS catalyzes two consecutive methylations involved in the conversion of 7-methylxanthine to caffeine, but is inactive toward xanthosine, indicating that the first methylation proceeds via a different enzyme. Heterologous expression of the CS cDNA showed that the enzyme was active toward 7-methylxanthine, paraxanthine,... 

Caffeine, a purine alkaloid, is one of the most widely known natural products. Caffeine is ingested as a natural component of coffee, tea, and cocoa, and the impact of caffeine on human health has been studied extensively. The biosynthetic pathway of caffeine has been elucidated recently on the genetic level. Caffeine biosynthesis has been studied most widely in the plant species Coffea (coffee) and Camellia (tea) (168, 169). 

Mizuno K, Kato M, Irino F, Yoneyama N, Fujimura T, Ashihara H. The first committed step reaction of caffeine biosynthesis 7-methylxanthosine synthase is closely homologous to caffeine synthase in coffee. FEES Lett. 2003 547 56-60. 

Table 29.1 Genes of iV-methyltransferases involved in caffeine biosynthesis isolated from coffee (Coffea arabica) plants...

Caffeine synthase, the majority of SAH hydrolase activity, and parts of the adenine-salvage pathway are localized to chloroplasts. In coffee SAM synthase is confined to the cytosol and SAM synthase genes fi-om tobacco and parsley lack a transit peptide. However, SAM synthase from tea is a chloroplastic enzyme, encoded by a nuclear gene (Koshiishi et al. 2001). The proposed model for the subcellular localization of caffeine biosynthesis begins with the production of homocysteine and its conversion to methionine in the chloroplasts. Methionine is then converted to SAM in the cytosol and transported back into the chloroplast to serve as the methyl donor in caffeine biosynthesis. Purine alkaloids are stored in vacuoles where they are thought to form complexes with chlorogenic acids (Mosli-Waldhauser and Baumann 1996). 

The methylxanthines of interest are caffeine (1,3,7-trimethylxanthine), theophylline (1,3-dimethylxanthine), and theobromine (3,7-dimethylxanthine) and they occur in coffee, tea, mate, cocoa products, and cola beverages. This chapter is an introduction to their chemistry, isolation, and biosynthesis. While the class of methylxanthines is large and comprised of more members than these three, this chapter will essentially be limited to caffeine, theobromine, and theophylline. 

Several purine derivatives are found in nature, e.g. xanthine, hypoxanthine and uric acid. The pharmacologically important (CNS-stimulant) xanthine alkaloids, e.g. caffeine, theobromine and theophylline, are found in tea leaves, coffee beans and coco. The actual biosynthesis of purines involves construction of a pyrimidine ring onto a pre-formed imidazole system. 

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]... 

Camellia (Thea) sinensis (tea) and Cqffea arabica (coffee) plants. Caffeine was formed rapidly by extracts of green coffee berries, but little by more mature ones, and not at all by seedlings. Biosynthesis of caffein proceeds from 7-methylxanthosine in the presence of an active purine nucleoside phosphorylase or 7-methyl-A -nucleoside hydrolase. Methionine and 5-adenosylmethionine serve as precursors for the methyl groups of purine alkaloids. These act in the presence of methyltransferases on 7-methylxanthine (38) and theobromine (31) to produce caffeine. A pathway for the origin of these compounds in coffee and tea plants has been proposed (Suzuki et al., 1992 Waller and Dermer, 1981) (Fig. 37.10). 

Little is known about the physiological significance of caffeine formation or the role of caffeine in plants. Perhaps some of these answers will be forthcoming during the next few decades as researchers begin to probe into the metabolism (biosynthesis and catabolism) of caffeine and relate it to purine nucleotide, nucleic acid, and other types of metabolism in coffee plants. 

Ashihara, H. et al. (1996) Biosynthesis of caffeine in leaves of coffee. Plant Physiol 111,... 

---