Chlorogenic acid synthase

Caffeine (Fig. 11.3), widely used by humans as a stimulatory drug, has so far been detected only in a few plant species. The biological roles of caffeine are believed to be in defense against herbivory or as an allelopathic response to potential competitors.83 Caffeine is derived from the purine alkaloid xanthosine. From xanthosine, three methylations are necessary to produce caffeine. First, xanthosine is methylated on N7 by 7-methylxanthosine synthase (MXS or 7NMT) to produce 7-methylxanthosine, which is enzymatically hydrolyzed to produce 7-methylxanthine and ribose.85,86 The methylations of N1 and N3 of 7-methylxanthine to produce 1,3,7-trimethylxanthine (caffeine) occur in young leaves of tea, and the same enzyme, caffeine synthase, apparently catalyzes both reactions.55 In coffee plants, caffeine is mainly found in the beans but also occurs in the leaves. Caffeine is stored in the vacuoles of coffee leaves as a complex with polyphenols such as chlorogenic acid.87 In contrast to tea, coffee plants appear to have separate enzymes for each step of N-methylation.57... 

Flax Petunia hybrid/ overexpression CaMV35S/CHS + CHI + DFR Chalcone synthase, chalcone isomerase, and dihydroflavonol 4-reductase Chalcones, flavanones, chlorogenic acid Higher in flavonoids (kaempferol), phenolic acids (coumaric, ferulic, synaptic acids) in seedcake extracts [65]... 

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). 

Phenolic compounds include a wide range of secondary metabolites that are biosynthesised from carbohydrates through the shikimate pathway [14]. This is the biosynthetic route to the aromatic amino acids, phenylalanine, tyrosine, and tryptophan, and only occurs in microorganisms and plants. In the first step, the glycolytic intermediate phosphoenol pyruvate and the pentose phosphate intermediate erythrose-4-phosphate are condensed to 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP), a step catalysed by DAHP synthase. Intermediates of the shikimate pathway are 3-dehydroquinate, shikimate, and chorismate (Fig. 1). Phenylalanine is biosynthesised from chorismate, and from phenylalanine all the phenylpropanoids. Quinate is produced from 3-dehydroquinate and incorporated into chlorogenic and isochlorogenic acids (caffeoyl quinic acids) by combination with caffeic acid. Gallic acid is produced from shikimate. 

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