CHAPTER 4: Coffee Plant Biochemistry

Primary and secondary metabolism in Coffea plants is reviewed. After a brief introduction of carbohydrate and nitrogen metabolism, the biosyntheses of three major secondary metabolites in coffee, caffeine, trigonelline and chlorogenic acids, are reviewed. Caffeine is a purine alkaloid and produced from purine nucleotides with the four-step biosynthetic pathway from xanthosine being catalyzed by three enzymes belonging to the SABATH family of N-methyltransferases. Catabolism of caffeine occurs in only a limited number of Coffea species. Trigonelline is a pyridine alkaloid produced from a pyridine nucleotide, NAD. Nicotinic acid, an intermediate of the pyridine cycle, serves as the substrate for trigonelline synthase. This enzyme is also a SABATH family N-methyltransferase and very similar to the enzymes operating in the caffeine biosynthetic pathway. Biosynthesis of chlorogenic acids begins with the core phenylpropanoid pathway. The route to 5-O-caffeoylquinic acid (5-CQA) from p-coumaroyl-CoA in Coffea plants is p-coumaroyl-CoA → 5-O-p-coumaroylshikimic acid → 5-O-caffeoylshikimic acid → caffeoyl-CoA → 5-CQA. Occurrence and plant physiological aspects of the metabolism of the three secondary metabolites are also described.