Catechins, possessing a 2-phenylbenzodihydropyran structure, belong to the flavanol class of compounds. The biosynthesis of catechins in tea plants mainly consists of four parts: the shikimic acid pathway, the phenylpropane pathway, the synthesis of non-ester catechins, and the synthesis of ester catechins. In the shikimic acid pathway, erythrose 4-phosphate (E4P) produced by the pentose phosphate pathway and phosphoenolpyruvate (PEP) produced by the glycolysis pathway are synthesized into 3-deoxy-D-7-phosphoarabinoheptanulose synthase (DAHPS) to produce 3-deoxy-D-7-phosphoarabinoheptanulose acid (DHAP). Subsequently, under the action of other enzymes, a series of reactions yield aryl amino acids, vitamins, lignin, phenolic substances, alkaloids, and other products. Among these, phenylalanine, under the action of phenylalanine ammonia-lyase (PAL), enters the phenylpropane pathway to synthesize p-coumaroyl-CoA. One molecule of coumaroyl-CoA and three molecules of malonyl-CoA synthesize naringenin chalcone under the action of chalcone synthase (CHS), and then cyclize under the action of chalcone isomerase (CHI) to give naringenin. Chalcone is an important precursor for the synthesis of compounds such as catechins and anthocyanins, and is the basic molecular skeleton of catechin compounds. This O1 and C2 linkage to form a pyran ring provides important information for the chemical synthesis of catechins. Naringenin introduces one and two hydroxyl groups into the B ring under the action of flavonoid 3'-hydroxylase (F3'H) and flavonoid 3',5'-hydroxylase (5'H) to give flavanones, which are then reduced by flavanone 3-hydroxylase (F3H) and dihydroflavanol 4-reductase (DFR) to synthesize colorless anthocyanins. Colorless anthocyanins can be directly reduced by reductase (LAR) to non-phenotypic catechins, namely catechins and GC, or can be continuously catalyzed by anthocyanin synthase (ANS) and anthocyanin reductase (ANR) to produce phenotypic catechins EC and EGC; isotope labeling experiments have found that there may also be an interconversion mechanism between EC and EGC.
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