Flavonoid ester synthesis

The 4-coumarate CoA ligase (4CL EC 6.2.1.12) enzyme activates 4-coumaric acid, caffeic acid, ferrulic acid, and (in some cases) sinapic acid by the formation of CoA esters that serve as branch-point metabolites between the phenylpropanoid pathway and the synthesis of secondary metabolites [46, 47]. The reaction has an absolute requirement for Mg " and ATP as cofactors. Multiple isozymes are present in all plants where it has been studied, some of which have variable substrate specificities consistent with a potential role in controlling accumulation of secondary metabolite end-products. Examination of a navel orange EST database (CitEST) for flavonoid biosynthetic genes resulted in the identification of 10 tentative consensus sequences that potentially represent a multi-enzyme family [29]. Eurther biochemical characterization will be necessary to establish whether these genes have 4CL activity and, if so, whether preferential substrate usage is observed. 

Chakon synthase, CHS a plant enzyme (formerly called flavanone synthase) catalysing the synthesis of chalcones from one molecule of the CoA ester of a substituted cinnamic add and 3 molecules of malo-nyl-CoA, a key reaction in flavonoid biosynthesis (see Stilbenes, Hg., and Flavonoid, Hg.). Spedfidty varies with the source of the enzyme, e.g. CHS from Tulipa stamens and Cosmos petals forms naringenin, eriodictyol or homoeriodictyol from 4-coumaroyl-... 

The reaction is catalyzed by sulfotransferases (cf. the synthesis of the sulfate esters of carbohydrates, D 1.1, Table 22, of indole derivatives, D 21, of flavonoids, D 23.3.3 and the synthesis of glucosinolates, D 9.4). In animals sulfate esters are of significance in the elimination of secondary products (E 1). They are excreted with the urine. 

A review of the rearrangement reaction of anthranilic acid esters, thioesters, or amides under acidic conditions to 2-aryl-3-substituted 4(l//)-quinolinone has been reported and summarizes limits and scope of the reaction in the synthesis of flavonoids. " Theoretical studies of the addition-cyclization-isomerization reaction of propargyl cyanamides with thiol and methanol have been reported to occur via five steps ... 

Analogous schemes, with some variations, may be used to accommodate the biosynthesis of esters and glycosides of caffeic acid, quercetin and hence the procyanidins (1, R = H). However several serious uncertainties arise when the origin of flavonoids such as myricetin and the prodelphinidins (1, R = OH) is considered. 3,4,5-Trihydroxycinnamic acid (5) is not an intermediate in lignin synthesis and, except as its various o-methyl ethers, it has not been encountered in nature. Its role as a putative precursor of the vicinal trihydroxyaryl (pyrogallol) group in... 

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