Biosynthesis of malonylated flavonoid glycosides on the basis of malonyltransferase activity in the petals of Clitoria ternatea

SummaryThe crude malonyltransferase from the petals of Clitoria ternatea was characterized enzymatically to investigate its role on the biosynthetic pathways of anthocyanins and flavonol glycosides. In C. ternatea, a blue flower cultivars (DB) and mauve flower variety (WM) accumulate polyacylated anthocyanins (ternatins) and delphinidin 3-O-(6″-O-malonyl)-β-glucoside which is one of the precursors of ternatins, respectively. Moreover, WM accumulates minor delphinidin glycosides – 3-O-β-glucoside, 3-O-(2″-O-α-rhamnosyl)-β-glucoside, 3-O-(2″-O-α-rhamnosyl-6″-O-malonyl)-β-glucoside of delphinidin. These glycosidic patterns for minor anthocyanins in WM are also found among the minor flavonol glycosides in all the varieties including a white flower variety (WW) although the major flavonol glycosides are 3-O-(2″-O-α-rhamnosyl)-β-glucoside, 3-O-(6″-O-α-rhamnosyl)-β-glucoside, 3-O-(2″,6″-di-O-α-rhamnosyl)-β-glucoside of kaempferol, quercetin, and myricetin. How do the enzymatic characteristics affect the variety of glycosidic patterns in the flavonoid glycoside biosynthesis among these varieties?While the enzyme from DB highly preferred delphinidin 3-O-β-glucoside in the presence of malonyl-CoA, it also has a preference for other anthocyanidin 3-O-β-glucosides. It could use flavonol 3-O-β-glucosides in much lower specific activities than anthocyanins; however, it could not utilize 3-O-(2″-O-α-rhamnosyl)-β-glucosides of anthocyanins and flavonols, and 3,3′-di- and 3,3′,5′-tri-O-β-glucoside of delphinidin – other possible precursors in ternatins biosynthesis. It highly preferred malonyl-CoA as an acyl donor in the presence of delphinidin 3-O-β-glucoside. The crude enzymes prepared from WM and WW had the same enzymatic characteristics.These results suggested that 3-O-(2″-O-α-rhamnosyl-6″-O-malonyl)-β-glucosides of flavonoids were synthesized via 3-O-(6″-O-malonyl)-β-glucosides rather than via 3-O-(2″-O-α-rhamnosyl)-β-glucosides, and that malonylation proceeded prior to glucosylation at the B-ring of delphinidin in the early biosynthetic steps towards ternatins. It seemed that the substrate specificities largely affected the difference in the accumulated amount of malonylated glycosides between anthocyanins and flavonols although they are not simply proportional to the accumulation ratio. This enzyme might join in the production of both malonylanthocyanins and flavonol malonylglycosides as a result of broad substrate specificities towards flavonoid 3-O-β-glucosides.