The pH-dependent substrate specificity of UDP-glucose anthocyandin 3-rhamnosylglucoside, 5-O-glucosyltransferase in petals of Silene dioica: the formation of anthocyanidin 3,5-diglucosides

Summary Depending upon the pH, the enzyme UDP-glucose : anthocyanidin 3-rhamnosylglucoside, 5-O-glucosyltransferase catalyzes the formation of either anthocyanidin 3-rhamnosylglucoside-5-glucosides or 3,5-diglucosides. Maximal formation of anthocyanidin 3-rhamnosylglucoside-5-glucosides takes place at pH 7.4; of anthocyanidin 3,5-diglucosides at pH 6.5. The reaction rate of the 5-O-glucosylation of cyanidin 3-rhamnosyl-glucoside at pH 7.4 was stimulated 1.6 fold by 1 mM CaCl 2 . Contrary, the glucosylation of cyanidin 3-glucoside at pH 6.5 was not stimulated by divalent metal ions. The «true Km» at pH 7.4 for cyanidin 3-rhamnosylglucoside is 3.6 mM. At pH 6.5 a Km value of 23.4 mM for cyanidin 3-glucoside was found. The affinity for UDP-glucose was not influenced by the pH; both at pH 6.5 and 7.4 the «true Km» for UDP-glucose was 0.6 mM.