ENERGETICS OF THE URIDINE 5'-DIPHOSPHOGLUCOSE - HYDROXY-CINNAMIC ACIDACYL-GLUCOSYLTRANSFERASE REACTION

A uridine 5'-diphosphoglucose: hydroxycinnamic acid acyl-glucosyltrans ferase (HCA-GT) which catalyses the formation of 1-O-sinapoyl-beta-glu cose (EC 2.4.1.120) in seedlings of Raphanus sativus has been partiall y purified. Kinetic analysis of the reversible enzymatic reaction reve aled a sequential binding mechanism for catalysis. Estimation of the r eaction equilibrium constant, calculated from the kinetic properties o f the forward and reverse reactions by means of a Haldane equation, ga ve a K(eq) of 0.21. From the estimated DELTAG(eq)0' of 3.9 kJ mol-1 of the reaction and the known standard free energy of hydrolysis for UDP -glucose and its hydrolysis constant, the corresponding values for 1-O -sinapoyl-beta-glucose of -35.7 kJ mol-1 and 1.45 x 10(6) were deduced . These results justify the conclusion that the 1-O-acylglucosides of hydroxycinnamic acids are high-energy acyl donor molecules with a high group-transfer potential in 1-O-acylglucose-dependent acyltransferase reactions involved in the formation of various hydroxycinnamic acid O -esters in plants.