BIOSYNTHESIS OF 1,2,3,6-TETRA-O-GALLOYL-BETA-D-GLUCOSE

An enzyme that catalysed the beta-glucogallin (1-O-galloyl-beta-D-glucose)-dependent galloylation of 1,2,6-tri-O-galloyl-beta-D-glucose to 1,2,3,6-tetra-O-galloyl-beta-D-glucose was partially purified from leaves of oak (Quercus robur). This acyltransferase had a M(r) of ca 380 000, and pH and temperature optima of 6.0 and 55-degrees, respectively, and was most stable between pH 4.0 and 6.5. In addition to the natural substrates beta-glucogallin (donor) and 1,2,6-trigalloylglucose (acceptor), 1,3,6-trigalloylglucose (which is not an intermediate in the biosynthesis of hydrolysable tannins in oak and sumac) was an equally efficient acceptor molecule; in both cases, 1,2,3,6-tetragalloylglucose was the reaction product. Based on the physiological role of this new enzyme, the systematic name 'beta-glucogallin: 1,2,6-tri-O-galloyl-beta-D-glucose 3-O-galloyltransferase' (EC 2.3.1.-) is proposed.