Enzymes in the Biosynthesis of Hydrolyzable Tannins

Abstract

The two classes of hydrolyzable tannins (Le., gallotannins and ellagitan- nins) have long been postulated to originate from 1,2,3,4,6-pentagalloyl- glucose as their common precursor. The biosynthesis of this pivotal ester was studied with enzymes from oak and sumac leaves. ,B-Glucogallin (1-O-galloyl-,B-D-glucose), the first intermediate of this metabolic se- quence, was formed from UDP-glucose and free gallic acid. Surpris- ingly, this monoester served also as acyl donor in a subsequent series of transacylation steps, yielding specifically substituted di-, tri-, tetra-, and finally, pentagalloylglucose derivatives. However, also higher sub- stituted galloylglucoses (particularly 1 ,6-digalloylglucose) acted as effi- cient acyl donors. The conversion of pentagalloylglucose to gallotan- nins by addition of depsidically bound galloyl groups depended again on ,B-glucogallin as an acyl donor. The biosynthesis of ellagitannins, in contrast, remains obscure, and this applies also to gallic acid, the fundamental phenolic unit of this class of natural products.