Changes in targeted metabolites, enzyme activities and transcripts at different developmental stages of tea leaves: a study for understanding the biochemical basis of tea shoot plucking
Catechins (flavan-3-ols), the most important secondary metabolites in the tea plant, have positive effects on human health and are crucial in defense against pathogens of the tea plant. The aim of this study was to elucidate the biosynthetic pathway of galloylated catechins in the tea plant. The results suggested that galloylated catechins were biosynthesized via 1-O-glucose ester-dependent two-step reactions by acyltransferases, which involved two enzymes, UDP-glucose:galloyl-1-O-β-D-glucosyltransferase (UGGT) and a newly discovered enzyme, epicatechin:1-O-galloyl-β-D-glucose O-galloyltransferase (ECGT). In the first reaction, the galloylated acyl donor β-glucogallin was biosynthesized by UGGT from gallic acid and uridine diphosphate glucose. In the second reaction, galloylated catechins were produced by ECGT catalysis from β-glucogallin and 2,3-cis-flavan-3-ol. 2,3-cis-Flavan-3-ol and 1-O-galloyl-β-D-glucose were appropriate substrates of ECGT rather than 2,3-trans-flavan-3-ol and 1,2,3,4,6-pentagalloylglucose. Purification by more than 1641-fold to apparent homogeneity yielded ECGT with an estimated molecular mass of 241 to 121 kDa by gel filtration. Enzyme activity and SDS-PAGE analysis indicated that the native ECGT might be a dimer, trimer, or tetramer of 60- and/or 58-kDa monomers, and these monomers represent a heterodimer consisting of pairs of 36- or 34- of and 28-kDa subunits. MALDI-TOF-TOF MS showed that the protein SCPL1199 was identified. Epigallocatechin and epicatechin exhibited higher substrate affinities than β-glucogallin. ECGT had an optimum temperature of 30 °C and maximal reaction rates between pH 4.0 and 6.0. The enzyme reaction was inhibited dramatically by phenylmethylsulfonyl fluoride, HgCl(2), and sodium deoxycholate.