Cloning of β-primeverosidase from tea leaves, a key enzyme in tea aroma formation

Abstract

A β-primeverosidase from tea (Camellia sinensis) plants is a unique disaccharide-specific glycosidase, which hydrolyzes aroma precursors of β-primeverosides (6-O-β-D-xylopyranosyl-β-D-glucopyranosides) to liberate various aroma compounds, and the enzyme is deeply concerned with the floral aroma formation in oolong tea and black tea during the manufacturing process. The β-primeverosidase was purified from fresh leaves of a cultivar for green tea (C. sinensis var sinensis cv Yabukita), and its partial amino acid sequences were determined. The β-primeverosidase cDNA has been isolated from a cDNA library of cv Yabukita using degenerate oligonucleotide primers. The cDNA insert encodes a polypeptide consisting of an N-terminal signal peptide of 28 amino acid residues and a 479-amino acid mature protein. The β-primeverosidase protein sequence was 50% to 60% identical to β-glucosidases from various plants and was classified in a family 1 glycosyl hydrolase. The mature form of the β-primeverosidase expressed in Escherichia coli was able to hydrolyze β-primeverosides to liberate a primeverose unit and aglycons, but did not act on 2-phenylethyl β-D-glucopyranoside. These results indicate that the β-primeverosidase selectively recognizes the β-primeverosides as substrates and specifically hydrolyzes the β-glycosidic bond between the disaccharide and the aglycons. The stereochemistry for enzymatic hydrolysis of 2-phenylethyl β-primeveroside by the β-primeverosidase was followed by 1H-nuclear magnetic resonance spectroscopy, revealing that the enzyme hydrolyzes the β-primeveroside by a retaining mechanism. The roles of the β-primeverosidase in the defense mechanism in tea plants and the floral aroma formation during tea manufacturing process are also discussed.