A furcatin hydrolyzing glycosidase of Viburnum furcatum blume

Acuminosylation of Tyrosol by a Commercial Diglycosidase

A commercial glycosidase mixture obtained from Penicillium multicolor (Aromase H2) was found to comprise a specific diglycosidase activity, β-acuminosidase, alongside undetectable levels of β-apiosidase. The enzyme was tested in the transglycosylation of tyrosol using 4-nitrophenyl β-acuminoside as the diglycosyl donor. The reaction was not chemoselective, providing a mixture of Osmanthuside H and its counterpart regioisomer 4-(2-hydroxyethyl)phenyl β-acuminoside in 58% yield. Aromase H2 is therefore the first commercial β-acuminosidase which is also able to glycosylate phenolic acceptors.

Apiose-Relevant Glycosidases

Apiose is a branched pentose naturally occurring either as a component of the plant cell wall polysaccharides or as a sugar moiety present in numerous plant secondary metabolites such as flavonoid and phenylethanoid glycosides, substrates in plant defense systems or as glycosylated aroma precursors. The enzymes catalyzing hydrolysis of such apiosylated substances (mainly glycosidases specific towards apiose or acuminose) have promising applications not only in hydrolysis (flavor development), but potentially also in the synthesis of apiosides and apioglucosides with pharmaceutical relevance. This review summarizes the actual knowledge of glycosidases recognizing apiose and their potential application in biocatalysis.

Computational and experimental analyses of furcatin hydrolase for substrate specificity studies of disaccharide-specific glycosidases

Disaccharide-specific glycosidases (diglycosidases) are unique glycoside hydrolases, as their substrate specificities differ from those of monosaccharide-specific beta-glycosidases (monoglycosidases), in spite of similarities in their sequences and reaction mechanisms. Diglycosidases selectively hydrolyse the beta-glycosidic bond between glycone and aglycone of disaccharide glycosides, but do not cleave the bond between two saccharides, and barely hydrolyse monosaccharide glycosides. We analysed the substrate recognition mechanisms of diglycosidases by computational and experimental methods, using furcatin hydrolase (FH) (EC 3.2.1.161) derived from Viburnum furcatum. Amino acid sequence comparisons and model structure building revealed two residues, Ala419 and Ser504 of FH, as candidates determining the substrate specificity. These residues were specifically conserved in the diglycosidases. The model structure suggested that Ala419 is involved in the aglycone recognition, whereas Ser504 recognizes the external saccharide of the glycone. Mutations at these sites drastically decreased the diglycosidase activity. The mechanism by which the diglycosidases acquired their substrate specificity is discussed, based on these observations.

Furcatin Hydrolase from Viburnum furcatum Blume Is a Novel Disaccharide-specific Acuminosidase in Glycosyl Hydrolase Family 1

Furcatin hydrolase (FH) is a unique disaccharide-specific acuminosidase, which hydrolyzes furcatin (p-allylphenyl 6-O-beta-D-apiofuranosyl-beta-D-glucopyranoside (acuminoside)) into p-allylphenol and the disaccharide acuminose. We have isolated a cDNA coding for FH from Viburnum furcatum leaves. The open reading frame in the cDNA encoded a 538-amino acid polypeptide including a putative chloroplast transit peptide. The deduced protein showed 64% identity with tea leaf beta-primeverosidase, which is another disaccharide glycosidase specific to beta-primeverosides (6-O-beta-D-xylopyranosyl-beta-D-glucopyranosides). The deduced FH also shared greater than 50% identity with various plant beta-glucosidases in glycosyl hydrolase family 1. The recombinant FH expressed in Escherichia coli exhibited the highest level of activity toward furcatin with a Km value of 2.2 mm and specifically hydrolyzed the beta-glycosidic bond between p-allylphenol and acuminose, confirming FH as a disaccharide glycosidase. The FH also hydrolyzed beta-primeverosides and beta-vicianoside (6-O-alpha-L-arabinopyranosyl-beta-D-glucopyranoside) but poorly hydrolyzed beta-gentiobiosides (6-O-beta-D-glucopyranosyl-beta-d-glucopyranosides), indicating high substrate specificity for the disaccharide glycone moiety. The FH exhibited activity toward p-allylphenyl beta-D-glucopyranoside containing the same aglycone as furcatin but little activity toward the other beta-D-glucopyranosides. Stereochemical analysis using 1H NMR spectroscopy revealed that FH is a retaining glycosidase. The subcellular localization of FH was analyzed using green fluorescent protein fused with the putative N-terminal signal peptide, indicating that FH is localized to the chloroplast. Phylogenetic analysis of plant beta-glucosidases revealed that FH clusters with beta-primeverosidase, and this suggests that the disaccharide glycosidases will form a new subfamily in glycosyl hydrolase family 1.

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

A beta-primeverosidase from tea (Camellia sinensis) plants is a unique disaccharide-specific glycosidase, which hydrolyzes aroma precursors of beta-primeverosides (6-O-beta-D-xylopyranosyl-beta-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 beta-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 beta-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 beta-primeverosidase protein sequence was 50% to 60% identical to beta-glucosidases from various plants and was classified in a family 1 glycosyl hydrolase. The mature form of the beta-primeverosidase expressed in Escherichia coli was able to hydrolyze beta-primeverosides to liberate a primeverose unit and aglycons, but did not act on 2-phenylethyl beta-D-glucopyranoside. These results indicate that the beta-primeverosidase selectively recognizes the beta-primeverosides as substrates and specifically hydrolyzes the beta-glycosidic bond between the disaccharide and the aglycons. The stereochemistry for enzymatic hydrolysis of 2-phenylethyl beta-primeveroside by the beta-primeverosidase was followed by (1)H-nuclear magnetic resonance spectroscopy, revealing that the enzyme hydrolyzes the beta-primeveroside by a retaining mechanism. The roles of the beta-primeverosidase in the defense mechanism in tea plants and the floral aroma formation during tea manufacturing process are also discussed.

Glycoconjugated aroma compounds: occurrence, role and biotechnological transformation

The present paper reviews the occurrence of glycosidically bound aroma compounds in the plant kingdom and discusses different hypotheses concerning their role in plants. Emphasis is on biotechnological methods for flavor release and flavor enhancement through enzymatic hydrolysis of glycoconjugated aroma substances.