"Sweet Flavonoids": Glycosidase-Catalyzed Modifications

Abstract

Natural flavonoids, especially in their glycosylated forms, are the most abundant phenolic compounds found in plants, fruit, and vegetables. They exhibit a large variety of beneficial physiological effects, which makes them generally interesting in a broad spectrum of scientific areas. In this review, we focus on recent advances in the modifications of the glycosidic parts of various flavonoids employing glycosidases, covering both selective trimming of the sugar moieties and glycosylation of flavonoid aglycones by natural and mutant glycosidases. Glycosylation of flavonoids strongly enhances their water solubility and thus increases their bioavailability. Antioxidant and most biological activities are usually less pronounced in glycosides, but some specific bioactivities are enhanced. The presence of l-rhamnose (6-deoxy-α-l-mannopyranose) in rhamnosides, rutinosides (rutin, hesperidin) and neohesperidosides (naringin) plays an important role in properties of flavonoid glycosides, which can be considered as "pro-drugs". The natural hydrolytic activity of glycosidases is widely employed in biotechnological deglycosylation processes producing respective aglycones or partially deglycosylated flavonoids. Moreover, deglycosylation is quite commonly used in the food industry aiming at the improvement of sensoric properties of beverages such as debittering of citrus juices or enhancement of wine aromas. Therefore, natural and mutant glycosidases are excellent tools for modifications of flavonoid glycosides.

Keywords: catechin; enzymatic hydrolysis; hesperetin; icariin; naringenin; puerarin; quercetin; rhamnosidase; rutinosidase; transglycosylation.

Figure 1

Structures of selected biotechnologically valuable flavonoids (in bold) and their glycosides. Glc: d-glucopyranosyl, Rha: l-rhamnopyranosyl (6-deoxy-l-mannopyranosyl), Gal: d-galactopyranosyl, Arap: l-arabinopyranosyl, Araf: l-arabinofuranosyl, Xyl: d-xylopyranosyl, Rut: rutinosyl (6-O-α-l-rhamnopyranosyl-β-d-glucopyranosyl), Neo: neohesperidosyl (2-O-α-l-rhamnopyranosyl-d-glucopyranosyl), α-Rha-β-Glc: 2-O-β-d-glucopyranosyl-α-l-rhamnopyranosyl, α-Rha-β-Xyl: 2-O-β-d-xylopyranosyl-α-l-rhamnopyranosyl, α-Rha-α-Rha: 2-O-α-l-rhamnopyranosyl-α-l-rhamnopyranosyl.

Scheme 1

Catalytic mechanisms of glycosidases. (A) Retaining β-glycosidase. (B) Inverting β-glycosidase. (C) Glycosynthase originated from a retaining β-glycosidase. The aglycone and fluor atom are highlighted by violet and dark red, respectively.

Scheme 2

Hydrolytic and transglycosylation reactions performed by glycosidases. The aglycone, transferred sugar moiety and water (and related processes) are highlighted in green, dark red and violet, respectively.

Scheme 3

Deglycosylation reactions of rutin, hesperidin and naringin catalyzed by glycosidases and their respective products. β-d-Glucopyranose, α-l-rhamnopyranose and β-rutinose (and related enzymes) are highlighted by green, dark red, and violet colors, respectively.

Scheme 4

Rutinosidase-catalyzed transglycosylations. (A) Transglycosylation reaction using rutin as glycosyl donor and catechol as an acceptor catalyzed by rutinosidase from Aspergillus niger [19]. (B) Transglycosylation reaction using rutin as glycosyl donor and coniferyl alcohol as an acceptor catalyzed by rutinosidase from A. niger; the transglycosylation product is subsequently cleaved by α-l-rhamnosidase from A. terreus to yield coniferin [106]. (C) Transglycosylation reaction using hesperidin as glycosyl donor and 4-methylumbelliferone as an acceptor catalyzed by rutinosidase from Acremonium sp. [103]. β-d-Glucopyranose, α-l-rhamnopyranose and β-rutinose (and related enzymes) are highlighted by green, dark red, and violet colors, respectively.

Scheme 5

Enzymatic glycosylations of (+)-catechin. (A) Transglycosylation reaction using sucrose as donor catalyzed by amylosucrase from Deinococcus geothermalis [108]. (B) Transglycosylation reaction using dextrin as donor catalyzed by α-amylase from Bacillus sp. [110]. α-d-Glucopyranose, β-d-fructofuranose and glycosylating enzymes are highlighted by green, dark red, and violet colors, respectively.

Figure 2

Structures of oligoglycosylated conjugates of flavonoids obtained by transglycosylation reactions. The conjugated sugar moieties are shown in green (β-d-glucopyranose), violet (β-d-galactopyranose) and dark red (β-d-fructofuranose) color.