Enzymatic synthesis of β-xylosyl-oligosaccharides by transxylosylation using two beta-xylosidases of glycoside hydrolase family 3 from Aspergillus nidulans FGSC A4

Two beta-xylosidases of glycoside hydrolase family 3 (GH 3) from Aspergillus nidulans FGSC A4, BxlA and BxlB were produced recombinantly in Pichia pastoris and secreted to the culture supernatants in yields of 16 and 118 mg/L, respectively. BxlA showed about sixfold higher catalytic efficiency (k(cat)/K-m) than BxlB towards para-nitrophenyl beta-o-xylopyranoside (pNPX) and beta-1,4-xylo-oligosaccharides (degree of polymerisation 2-6).

For both enzymes k(cat)/K-m decreased with increasing beta-1,4-xylo-oligosaccharide chain length. Using pNPX as donor with 9 monosaccharides, 7 disaccharides and two sugar alcohols as acceptors 18 different p-xylosyl-oligosaccharides were synthesised in 2-36% (BxlA) and 6-66% (BxlB) yields by transxylosylation.

BxlA utilised the monosaccharides D-mannose, D-lyxose, D-talose, D-xylose, D-arabinose, L-fucose, D-glucose, D-galactose and D-fructose as acceptors, whereas BxlB used the same except for D-lyxose, D-arabinose and L-fucose. BxlB transxylosylated the disaccharides xylobiose, lactulose, sucrose, lactose and turanose in upto 35% yield, while BxlA gave inferior yields on these acceptors.

The regioselectivity was acceptor dependent and primarily involved beta-1,4 or 1,6 product linkage formation although minor products with different linkages were also obtained. Five of the 18 transxylosylation products obtained from D-lyxose, D-galactose, turanose and sucrose (two products) as acceptors were novel xylosyl-oligosaccharides, beta-D-Xylp-(1 -> 6)-D-Galp, beta-D-Xylp-(1 -> 4)-alpha-D-Glcp-(1 -> 3)- beta-D-Fruf. beta-D-Xylp-(1 -> 4)-alpha-D-Glcp-(1 -> 2)-beta-D-Fruf, and beta-D-Xylp-(1 -> 6)-beta-D-Fruf-(2 -> 1)-alpha-D-Glcp, as structure-determined by 2D NMR, indicating that GH3 beta-xylosidases are able to transxylosylate a larger variety of carbohydrate acceptors than earlier reported.

Furthermore, transxylosylation of certain acceptors resulted in mixtures. Some of these products are also novel, but the structures of the individual products could not be determined. (C) 2010 Elsevier Ltd. All rights reserved.