文摘
Two x3b2;-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 (kcat/Km) than BxlB towards para-nitrophenyl x3b2;-d-xylopyranoside (pNPX) and x3b2;-1,4-xylo-oligosaccharides (degree of polymerisation 2–6). For both enzymes kcat/Km decreased with increasing x3b2;-1,4-xylo-oligosaccharide chain length. Using pNPX as donor with 9 monosaccharides, 7 disaccharides and two sugar alcohols as acceptors 18 different x3b2;-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 x3b2;-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, x3b2;-d-Xylp-(1→4)-d-Lyxp, x3b2;-d-Xylp-(1→6)-d-Galp, x3b2;-d-Xylp-(1→4)-α-d-Glcp-(1→3)-x3b2;-d-Fruf, x3b2;-d-Xylp-(1→4)-α-d-Glcp-(1→2)-x3b2;-d-Fruf, and x3b2;-d-Xylp-(1→6)-x3b2;-d-Fruf-(2→1)-α-d-Glcp, as structure-determined by 2D NMR, indicating that GH3 x3b2;-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.