Action pattern of xylo-oligosaccharide hydrolysis by Schizophyllum commune xylanase A

The endo-1,4-beta-xylanase of the basidiomycete Schizophyllum commune, designated xylanase A, was studied to determine its action pattern, rates of reaction and bond-cleavage frequencies on xylo-oligomer and xylo-alditol substrates ranging in degree of polymerization (Dp) from xylotriose (X3) to xyloheptaose (X7). An HPLC method using a Dionex HPLC and Carbopac PA1 ion-exchange column with pulsed amperometric detection was developed to quantify both substrate loss and increase of products. Xylanase A had no detectable activity on xylobiose (X2) and low activity on xylotriose and xylotetraose (X4) but cleaved X5-X7 rapidly with X2 and X3 as major products. Initial rate data from hydrolyses of individual oligomers at 25 degrees C and pH 5.81 indicated that the Michaelis constant (Km) decreased with increasing chain length (n) of oligomer. Turnover number (kcat) increased with chain length up to n = 7 suggesting that the specificity region of xylanase A spans about seven xylose units. Bond-cleavage frequencies obtained from xylanase A hydrolysis of xylo-alditols indicated a strong preference for internal linkages of the xylose chain. The action pattern of xylanase A on reduced substrates suggests that the catalytic site is located assymetrically within the binding cleft of the enzyme.