Epoxyalkyl glycosides of d-xylose and xylo-oligosaccharides are active-site markers of xylanases from glycoside hydrolase family 11, not from family 10

摘要

pA series of Ω-epoxyalkyl glycosides of D-xylopyranose, xylobiose and xylotriose were tested as potential active-site-directed inhibitors of xylanases from glycoside hydrolase families 10 and 11. Whereas family-10 enzymes (iThermoascus aurantiacus/i Xyn and iClostridium thermocellum/i Xyn Z) are resistant to electrophilic attack of active-site carboxyl residues, glycoside hydrolases of family 11 (iThermomyces lanuginosus/i Xyn and iTrichoderma reesei/i Xyn II) are irreversibly inhibited. The apparent inactivation and association constants (ik/isubi/sub, 1/iK/isubi/sub) are one order of magnitude higher for the xylobiose and xylotriose derivatives. The effects of the aglycone chain length can clearly be described. Xylobiose and n-alkyl β-D-xylopyranosides are competitive ligands and provide protection against inactivation. MS measurements showed 1:1 stoichiometries in most labelling experiments. Electrospray ionization MS/MS analysis revealed the nucleophile Glusup86/sup as the modified residue in the iT. lanuginosus/i xylanase when 2,3-epoxypropyl β-D-xylopyranoside was used, whereas the acid/base catalyst Glusup178/sup was modified by the 3,4-epoxybutyl derivative. The active-site residues Glusup86/sup and Glusup177/sup in iT. reesei/i Xyn II are similarly modified, confirming earlier X-ray crystallographic data [Havukainen, T?rr?nen, Laitinen and Rouvinen (1996) Biochemistry 35, 9617-9624]. The inability of the Ω-epoxyalkyl xylo(oligo)saccharide derivatives to inactivate family-10 enzymes is discussed in terms of different ligand-subsite interactions./p