STEREOCHEMISTRY OF THE HYDROLYSIS OF GLYCOSIDIC LINKAGE BY ENDO-BETA-1,4-XYLANASES OF TRICHODERMA-REESEI

Methyl beta-D-xylotrioside was used as a non-reducing substrate to investigate the stereochemistry of hydrolysis of beta-1,4-xylopyranosidic linkage by purified endo-beta-1,4-xylanases (EC 3.2.1.8) of Trichoderma reesei, employing H-1 NMR spectroscopy. The fungus produces one acidic species (pI 4.8-5.5), designated as EXI, and one alkaline species (pI 8.5-9.0), designated as EXII. Both enzymes were found to cleave the xylotrioside predominantly to methyl beta-D-xyloside and xylobiose. Monitoring of the intensity of the H-1 signals of alpha- and beta-xylobiose during the time course of hydrolysis clearly showed that both enzymes liberate the beta-anomer of xylobiose, i.e. a product with anomeric configuration identical with that of the cleaved glycosidic linkage. This means that both EXI and EXII belong to the so-called retaining glycanases that utilize the double displacement reaction mechanism of hydrolysis.