LAUSR

Substrate specificities of glycoside hydrolase families 8 (Rex), 39 ( Bh Xyl39), and 52 ( Bh Xyl52) β-xylosidases from Bacillus halodurans C-125 were investigated. Bh Xyl39 hydrolyzed xylotriose most efficiently among the linear xylooligosaccharides. The activity decreased in the order of xylohexaose > xylopentaose > xylotetraose and it had little effect on xylobiose. In contrast, Bh Xyl52 hydrolyzed xylobiose and xylotriose most efficiently, and its activity decreased when the main chain became longer as follows: xylotetraose > xylopentaose > xylohexaose. Rex produced O -β-D-xylopyranosyl-(1 → 4)-[ O -α-L-arabinofuranosyl-(1 → 3)]- O -β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranose (Ara 2 Xyl 3 ) and O -β-D-xylopyranosyl-(1 → 4)-[ O -4- O -methyl-α-D-glucuronopyranosyl-(l → 2)]-β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranose (MeGlcA 2 Xyl 3 ), which lost a xylose residue from the reducing end of O -β-D-xylopyranosyl-(1 → 4)-[ O -α-L-arabinofuranosyl-(1 → 3)]- O -β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranose (Ara 3 Xyl 4 ) and O -β-D-xylopyranosyl-(1 → 4)-[ O -4- O -methyl-α-D-glucuronopyranosyl-(1 → 2)]-β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranosyl-(1 → 4)-β-D-xylopyranose (MeGlcA 3 Xyl 4 ). It was considered that there is no space to accommodate side chains at subsite −1. Bh Xyl39 rapidly hydrolyzes the non-reducing-end xylose linkages of MeGlcA 3 Xyl 4 , while the arabinose branch does not significantly affect the enzyme activity because it degrades Ara 3 Xyl 4 as rapidly as unmodified xylotetraose. The model structure suggested that Bh Xyl39 enhanced the activity for MeGlcA 3 Xyl 4 by forming a hydrogen bond between glucuronic acid and Lys265. Bh Xyl52 did not hydrolyze Ara 3 Xyl 4 and MeGlcA 3 Xyl 4 because it has a narrow substrate binding pocket and 2- and 3-hydroxyl groups of xylose at subsite +1 hydrogen bond to the enzyme.