Beta-D-xylosidase from Selenomonas ruminantium: catalyzed reactions with natural and artificial substrates.

Catalytically efficient beta-D-xylosidase from Selenomonas ruminantium (SXA) exhibits pK (a)s 5 and 7 (assigned to catalytic base, D14, and catalytic acid, E186) for k (cat)/K (m) with substrates 1,4-beta-D-xylobiose (X2) and 1,4-beta-D-xylotriose (X3). Catalytically inactive, dianionic SXA (D14(-)E186(-)) has threefold lower affinity than catalytically active, monoanionic SXA (D14(-)E186(H)) for X2 and X3, whereas D14(-)E186(-) has twofold higher affinity than D14(-)E186(H) for 4-nitrophenyl-beta-D-xylopyranoside (4NPX), and D14(-)E186(-) has no affinity for 4-nitrophenyl-alpha-L-arabinofuranoside. Anomeric isomers, alpha-D-xylose and beta-D-xylose, have similar affinity for SXA. 4-Nitrophenol competitively inhibits SXA-catalyzed hydrolysis of 4NPX. SXA steady-state kinetic parameters account for complete progress curves of SXA-catalyzed hydrolysis reactions.