β-D-Xylosidase from Selenomonas ruminantium: Thermodynamics of Enzyme-Catalyzed and Noncatalyzed Reactions

beta-D-Xylosidase/alpha-L-arabinofuranosidase from Selenomonas ruminantium is the most active enzyme known for catalyzing hydrolysis of 1,4-beta-D-xylooligosaccharides to D-xylose. Temperature dependence for hydrolysis of 4-nitrophenyl-beta-D-xylopyranoside (4NPX), 4-nitrophenyl-alpha-L-arabinofuranoside (4NPA), and 1,4-beta-D-xylobiose (X2) was determined on and off (k(non)) the enzyme at pH 5.3, which lies in the pH-independent region for k(cat) and k(non). Rate enhancements (k(cat)/k(non)) for 4NPX, 4NPA, and X2 are 4.3 x 10(11), 2.4 x 10(9), and 3.7 x 10(12), respectively, at 25 degrees C and increase with decreasing temperature. Relative parameters k(cat)(4NPX)/k(cat)(4NPA), k(cat)(4NPX)/k(cat)(X2), and (k(cat)/K-m)(4NPX)/(k(cat)/K-m)(X2) increase and (k(cat)/K-m)(4NPX)/(k(cat)/K-m)(4NPA), (1/K-m)(4NPX)/(1/K-m)(4NPA), and (1/K-m)(4NPX)/(1/K-m)(X2) decrease with increasing temperature.