Biochemical characterization of a novel dual-function arabinofuranosidase/xylosidase isolated from a compost starter mixture.

The gene encoding a glycoside hydrolase family 43 enzyme termed deAX was isolated and subcloned from a culture seeded with a compost starter mixed bacterium population, expressed with a C-terminal His(6)-tag, and purified to apparent homogeneity. deAX was monomeric in solution and had a broad pH maximum between pH 5.5 and pH 7. A twofold greater k (cat)/K (m) for the p-nitrophenyl derivative of alpha-L: -arabinofuranose versus that for the isomeric substrate beta-D-xylopyranose was due to an appreciably lower K (m) for the arabinofuranosyl substrate. Substrate inhibition was observed for both 4-methylumbelliferryl arabinofuranoside and the xylopyranoside cogener. While no loss of activity was observed over 4 h at 40 degrees C, the observed t (1/2) value rapidly decreased from 630 min at 49 degrees C to 47 min at 53 degrees C. The enzyme exhibited end-product inhibition, with a K (i) for xylose of 145 mM, 18.5 mM for arabinose, and 750 mM for glucose. Regarding natural substrate specificity, deAX had arabinofuranosidase activity on sugar beet arabinan, 1,5-alpha-L-arabinobiose, and 1,5-alpha-L-arabinotriose, and wheat and rye arabinoxylan, while xylosidase activity was detected for the substrates xylobiose, xylotriose, xylotetraose, and arabinoxylan from beech and birch. Thus, deAX can be classified as a dual-function xylosidase/arabinofuranosidase with respect to both artificial and natural substrate specificity.