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 His₆-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 α-l-arabinofuranose versus that for the isomeric substrate β-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°C, the observed t _1/2 value rapidly decreased from 630 min at 49°C to 47 min at 53°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-α-l-arabinobiose, and 1,5-α-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.