Highly active β-xylosidases of glycoside hydrolase family 43 operating on natural and artificial substrates

The hemicellulose xylan constitutes a major portion of plant biomass, a renewable feedstock available for conversion to biofuels and other bioproducts. β-xylosidase operates in the deconstruction of the polysaccharide to fermentable sugars. Glycoside hydrolase family 43 is recognized as a source of highly active β-xylosidases, some of which could have practical applications. The biochemical details of four GH43 β-xylosidases (those from Alkaliphilus metalliredigens QYMF, Bacillus pumilus, Bacillus subtilis subsp. subtilis str. 168, and Lactobacillus brevis ATCC 367) are examined here. Sedimentation equilibrium experiments indicate that the quaternary states of three of the enzymes are mixtures of monomers and homodimers (B. pumilus) or mixtures of homodimers and homotetramers (B. subtilis and L. brevis). k _cat and k _cat/K _m values of the four enzymes are higher for xylobiose than for xylotriose, suggesting that the enzyme active sites comprise two subsites, as has been demonstrated by the X-ray structures of other GH43 β-xylosidases. The K _i values for d-glucose (83.3-357 mM) and d-xylose (15.6-70.0 mM) of the four enzymes are moderately high. The four enzymes display good temperature (K _t ^0.5 ∼ 45 C) and pH stabilities (>4.6 to <10.3). At pH 6.0 and 25 C, the enzyme from L. brevis ATCC 367 displays the highest reported k _cat and k _cat/K _m on natural substrates xylobiose (407 s ^-1, 138 s^-1 mM^-1), xylotriose (235 s ^-1, 80.8 s^-1 mM^-1), and xylotetraose (146 s^-1, 32.6 s^-1 mM^-1).