Molecular cloning and characterization of a novel β-1,3-xylanase possessing two putative carbohydrate-binding modules from a marine bacterium Vibrio sp. strain AX-4

We cloned a novel β-1,3-xylanase gene, consisting of a 1728-bp open reading frame encoding 576 amino acid residues, from a marine bacterium, Vibrio sp. strain AX-4. Sequence analysis revealed that the β-1,3-xylanase is a modular enzyme composed of a putative catalytic module belonging to glycoside hydrolase family 26 and two putative carbohydrate-binding modules belonging to family 31. The recombinant enzyme hydrolysed β-1,3-xylan to yield xylo-oligosaccharides with different numbers of xylose units, mainly xylobiose, xylotriose and xylotetraose. However, the enzyme did not hydrolyse β-1,4-xylan, β-1,4-mannan, β-1,4-glucan, β-1,3-xylobiose or p-nitrophenyl-β-xyloside. When β-1,3-xylo-oligosaccharides were used as the substrate, the k_cat value of the enzyme for xylopentaose was found to be 40 times higher than that for xylotetraose, and xylotriose was extremely resistant to hydrolysis by the enzyme. A PSI-BLAST search revealed two possible catalytic Glu residues (Glu-138 as an acid/base catalyst and Glu-234 as a nucleophile), both of which are generally conserved in glycoside hydrolase superfamily A. Replacement of these two conserved Glu residues with Asp and Gln resulted in a significant decrease and complete loss of enzyme activity respectively, without a change in their CD spectra, suggesting that these Glu residues are the catalytic residues of β-1,3-xylanase. The present study also clearly shows that the non-catalytic putative carbohydrate-binding modules play an important role in the hydrolysis of insoluble β-1,3-xylan, but not that of soluble glycol-β-1,3-xylan. Furthermore, repeating a putative carbohydrate-binding module strongly enhanced the hydrolysis of the insoluble substrate.