Characterization of a family 43 β-xylosidase from the xylooligosaccharide utilizing putative probiotic Weissella sp. strain 92

Abstract

In this work, we present the first XOS degrading glycoside hydrolase from Weissella, WXyn43, a two-domain enzyme from GH43. The gene was amplified from genomic DNA of the XOS utilizing Weissella strain 92, classified under the species-pair Weissella cibaria/W.confusa, and expressed in Escherichia coli. The enzyme is lacking a putative signal peptide and is, from a homology model, shown to be composed of an N-terminal 5-fold β-propeller catalytic domain and a C-terminal β-sandwich domain of unknown function. WXyn43 hydrolyzed short (1-4)-β-d-xylooligosaccharides, with similar kcat/KM for xylobiose (X2) and xylotriose (X3) and clearly lower efficiency in xylotetraose (X4) conversion. WXyn43 displays the highest reported kcat for conversion of X3 (900 s-1 at 37°C) and X4 (770 s-1), and kcat for hydrolysis of X2 (907 s-1) is comparable with or greater than the highest previously reported. The purified enzyme adopted a homotetrameric state in solution, while a truncated form with isolated N-terminal catalytic domain adopted a mixture of oligomeric states and lacked detectable activity. The homology model shows that residues from both domains are involved in monomer-monomer hydrogen bonds, while the bonds creating dimer-dimer interactions only involved residues from the N-terminal domain. Docking of X2 and X3 in the active site shows interactions corresponding to subsites -1 and +1, while presence of a third subsite is unclear, but interactions between a loop and the reducing-end xylose of X3 may be present.