Identification and structural characterization of a novel acetyl xylan esterase fromAspergillus oryzae

Abstract

AbstractAcetyl xylan esterase plays a crucial role in the degradation of xylan, the major plant hemicellulose, by liberating acetic acid from the backbone polysaccharides. Acetyl xylan esterase B fromAspergillus oryzae, designatedAoAXEB, was biochemically and structurally investigated. TheAoAXEB-encoding gene with a native signal peptide was successfully expressed inPichia pastorisas an active extracellular protein. The purified recombinant protein had pH and temperature optima of 8.0 and 30 °C, respectively, and was stable up to 35°C. The optimal substrate for hydrolysis by purified recombinantAoAXEB among a panel of α-naphthyl esters was α-naphthyl acetate. RecombinantAoAXEB catalyzes the release of acetic acid from wheat arabinoxylan. The release of acetic acid from wheat arabinoxylan increases synergistically with xylanase addition. No activity was detected using the methyl esters of ferulic,p-coumaric, caffeic, or sinapic acids. The crystal structures ofAoAXEB in the apo and succinate complexes were determined at resolutions of 1.75 and 1.90 Å, respectively. AlthoughAoAXEB has been classified in the Esterase_phb family in the ESTerases and alpha/beta-Hydrolase Enzymes and Relatives (ESTHER) database, its structural features partly resemble those of ferulic acid esterase in the FaeC family. Phylogenetic analysis also indicated thatAoAXEB is located between the clades of the two families. Docking analysis provided a plausible binding mode for xylotriose substrates acetylated at the 2- or 3-hydroxy position. This study expands the repertoire of side chain-degrading enzymes required for complete plant biomass degradation.