Characterization of a recombinant α-glucuronidase from Aspergillus fumigatus

The degradation of xylan requires the action of glycanases and esterases which hydrolyse, in a synergistic fashion, the main chain and the different substituents which decorate its structure. Among the xylanolytic enzymes acting on side-chains are the α-glucuronidases (AguA) (E.C. 3.2.1.139) which release methyl glucuronic acid residues. These are the least studies among the xylanolytic enzymes. In this work, the gene and cDNA of an α-glucuronidase from a newly isolated strain of Aspergillus fumigatus have been sequenced, and the gene has been expressed in Pichia pastoris. The gene is 2523 bp long, has no introns and codes for a protein of 840 amino acid residues including a putative signal peptide of 19 residues. The mature protein has a calculated molecular weight of 91,725 and shows 99 % identity with a putative α-glucuronidase from A. fumigatus A1163. The recombinant enzyme was expressed with a histidine tag and was purified to near homogeneity with a nickel nitriloacetic acid (Ni-NTA) column. The purified enzyme has a molecular weight near 100,000. It is inactive using birchwood glucuronoxylan as substrate. Activity is observed in the presence of xylooligosaccharides generated from this substrate by a family 10 endoxylanase and when a mixture of aldouronic acids are used as substrates. If, instead, family 11 endoxylanase is used to generate oligosaccharides, no activity is detected, indicating a different specificity in the cleavage of xylan by family 10 and 11 endoxylanases. Enzyme activity is optimal at 37 °C and pH 4.5-5. The enzyme binds cellulose, thus it likely possesses a carbohydrate binding module. Based on its properties and sequence similarities the catalytic module of the newly described α-glucuronidase can be classified in family 67 of the glycosyl hydrolases. The recombinant enzyme may be useful for biotechnological applications of α-glucuronidases.