A novel glucuronoyl esterase from Aspergillus fumigatus—the role of conserved Lys residue in the preference for 4-O-methyl glucuronoyl esters

Hung Hiep Huynh, Nozomi Ishii, Ichiro Matsuo, Manabu Arioka

Cellulose in plant cell walls is mainly covered by hemicellulose and lignin, and thus efficient removal of these components is thought to be a key step in the optimal utilization of lignocellulose. The recently discovered carbohydrate esterase (CE) 15 family of glucuronoyl esterases (GEs) which cleave the linkages between the free carboxyl group of D-glucuronic acid in hemicellulose and the benzyl groups in lignin residues could contribute to this process. Herein, we report the identification, functional expression, and enzymatic characterization of a GE, AfGE, from the filamentous fungus Aspergillus fumigatus. AfGE was heterologously expressed in Aspergillus oryzae, and the purified enzyme displayed the ability to degrade the synthetic substrates mimicking the ester linkage between hemicellulose and lignin. AfGE is a potentially industrially applicable enzyme due to its characteristic as a thermophilic enzyme with the favorable temperature of 40-50 °C at pH 5. Molecular modeling and site-directed mutagenesis studies of AfGE demonstrated that Lys209 plays an important role in the preference for the substrates containing 4-O-methyl group in the glucopyranose ring.