An unusual feruloyl esterase from Aspergillus oryzae: two tryptophan residues play a crucial role for the activity

Abstract

The genome of Aspergillus oryzae encodes a hypothetical protein (annotated as AO090701000884), designated AoFaeD, which exhibits sequence identity to feruloyl esterases from Aspergillus clavatus, Coprinopsis cinerea, Neurospora crassa, and Pseudomonas fluorescens subsp. cellulosa. In this study, we cloned the AofaeD coding sequence into Pichia pastoris and demonstrated heterologous expression and secretion of active recombinant enzyme (rAoFaeD) as a 30-kDa protein in the culture medium. Purified rAoFaeD exhibited optimal activity at pH 7.0 and at 45°C, but was stable at a pH range of 7.0–10.0 and a temperature of 40°C. While wild-type rAoFaeD failed to cleave the methyl esters of ferulic acid (MFA), p-coumaric acid (MpCA), caffeic acid (MCA), and sinapic acid (MSA) and ethyl ester of ferulic acid (EFA), enzyme exhibited esterase activity when wheat arabinoxylan was used as a substrate, and this reaction was enhanced synergistically by the addition of xylanase. Notably, rAoFaeD variants in which one of the tryptophan residues (Trp142 or Trp144) located adjacent to the putative catalytic serine residue at position 143 (Ser143) was replaced with phenylalanine and tyrosine (W142F and W144Y), respectively, exhibited hydrolytic activity toward MFA, MpCA, MCA, MSA, and EFA. Moreover, these variants exhibited enhanced production of ferulic acid from wheat arabinoxylan compared to the wild-type protein, and the activity of these proteins was enhanced by the addition of xylanase.