Novel structural features in the GMC family of oxidoreductases revealed by the crystal structure of fungal aryl-alcohol oxidase

Lignin biodegradation, a key step in carbon recycling in land ecosystems, is carried out by white-rot fungi through an H2O2-dependent process defined as enzymatic combustion. Pleurotus eryngii is a selective lignin-degrading fungus that produces H2O2 during redox cycling of p-anisylic compounds involving the secreted flavoenzyme aryl-alcohol oxidase (AAO). Here, the 2.4 angstrom resolution X-ray crystal structure of this oxidoreductase, which catalyzes dehydrogenation reactions on various primary polyunsaturated alcohols, yielding the corresponding aldehydes, is reported. The AAO crystal structure was solved by single-wavelength anomalous diffraction of a selenomethionine derivative obtained by Escherichia coli expression and in vitro folding. This monomeric enzyme is composed of two domains, the overall folding of which places it into the GMC (glucose-methanol-choline oxidase) oxidoreductase family, and a noncovalently bound FAD cofactor. However, two additional structural elements exist in the surroundings of its active site that modulate the access of substrates; these are absent in the structure of the model GMC oxidoreductase glucose oxidase. The folding of these novel elements gives rise to a funnel-like hydrophobic channel that connects the solvent region to the buried active-site cavity of AAO. This putative active-site cavity is located in front of the re side of the FAD isoalloxazine ring and near two histidines (His502 and His546) that could contribute to alcohol activation as catalytic bases. Moreover, three aromatic side chains from two phenylalanines (Phe397 and Phe502) and one tyrosine (Tyr92) at the inner region of the channel form an aromatic gate that may regulate the access of the enzyme substrates to the active site as well as contribute to the recognition of the alcohols that can effectively be oxidized by AAO.