Enzymatic properties of a non-classical aldoxime dehydratase capable of producing alkyl and arylalkyl nitriles.

Nitriles are of significant interest in the flavor and fragrance industries with potential application in cosmetics due to their higher stability than analogous aldehydes. However, the traditional methods to prepare nitriles need toxic reagents and hash conditions. This work aimed to develop a chemoenzymatic strategy to synthesize nitriles from natural aldehydes with aldoxime as the intermediate. A non-classical aldoxime dehydratase (Oxd) was discovered from the fungus Aspergillus ibericus (OxdAsp) to catalyze the dehydration of aldoximes to corresponding nitriles under mild conditions. The amino acid sequence of OxdAsp exhibits an approximately 20% identity with bacterial Oxds. OxdAsp contains a heme prosthetic group bound with the axial H287 in the catalytic pocket. The structure models of OxdAsp with substrates suggest that its catalytic triad is Y138-R141-E192, which is different from the classically bacterial Oxds of His-Arg-Ser/Thr. The catalytic mechanism of OxdAsp was proposed based on the mutagenesis of key residues. The hydroxyl group of the substrate is fixed by E192 to increase its basicity. Y138 acts as a general acid-based catalyst, and its phenolic proton is polarized by the adjacent R141. The protonated Y138 would donate a proton to the hydroxyl group of the substrate and eliminate a water molecule from aldoxime to produce nitrile. The recombinant OxdAsp can efficiently dehydrate citronellal oxime and cinnamaldoxime to citronellyl nitrile and cinnamonitrile in aqueous media, which are applied as fragrance ingredients in the food and cosmetic fields. KEY POINTS: • A novel aldoxime dehydratase from the Aspergillus genus was first characterized as a heme-binding protein. • The catalytic mechanism was predicted based on the molecular interactions of the catalytic pocket with the substrate. • A chemoenzymatic strategy was developed to synthesize nitriles from natural aldehydes with aldoxime as the intermediate.