Substrate Specificity of Flavin‐Dependent Vanillyl‐Alcohol Oxidase from Penicillium Simplicissimum

The substrate specificity of the flavoprotein vanillyl‐alcohol oxidase from Penicillium simplicissimum was investigated. Vanillyl‐alcohol oxidase catalyzes besides the oxidation of 4‐hydroxybenzyl alcohols, the oxidative deamination of 4‐hydroxybenzylamines and the oxidative demethylation of 4‐(methoxy‐methyl)phenols. During the conversion of vanillylamine to vanillin, a transient intermediate, most probably vanillylimine, is observed. Vanillyl‐alcohol oxidase weakly interacts with 4‐hydroxyphenylglycols and a series of catechol‐amines. These compounds are converted to the corresponding ketones. Both enantiomers of (nor)epineph‐rine are substrates for vanillyl‐alcohol oxidase, but the R isomer is preferred. Vanillyl‐alcohol oxidase is most active with chavicol and eugenol. These 4‐allylphenols are converted to coumaryl alcohol and coniferyl alcohol, respectively. Isotopic labeling experiments show that the oxygen atom inserted at the Cγ atom of the side chain is derived from water. The 4‐hydroxycinnamyl alcohol products and the substrate analog isoeugenol are competitive inhibitors of vanillyl alcohol oxidation. The binding of isoeugenol to the oxidized enzyme perturbs the optical spectrum of protein‐bound FAD. pH‐dependent binding studies suggest that vanillyl‐alcohol oxidase preferentially binds the phenolate form of isoeugenol (p K a <6, 25°C). From this and the high pH optimum for turnover, a hydride transfer mechanism involving a P ‐quinone methide intermediate is proposed for the vanillyl‐alcohol‐oxidase‐catalyzed conversion of 4‐allylphenols.