title: Kinetic mechanism of vanillyl-alcohol oxidase with short-chain 4-alkylphenols.
abstract: The kinetic mechanism of vanillyl-alcohol oxidase with 4-methylphenol, 4-ethylphenol, 4-propylphenol and their C alpha-deuterated analogs has been studied at pH 7.5 and 25 degrees C. Conversion of 4-methylphenol is extremely slow (0.005 s(-1)) while the enzyme is largely in the reduced form during turnover. 4-Ethylphenol and 4-propylphenol are readily converted while the enzyme is mainly in the oxidized form during turnover. The deuterium kinetic isotope effect for overall catalysis ranges between 7-10 whereas the intrinsic deuterium kinetic isotope effect for flavin reduction ranges over 9-10. With all three 4-alkylphenols, flavin reduction appeared to be a reversible process with the rate of reduction being in the same range as the rate for the reverse reaction. During the reductive half-reaction of vanillyl-alcohol oxidase with 4-ethylphenol and 4-propylphenol, a transient intermediate is formed with an absorbance maximum at 330 nm. This intermediate has been tentatively identified as the p-quinone methide of the aromatic substrate in complex with reduced enzyme. It is concluded that vanillyl-alcohol oxidase catalysis with 4-ethylphenol and 4-propylphenol favors an ordered sequential binding mechanism in which the rate of flavin reduction determines the turnover rate while the reduced enzyme-p-quinone methide binary complex rapidly reacts with dioxygen. During the reaction of vanillyl-alcohol oxidase with 4-methylphenol, a fluorescent enzyme species is stabilized. Based on its spectal characteristics and crystallographic data [Mattevi, A., Fraaije, M. W., Mozzarelli, A., Olivi, L., Coda, A. & van Berkel, W. J. H. (1997) Structure 5, 907-920], it is proposed that this species represents a covalent 5-(4'-hydroxybenzyl)-FAD adduct. With 4-ethylphenol and 4-propylphenol, similar N5 flavin adducts may be formed but their rate of formation is too slow to be of catalytic relevance.
authors.affiliation: Department of Biochemistry, Wageningen Agricultural University, The Netherlands.
authors.forename: M W
authors.initials: MW
authors.lastname: Fraaije
authors.forename: R H
authors.initials: RH
authors.lastname: van den Heuvel
authors.forename: J C
authors.initials: JC
authors.lastname: Roelofs
authors.forename: W J
authors.initials: WJ
authors.lastname: van Berkel
mesh_terms: D000429:Alcohol Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010636:Phenols / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013379:Substrate Specificity
chemical_list: D010636:Phenols; D000429:Alcohol Oxidoreductases; C076865:vanillyl-alcohol oxidase