Direction of the reactivity of vanillyl-alcohol oxidase with 4-alkylphenols

The covalent flavoprotein vanillyl-alcohol oxidase (VAO) predominantly converts short-chain 4-alkylphenols, like 4-ethylphenol, to (R)-1-(4'-hydroxyphenyl)alcohols and medium-chain 4-alkylphenols, like 4-butylphenol, to 1-(4'-hydroxyphenyl)alkenes. Crystallographic studies have indicated that the active site residue Asp170 is involved in determining the efficiency of substrate hydroxylation. To test this hypothesis, we have addressed the reactivity of Asp170 variants with 4-alkylphenols. The substrate preference of Asp170Glu was similar to wild type VAO. However, Asp170Ser was most active with branched-chain 4-alkylphenols. The hydroxylation efficiency of the Asp170 variants was dependent on the bulkiness of the newly introduced side chain. The Glu170 mutation favored the production of alkenes, whereas the Ser170 mutation stimulated the formation of alcohols.