Vanillyl-alcohol oxidase (VAO) from Penicillium simplicissimum is an inducible flavoprotein that is active with a wide range of phenolic compounds. The enzyme is the prototype of a newly recognized family of structurally related oxidoreductases, whose members share a conserved FAD-binding domain. The flavin cofactor in VAO is covalently linked to His422 of the cap domain. Studies from His422 variants revealed that deletion of the histidyl-flavin bond does not result in any significant structural change. However, the covalent interaction increases the redox potential of the flavin, facilitating substrate oxidation. His61, located in the FAD-binding domain, is involved in the autocatalytic process of covalent flavinylation. This could be nicely demonstrated by creating the H61T mutant enzyme which binds the flavin in a non-covalently mode. Similar to the noncovalent His422 variants, H61T is 10-fold less active than wild-type VAO. From this and the similar crystal structures of apo and holo H61T it is concluded that the FAD binds to a preorganized binding site where His61 activates His422 for autocatalytic flavinylation. © 2002 Published by Elsevier Science B.V.
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