Oxygen Reactions in ρ-Hydroxybenzoate Hydroxylase Utilize the H-Bond Network during Catalysis

Abstract

'para'-Hydroxybenzoate hydroxylase is a flavoprotein monooxygenase that catalyses a reaction in two parts: reduction of the flavin adenine dinucleotide (FAD) in the enzyme by reduced nicotinamide adenine dinucleotide phosphate (NADPH) in response to binding ρ-hydroxybenzoate to the enzyme and oxidation of reduced FAD with oxygen to form a hydroperoxide, which then oxygenates ρ-hydroxybenzoate. These different reactions are coordinated through conformational rearrangements of the protein and isoalloxazine ring during catalysis. Earlier research showed that reduction of FAD occurs when the isoalloxazine of the FAD moves to the surface of the protein to allow hydride transfer from NADPH. This move is coordinated with protein rearrangements that are triggered by deprotonation of buried ρ-hydroxybenzoate through a H-bond network that leads to the surface of the protein. In this paper, we examine the involvement of this same H-bond network in the oxygen reactions - the initial formation of a flavin-C4a-hydroperoxide from the reaction between oxygen and reduced flavin, the electrophilic attack of the hydroperoxide upon the substrate to form product, and the elimination of water from the flavin-C4a-hydroxide to form oxidized enzyme in association with product release. These reactions were measured through absorbance and fluorescence changes in the FAD during the reactions. Results were collected over a range of pH for the reactions of wild-type enzyme and a series of mutant enzymes with the natural substrate and substrate analogues. We discovered that the rate of formation of the flavin hydroperoxide is not influenced by pH change, which indicates that the proton required for this reaction does not come from the H-bond network. The rate of the hydroxylation reaction increases with pH in a manner consistent with a pKa of 7.1. We conclude that the H-bond network abstracts the phenolic proton from ρ-hydroxybenzoate in the transition state of oxygen transfer. The rate of formation of oxidized enzyme increases with pH in a manner consistent with a pKa of 7.1, indicating the involvement of the H-bond network. We conclude that product deprotonation enhances the rate of a specific conformational change required for both product release and the elimination of water from C4a-OH-FAD.