Interpretation of the spectra observed during oxidation of p-hydroxybenzoate hydroxylase reconstituted with modified flavins

Tuning the Quantum Chemical Properties of Flavins via Modification at C8

Flavins are central to countless enzymes but display different reactivities depending on their environments. This is understood to reflect modulation of the flavin electronic structure. To understand changes in orbital natures, energies, and correlation over the ring system, we begin by comparing seven flavin variants differing at C8, exploiting their different electronic spectra to validate quantum chemical calculations. Ground state calculations replicate a Hammett trend and reveal the significance of the flavin π-system. Comparison of higher-level theories establishes CC2 and ACD(2) as methods of choice for characterization of electronic transitions. Charge transfer character and electron correlation prove responsive to the identity of the substituent at C8. Indeed, bond length alternation analysis demonstrates extensive conjugation and delocalization from the C8 position throughout the ring system. Moreover, we succeed in replicating a particularly challenging UV/Vis spectrum by implementing hybrid QM/MM in explicit solvents. Our calculations reveal that the presence of nonbonding lone pairs correlates with the change in the UV/Vis spectrum observed when the 8-methyl is replaced by NH₂, OH, or SH. Thus, our computations offer routes to understanding the spectra of flavins with different modifications. This is a first step toward understanding how the same is accomplished by different binding environments.

Phenolic hydroxylases

Many flavin-dependent phenolic hydroxylases (monooxygenases) have been extensively investigated. Their crystal structures and reaction mechanisms are well understood. These enzymes belong to groups A and D of the flavin-dependent monooxygenases and can be classified as single-component and two-component flavin-dependent monooxygenases. The insertion of molecular oxygen into the substrates catalyzed by these enzymes is beneficial for modifying the biological properties of phenolic compounds and their derivatives. This chapter provides an in-depth discussion of the structural features of single-component and two-component flavin-dependent phenolic hydroxylases. The reaction mechanisms of selected enzymes, including 3-hydroxy-benzoate 4-hydroxylase (PHBH) and 3-hydroxy-benzoate 6-hydroxylase as representatives of single-component enzymes and 3-hydroxyphenylacetate 4-hydroxylase (HPAH) as a representative of two-component enzymes, are discussed in detail. This chapter comprises the following four main parts: general reaction, structures, reaction mechanisms, and enzyme engineering for biocatalytic applications. Enzymes belonging to the same group catalyze similar reactions but have different unique structural features to control their reactivity to substrates and the formation and stabilization of C4a-hydroperoxyflavin. Protein engineering has been employed to improve the ability to use these enzymes to synthesize valuable compounds. A thorough understanding of the structural and mechanistic features controlling enzyme reactivity is useful for enzyme redesign and enzyme engineering for future biocatalytic applications.

Hydrogen movements in the oxidative half-reaction of kynurenine 3-monooxygenase from Pseudomonas fluorescens reveal the mechanism of hydroxylation

Kynurenine 3-monoxygenase (KMO) catalyzes the conversion of l-kynurenine (L-Kyn) to 3-hydroxykynurenine (3-OHKyn) in the pathway for tryptophan catabolism. We have investigated the effects of pH and deuterium substitution on the oxidative half-reaction of KMO from P. fluorescens (PfKMO). The three phases observed during the oxidative half reaction are formation of the hydroperoxyflavin, hydroxylation and product release. The measured rate constants for these phases proved largely unchanging with pH, suggesting that the KMO active site is insulated from exchange with solvent during catalysis. A solvent inventory study indicated that a solvent isotope effect of 2-3 is observed for the hydroxylation phase and that two or more protons are in flight during this step. An inverse isotope effect of 0.84 ± 0.01 on the rate constant for the hydroxylation step with ring perdeutero-L-Kyn as a substrate indicates a shift from sp² to sp³ hybridization in the transition state leading to the formation of a non-aromatic intermediate. The pH dependence of transient state data collected for the substrate analog meta-nitrobenzoylalanine indicate that groups proximal to the hydroperoxyflavin are titrated in the range pH 5-8.5 and can be described by a pKa of 8.8. That higher pH values do not slow the rate of hydroxylation precludes that the pKa measured pertains to the proton of the hydroperoxflavin. Together, these observations indicate that the C4a-hydroperoxyflavin has a pKa ≫ 8.5, that a non-aromatic species is the immediate product of hydroxylation and that at least two solvent derived protons are in-flight during oxygen insertion to the substrate aromatic ring. A unifying mechanistic proposal for these observations is proposed.

Unique Biochemical and Sequence Features Enable BluB To Destroy Flavin and Distinguish BluB from the Flavin Monooxygenase Superfamily

Vitamin B₁₂ (cobalamin) is an essential micronutrient for humans that is synthesized by only a subset of bacteria and archaea. The aerobic biosynthesis of 5,6-dimethylbenzimidazole, the lower axial ligand of cobalamin, is catalyzed by the "flavin destructase" enzyme BluB, which fragments reduced flavin mononucleotide following its reaction with oxygen to yield this ligand. BluB is similar in sequence and structure to members of the flavin oxidoreductase superfamily, yet the flavin destruction process has remained elusive. Using stopped-flow spectrophotometry, we find that the flavin destructase reaction of BluB from Sinorhizobium meliloti is initiated with canonical flavin-O₂ chemistry. A C4a-peroxyflavin intermediate is rapidly formed in BluB upon reaction with O₂, and has properties similar to those of flavin-dependent hydroxylases. Analysis of reaction mixtures containing flavin analogues indicates that both formation of the C4a-peroxyflavin and the subsequent destruction of the flavin to form 5,6-dimethylbenzimidazole are influenced by the electronic properties of the flavin isoalloxazine ring. The flavin destruction phase of the reaction, which results from the decay of the C4a-peroxyflavin intermediate, occurs more efficiently at pH >7.5. Furthermore, the BluB mutants D32N and S167G are specifically impaired in the flavin destruction phase of the reaction; nevertheless, both form the C4a-peroxyflavin nearly quantitatively. Coupled with a phylogenetic analysis of BluB and related flavin-dependent enzymes, these results demonstrate that the BluB flavin destructase family can be identified by the presence of active site residues D32 and S167.

α,β-Dicarbonyl reduction is mediated by the Saccharomyces Old Yellow Enzyme

The undesirable flavor compounds diacetyl and 2,3-pentanedione are vicinal diketones (VDKs) formed by extracellular oxidative decarboxylation of intermediate metabolites of the isoleucine, leucine and valine (ILV) biosynthetic pathway. These VDKs are taken up by Saccharomyces and enzymatically converted to acetoin and 3-hydroxy-2-pentanone, respectively. Purification of a highly enriched diacetyl reductase fraction from Saccharomyces cerevisiae in conjunction with mass spectrometry identified Old Yellow Enzyme (Oye) as an enzyme capable of catalyzing VDK reduction. Kinetic analysis of recombinant Oye1p, Oye2p and Oye3p isoforms confirmed that all three isoforms reduced diacetyl and 2,3-pentanedione in an NADPH-dependent reaction. Transcriptomic analysis of S. cerevisiae (ale) and S. pastorianus (lager) yeast during industrial fermentations showed that the transcripts for OYE1, OYE2, arabinose dehydrogenase (ARA1), α-acetolactate synthase (ILV2) and α-acetohydroxyacid reductoisomerase (ILV5) were differentially regulated in a manner that correlated with changes in extracellular levels of VDKs. These studies provide insights into the mechanism for reducing VDKs and decreasing maturation times of beer which are of commercial importance.

Electron transferases

The flavin isoalloxazine ring in electron transferases functions in a redox capacity, being able to take up electrons from a donor to subsequently deliver them to an acceptor. The main characteristics of these flavoproteins, including their unique ability to mediate obligatory processes of two-electron transfers with those involving single-electron transfer, are here described. To illustrate the versatility of these proteins, the acquired knowledge of the function of the two electron transferases involved in the cyanobacterial photosynthetic electron transfer from photosystem I to NADP(+) is presented. Many aspects of their biochemistry and biophysics have been extensively characterized using site-directed mutagenesis, steady-state and transient kinetics, spectroscopy, calorimetry, X-ray crystallography, electron paramagnetic resonance, and computational methods.

Understanding the FMN cofactor chemistry within the Anabaena Flavodoxin environment

The chemical versatility of flavin cofactors within the flavoprotein environment allows them to play main roles in the bioenergetics of all type of organisms, particularly in energy transformation processes such as photosynthesis or oxidative phosphorylation. Despite the large diversity of properties shown by flavoproteins and of the biological processes in which they are involved, only two flavin cofactors, FMN and FAD (both derived from the 7,8-dimethyl-10-(1'-D-ribityl)-isoalloxazine), are usually found in these proteins. Using theoretical and experimental approaches we have carried out an evaluation of the effects introduced upon substituting the 7- and/or 8-methyls of the isoalloxazine ring in the chemical and oxido-reduction properties of the different atoms of the ring on free flavins and on the photosynthetic Anabaena Flavodoxin (a flavoprotein that replaces Ferredoxin as electron carrier from Photosystem I to Ferredoxin-NADP(+) reductase). In Anabaena Flavodoxin both the protein environment and the redox state contribute to modulate the chemical reactivity of the isoalloxazine ring. Anabaena apoflavodoxin is shown to be designed to stabilise/destabilise each one of the FMN redox states (but not of the analogues produced upon substitution of the 7- and/or 8-methyls groups) in the adequate proportions to provide Flavodoxin with the particular properties required for the functions in which it is involved in vivo. The 7- and/or 8-methyl groups of the ixoalloxazine can be discarded as the gate for electrons exchange in Anabaena Fld, but a key role in this process is envisaged for the C6 atom of the flavin and the backbone atoms of Asn58.

Dual role of FMN in flavodoxin function: electron transfer cofactor and modulation of the protein-protein interaction surface

Flavodoxin (Fld) replaces Ferredoxin (Fd) as electron carrier from Photosystem I (PSI) to Ferredoxin-NADP(+) reductase (FNR). A number of Anabaena Fld (AnFld) variants with replacements at the interaction surface with FNR and PSI indicated that neither polar nor hydrophobic residues resulted critical for the interactions, particularly with FNR. This suggests that the solvent exposed benzenoid surface of the Fld FMN cofactor might contribute to it. FMN has been replaced with analogues in which its 7- and/or 8-methyl groups have been replaced by chlorine and/or hydrogen. The oxidised Fld variants accept electrons from reduced FNR more efficiently than Fld, as expected from their less negative midpoint potential. However, processes with PSI (including reduction of Fld semiquinone by PSI, described here for the first time) are impeded at the steps that involve complex re-arrangement and electron transfer (ET). The groups introduced, particularly chlorine, have an electron withdrawal effect on the pyrazine and pyrimidine rings of FMN. These changes are reflected in the magnitude and orientation of the molecular dipole moment of the variants, both factors appearing critical for the re-arrangement of the finely tuned PSI:Fld complex. Processes with FNR are also slightly modulated. Despite the displacements observed, the negative end of the dipole moment points towards the surface that contains the FMN, still allowing formation of complexes competent for efficient ET. This agrees with several alternative binding modes in the FNR:Fld interaction. In conclusion, the FMN in Fld not only contributes to the redox process, but also to attain the competent interaction of Fld with FNR and PSI.

Flavin-Catalyzed Oxidation of Amines and Sulfides with Molecular Oxygen: Biomimetic Green Oxidation

Flavin-catalyzed green oxidation of heteroatom compounds such as sulfides and amines with molecular oxygen and even air in the presence of hydrazine monohydrate in a fluorous solvent such as 2,2,2-trifluoroethanol at room temperature gives the corresponding oxidation products highly efficiently and selectively along with water and molecular nitrogen, which are environmentally benign by-products. The proposed reaction mechanism is based on the kinetics, solvent effect, and redox properties of flavin catalysts.

Protein dynamics and electrostatics in the function of p-hydroxybenzoate hydroxylase

para-Hydroxybenzoate hydroxylase is a flavoprotein monooxygenase that catalyzes a reaction in two parts: reduction of the enzyme cofactor, FAD, by NADPH in response to binding p-hydroxybenzoate to the enzyme, then oxidation of reduced FAD by oxygen to form a hydroperoxide, which oxygenates p-hydroxybenzoate to form 3,4-dihydroxybenzoate. These diverse reactions all occur within a single polypeptide and are achieved through conformational rearrangements of the isoalloxazine ring and protein residues within the protein structure. In this review, we examine the complex dynamic behavior of the protein that enables regulated fast and specific catalysis to occur. Original research papers (principally from the past 15 years) provide the information that is used to develop a comprehensive overview of the catalytic process. Much of this information has come from detailed analysis of many specific mutants of the enzyme using rapid reaction technology, biophysical measurements, and high-resolution structures obtained by X-ray crystallography. We describe how three conformations of the enzyme provide a foundation for the catalytic cycle. One conformation has a closed active site for the conduct of the oxygen reactions, which must occur in the absence of solvent. The second conformation has a partly open active site for exchange of substrate and product, and the third conformation has a closed protein structure with the isoalloxazine ring rotated out to the surface for reaction with NADPH, which binds in a surface cleft. A fundamental feature of the enzyme is a H-bond network that connects the phenolic group of the substrate in the buried active site to the surface of the protein. This network serves to protonate and deprotonate the substrate and product in the active site to promote catalysis and regulate the coordination of conformational states for efficient catalysis.

Altered balance of half-reactions in p-hydroxybenzoate hydroxylase caused by substituting the 2'-carbon of FAD with fluorine

Apo-p-hydroxybenzoate hydroxylase was reconstituted using 2'-fluoro-2'-deoxy-arabino-FAD, a synthetic flavin in which the hydroxyl of the 2'-center of the ribityl chain was replaced with fluorine in an inverted configuration. The absorbance spectral changes caused by the binding of either p-hydroxybenzoate (pOHB) or 2,4-dihydroxybenzoate (2,4-diOHB) indicated that the isoalloxazine of the artificial flavin adopts the more solvent-exposed "out" conformation rather than the partially buried "in" conformation near the aromatic substrate. In contrast, the flavin of the natural enzyme adopts the in conformation when pOHB is bound. Much of the behavior of the artificial enzyme can be rationalized in light of the preference of the flavin for the out conformation, including the weaker binding of pOHB, the tighter binding of 2,4-diOHB, and the slower reactions involved in the hydroxylation of pOHB and 2,4-diOHB. Particularly noteworthy is the enhancement of the reduction of the flavin by NADPH when pOHB is bound to the active site, consistent with the recent finding that the reaction occurs when the flavin adopts the out conformation (Palfey, B. A., Moran, G. R., Entsch, B., Ballou, D. P., and Massey, V. (1999) Biochemistry 38, 1153-1158). Thus, whereas the change that induces the out conformation is detrimental to the oxidative half-reaction, it improves the reductive half-reaction, showing that the control of the flavin position in p-hydroxybenzoate hydroxylase represents a compromise between the conflicting needs of two chemically disparate half-reactions, and demonstrating that the 2'-hydroxyl of FAD can serve as a critical control element in flavoenzyme catalysis.

The covalent FAD of monoamine oxidase: structural and functional role and mechanism of the flavinylation reaction

The family of flavoenzymes in which the flavin coenzyme redox cofactor is covalently attached to the protein through an amino acid side chain is covered in this review. Flavin-protein covalent linkages have been shown to exist through each of five known linkages: (a) 8alpha-N(3)-histidyl, (b) 8alpha-N(1)-histidyl, (c) 8alpha-S-cysteinyl, (d) 8alpha-O-tyrosyl, or (e) 6-S-cysteinyl with the flavin existing at either the flavin mononucleotide or flavin adenine dinucleotide (FAD) levels. This class of enzymes is widely distributed in diverse biological systems and catalyzes a variety of enzymatic reactions. Current knowledge on the mechanism of covalent flavin attachment is discussed based on studies on the 8alpha-S-cysteinylFAD of monoamine oxidases A and B, as well as studies on other flavoenzymes. The evidence supports an autocatalytic quinone-methide mechanism of protein flavinylation. Proposals to explain the structural and mechanistic advantages of a covalent flavin linkage in flavoenzymes are presented. It is concluded that multiple factors are involved and include: (a) stabilization of the apoenzyme structure, (b) steric alignment of the cofactor in the active site to facilitate catalysis, and (c) modulation of the redox potential of the covalent flavin through electronic effects of 8alpha-substitution.

Synergistic interactions of multiple mutations on catalysis during the hydroxylation reaction of p-hydroxybenzoate hydroxylase: studies of the Lys297Met, Asn300Asp, and Tyr385Phe mutants reconstituted with 8-Cl-flavin

The oxygen transfer to p-hydroxybenzoate catalyzed by p-hydroxybenzoate hydroxylase (PHBH) has been shown to occur via a C4a-hydroperoxide of the flavin. Two factors are likely to be important in facilitating the transfer of oxygen from the C4a-hydroperoxide to the substrate. (a) The positive electrostatic potential of the active site partially stabilizes the negative charge centered on the oxygen of the flavin-C4a-alkoxide leaving group during the transition state [Ortiz-Maldonado, M., Ballou, D. P., and Massey, V. (1999) Biochemistry 38, 8124-8137]. (b) The hydrogen-bonding network ionizes the substrate to promote its nucleophilic attack on the electrophilic C4a-hydroperoxide intermediate [Entsch, B., Palfey, B. A., Ballou, D. P., and Massey, V. (1991) J. Biol. Chem. 266, 17341-17349]. This ionization is also aided by the positive electrostatic potential of the active site [Moran, G. R., Entsch, B., Palfey, B. A., and Ballou, D. P. (1997) Biochemistry 36, 7548-7556]. Substituents on the flavin can specifically affect the stability of the alkoxide leaving-group, whereas changes to specific enzyme residues can affect the charge in the active site and the hydrogen-bonding network. We have used wild-type (WT) PHBH and several mutant forms, all with normal FAD and with 8-Cl-FAD substituted for FAD, to assess the relative contributions of the two effects. Lys297Met and Asn300Asp have decreased positive charge in the active site, and these variants engender approximately 35-fold slower hydroxylation rates than the WT enzyme. Substitution of 8-Cl-FAD in these mutant forms gives approximately 1.8-fold increases in hydroxylation rates, compared with a > or =4.8-fold increase for WT with this flavin. The hydroxylation catalyzed by Tyr385Phe, a mutant enzyme form with a disrupted hydrogen-bonding network that compromises the ionization of the substrate without changing the positive charge of the active site, is stimulated 1.5-fold by substituting the enzyme with 8-Cl-FAD. The substrate, tetrafluoro-p-hydroxybenzoate, is fully ionized in WT PHBH, but this phenolate is a poor nucleophile because of the electron-withdrawing effects of the fluorine substituents. With tetrafluoro-p-hydroxybenzoate as the substrate, substitution of FAD with 8-Cl-FAD in the WT enzyme stabilizes the leaving alkoxide and leads to a 2.3-fold increase in the hydroxylation rate compared to that with FAD. Either the use of substrates that do not communicate with the proton network or the mutation of amino acid residues that perturb this interaction may prevent a necessary conformational change that allows proper orientation between reactants during the hydroxylation reaction or permits the essential protonation of the initially formed nascent flavin-C4a-peroxide anion. Thus, both activation of substrate by the proton network and stabilization of the leaving alkoxide appear to be important for oxygen transfer catalyzed by PHBH. The full effect of the substituents on the flavin (4.8-fold) can only be realized when the optimal transition state can be achieved, and this optimal state is not fully realized with the mutant forms.

Catalytic mechanism of 2-hydroxybiphenyl 3-monooxygenase, a flavoprotein from Pseudomonas azelaica HBP1

2-Hydroxybiphenyl 3-monooxygenase (EC 1.14.13.44) from Pseudomonas azelaica HBP1 is an FAD-dependent aromatic hydroxylase that catalyzes the conversion of 2-hydroxybiphenyl to 2, 3-dihydroxybiphenyl in the presence of NADH and oxygen. The catalytic mechanism of this three-substrate reaction was investigated at 7 degrees C by stopped-flow absorption spectroscopy. Various individual steps associated with catalysis were readily observed at pH 7.5, the optimum pH for enzyme turnover. Anaerobic reduction of the free enzyme by NADH is a biphasic process, most likely reflecting the presence of two distinct enzyme forms. Binding of 2-hydroxybiphenyl stimulated the rate of enzyme reduction by NADH by 2 orders of magnitude. The anaerobic reduction of the enzyme-substrate complex involved the formation of a transient charge-transfer complex between the reduced flavin and NAD(+). A similar transient intermediate was formed when the enzyme was complexed with the substrate analog 2-sec-butylphenol or with the non-substrate effector 2,3-dihydroxybiphenyl. Excess NAD(+) strongly stabilized the charge-transfer complexes but did not give rise to the appearance of any intermediate during the reduction of uncomplexed enzyme. Free reduced 2-hydroxybiphenyl 3-monooxygenase reacted rapidly with oxygen to form oxidized enzyme with no appearance of intermediates during this reaction. In the presence of 2-hydroxybiphenyl, two consecutive spectral intermediates were observed which were assigned to the flavin C(4a)-hydroperoxide and the flavin C(4a)-hydroxide, respectively. No oxygenated flavin intermediates were observed when the enzyme was in complex with 2, 3-dihydroxybiphenyl. Monovalent anions retarded the dehydration of the flavin C(4a)-hydroxide without stabilization of additional intermediates. The kinetic data for 2-hydroxybiphenyl 3-monooxygenase are consistent with a ternary complex mechanism in which the aromatic substrate has strict control in both the reductive and oxidative half-reaction in a way that reactions leading to substrate hydroxylation are favored over those leading to the futile formation of hydrogen peroxide. NAD(+) release from the reduced enzyme-substrate complex is the slowest step in catalysis.

Use of free energy relationships to probe the individual steps of hydroxylation of p-hydroxybenzoate hydroxylase: studies with a series of 8-substituted flavins

We report Hammett correlations, using 8-substituted flavins, to clarify the mechanism of hydroxylation by p-hydroxybenzoate hydroxylase (PHBH). The 8-position of the FAD isoalloxazine ring was chosen for modifications, because in PHBH it has minimal interactions with the protein, and it is accessible to solvent and away from the site of hydroxylation. Although two intermediates, a flavin-C4a-hydroperoxide and a flavin-C4a-hydroxide, are known to participate in hydroxylation, the mechanism of oxygen transfer remains controversial. Mechanisms as diverse as electrophilic aromatic substitution, diradical formation, and isoalloxazine ring opening have been proposed. In the studies reported here, it was possible to monitor spectrally each of the individual steps involved in hydroxylation, because the FAD cofactor acts as a reporter group. Thus, with PHBH, substituted separately with nine derivatives of FAD altered in the 8-position, quantitative structure-reactivity relationships (QSAR) have been applied to probe the mechanisms of formation of the flavin-C4a-hydroperoxide, the conversion to the flavin-C4a-hydroxide with concomitant oxygen transfer to the substrate, and the dehydration of the flavin-C4a-hydroxide to form oxidized FAD. The individual chemical steps in the mechanism of PHBH were not altered when using any of the modified flavins, and normal products were obtained; however, the rates of individual steps were affected, and depended on the electronic properties of the 8-substituent. Increased hydroxylation rates were observed when a more electrophilic flavin-C4a-hydroperoxide (i.e., with an electron-withdrawing substituent at the 8-position) is bound to PHBH. On the basis of QSAR analysis, we conclude that the mechanism of the hydroxylation step is best described by electrophilic aromatic substitution.

Mechanistic insights into p-hydroxybenzoate hydroxylase from studies of the mutant Ser212Ala

In the crystal structure of native p-hydroxybenzoate hydroxylase, Ser212 is within hydrogen bonding distance (2.7 A) of one of the carboxylic oxygens of p-hydroxybenzoate. In this study, we have mutated residue 212 to alanine to study the importance of the serine hydrogen bond to enzyme function. Comparisons between mutant and wild type (WT) enzymes with the natural substrate p-hydroxybenzoate showed that this residue contributes to substrate binding. The dissociation constant for this substrate is 1 order of magnitude higher than that of WT, but the catalytic process is otherwise unchanged. When the alternate substrate, 2,4-dihydroxybenzoate, is used, two products are formed (2,3,4-trihydroxybenzoate and 2,4, 5-trihydroxybenzoate), which demonstrates that this substrate can be bound in two orientations. Kinetic studies provide evidence that the intermediate with a high extinction coefficient previously observed in the oxidative half-reaction of the WT enzyme with this substrate is composed of contributions from both the dienone form of the product and the C4a-hydroxyflavin. During the reduction of the enzyme-2,4-dihydroxybenzoate complex by NADPH with 2, 4-dihydroxybenzoate, a rapid transient increase in flavin absorbance is observed prior to hydride transfer from NADPH to FAD. This is direct evidence for movement of the flavin before reduction occurs.

Electrostatic effects on substrate activation in para-hydroxybenzoate hydroxylase: studies of the mutant lysine 297 methionine

p-Hydroxybenzoate hydroxylase (EC 1.14.13.2) is a flavoprotein monooxygenase that catalyzes the incorporation of one atom of molecular oxygen into p-hydroxybenzoate to form 3,4-dihydroxybenzoate. The enzyme activates the substrate at the 3 position to electrophilic substitution by lowering the pKa of the phenolic oxygen. The results presented here indicate that regions of positive potential in the active site facilitate this substrate activation, which is necessary for rapid hydroxylation. We have neutralized a positive point charge by mutating lysine 297 to methionine (K297M). This mutation changes an amino acid near the active site, but not directly in contact with the flavin or the substrate. A variety of transient state kinetic and static parameters have been determined with two substrates. The results indicate that the K297M mutant does not activate the substrate through phenolic ionization to the same extent as wild-type (WT) and yet remains a competent hydroxylase. However, catalysis by the mutant is slow compared to that of WT, particularly in the oxidative half-reaction. Thus, normally quite labile oxygenated flavin intermediates encountered in the hydroxylation pathway of WT p-hydroxybenzoate hydroxylase are stabilized and their decay is rate limiting in the K297M turnover. Electrostatic potential calculations offer an explanation for the lack of substrate activation. The stability of the oxidative reaction intermediates seems to be related to a lower degree of substrate activation.