Crystal Structure Analysis of Recombinant Rat Kidney Long Chain Hydroxy Acid Oxidase,

Prochlorococcus marinus responses to light and oxygen

Prochlorococcus marinus, the smallest picocyanobacterium, comprises multiple clades occupying distinct niches, currently across tropical and sub-tropical oligotrophic ocean regions, including Oxygen Minimum Zones. Ocean warming may open growth-permissive temperatures in new, poleward photic regimes, along with expanded Oxygen Minimum Zones. We used ocean metaproteomic data on current Prochlorococcus marinus niches, to guide testing of Prochlorococcus marinus growth across a matrix of peak irradiances, photoperiods, spectral bands and dissolved oxygen. MED4 from Clade HLI requires greater than 4 h photoperiod, grows at 25 μmol O2 L-1 and above, and exploits high cumulative diel photon doses. MED4, however, relies upon an alternative oxidase to balance electron transport, which may exclude it from growth under our lowest, 2.5 μmol O2 L-1, condition. SS120 from clade LLII/III is restricted to low light under full 250 μmol O2 L-1, shows expanded light exploitation under 25 μmol O2 L-1, but is excluded from growth under 2.5 μmol O2 L-1. Intermediate oxygen suppresses the cost of PSII photoinactivation, and possibly the enzymatic production of H2O2 in SS120, which has limitations on genomic capacity for PSII and DNA repair. MIT9313 from Clade LLIV is restricted to low blue irradiance under 250 μmol O2 L-1, but exploits much higher irradiance under red light, or under lower O2 concentrations, conditions which slow photoinactivation of PSII and production of reactive oxygen species. In warming oceans, range expansions and competition among clades will be governed not only by light levels. Short photoperiods governed by latitude, temperate winters, and depth attenuation of light, will exclude clade HLI (including MED4) from some habitats. In contrast, clade LLII/III (including SS120), and particularly clade LLIV (including MIT9313), may exploit higher light niches nearer the surface, under expanding OMZ conditions, where low O2 relieves the stresses of oxidation stress and PSII photoinhibition.

Structure of lactate oxidase from Enterococcus hirae revealed new aspects of active site loop function: Product-inhibition mechanism and oxygen gatekeeper

l-Lactate oxidase (LOx) is a flavin mononucleotide (FMN)-dependent triose phosphate isomerase (TIM) barrel fold enzyme that catalyzes the oxidation of l-lactate using oxygen as a primary electron acceptor. Although reductive half-reaction mechanism of LOx has been studied by structure-based kinetic studies, oxidative half-reaction and substrate/product-inhibition mechanisms were yet to be elucidated. In this study, the structure and enzymatic properties of wild-type and mutant LOxs from Enterococcus hirae (EhLOx) were investigated. EhLOx structure showed the common TIM-barrel fold with flexible loop region. Noteworthy observations were that the EhLOx crystal structures prepared by co-crystallization with product, pyruvate, revealed the complex structures with "d-lactate form ligand," which was covalently bonded with a Tyr211 side chain. This observation provided direct evidence to suggest the product-inhibition mode of EhLOx. Moreover, this structure also revealed a flip motion of Met207 side chain, which is located on the flexible loop region as well as Tyr211. Through a saturation mutagenesis study of Met207, one of the mutants Met207Leu showed the drastically decreased oxidase activity but maintained dye-mediated dehydrogenase activity. The structure analysis of EhLOx Met207Leu revealed the absence of flipping in the vicinity of FMN, unlike the wild-type Met207 side chain. Together with the simulation of the oxygen-accessible channel prediction, Met207 may play as an oxygen gatekeeper residue, which contributes oxygen uptake from external enzyme to FMN. Three clades of LOxs are proposed based on the difference of the Met207 position and they have different oxygen migration pathway from external enzyme to active center FMN.

Dual activities of oxidation and oxidative decarboxylation by flavoenzymes

Specific flavoenzyme oxidases catalyze oxidative decarboxylation in addition to their classical oxidation reactions in the same active sites. The mechanisms underlying oxidative decarboxylation by these enzymes and how they control their two activities are not clearly known. This article reviews the current state of knowledge of four enzymes from the l-amino acid oxidase and l-hydroxy acid oxidase families, including l-tryptophan 2-monooxygenase, l-phenylalanine 2-oxidase and l-lysine oxidase/monooxygenase and lactate monooxygenase which catalyze substrate oxidation and oxidative decarboxylation. Apart from specific interactions to allow substrate oxidation by the flavin cofactor, specific binding of oxidized product in the active sites appears to be important for enabling subsequent decarboxylation by these enzymes. Based on recent findings of l-lysine oxidase/monooxygenase, we propose that nucleophilic attack of H2O2 on the imino acid product is the mechanism enabling oxidative decarboxylation.

Kinetic and Bioinformatic Characterization of d-2-Hydroxyglutarate Dehydrogenase from Pseudomonas aeruginosa PAO1

d-2-Hydroxyglutarate dehydrogenase from Pseudomonas aeruginosa PAO1 (PaD2HGDH) catalyzes the oxidation of d-2-hydroxyglutarate to 2-ketoglutarate, which is a necessary step in the serine biosynthetic pathway. The dependence of P. aeruginosa on PaD2HGDH makes the enzyme a potential therapeutic target against P. aeruginosa. In this study, recombinant His-tagged PaD2HGDH was expressed and purified to high levels from gene PA0317, which was previously annotated as an FAD-binding PCMH-type domain-containing protein. The enzyme cofactor was identified as FAD with fluorescence emission after phosphodiesterase treatment and with mass spectrometry analysis. PaD2HGDH had a k_cat value of 11 s^-1 and a K_m value of 60 μM with d-2-hydroxyglutarate at pH 7.4 and 25 °C. The enzyme was also active with d-malate but did not react with molecular oxygen. Steady-state kinetics with d-malate and phenazine methosulfate as an electron acceptor established a mechanism that was consistent with ping-pong bi-bi steady-state kinetics at pH 7.4. A comparison of the k_cat/K_m values with d-2-hydroxyglutarate and d-malate suggested that the C5 carboxylate of d-2-hydroxyglutarate is important for the substrate specificity of the enzyme. Other homologues of the enzyme have been previously grouped in the VAO/PMCH family of flavoproteins. PaD2HGDH shares fully conserved residues with other α-hydroxy acid oxidizing enzymes, and these conserved residues are found in the active site of the PaD2HDGH homology model. An Enzyme Function Initiative-Enzyme Similarity Tool Sequence Similarity Network analysis suggests a functional difference between PaD2HGDH and human D2HGDH, and no relationship with VAO. A phylogenetic tree analysis of PaD2HGDH, VAO, and human D2HGDH establishes genetic diversity among these enzymes.

Biochemical and structural explorations of α-hydroxyacid oxidases reveal a four-electron oxidative decarboxylation reaction

Structural and enzymological explorations of α-hydroxyacid oxidases uncover new flavin mononucleotide-mediated reactions and intermediates.

p-Hydroxymandelate oxidase (Hmo) is a flavin mononucleotide (FMN)-dependent enzyme that oxidizes mandelate to benzoylformate. How the FMN-dependent oxidation is executed by Hmo remains unclear at the molecular level. A continuum of snapshots from crystal structures of Hmo and its mutants in complex with physiological/nonphysiological substrates, products and inhibitors provides a rationale for its substrate enantioselectivity/promiscuity, its active-site geometry/reactivity and its direct hydride-transfer mechanism. A single mutant, Y128F, that extends the two-electron oxidation reaction to a four-electron oxidative decarboxylation reaction was unexpectedly observed. Biochemical and structural approaches, including biochemistry, kinetics, stable isotope labeling and X-ray crystallo­graphy, were exploited to reach these conclusions and provide additional insights.

Studies on the L-2-hydroxy-acid oxidase 2 catalyzed metabolism of S-mandelic acid and its analogues

Mandelic acid (MA) is generally used as a biomarker of the exposure of styrene, which is classified as a class of hazardous environmental pollutants, and also used as an important chiral intermediate in pharmaceutical industry. The previous studies have found the excretion of phenylglyoxylic acid (PGA) in human and rat, a metabolite of MA, was mainly from S-MA rather than R-MA. The metabolic mechanism, however, is not clear. In order to explore the possible metabolic mechanism, the enzyme types involved in the stereoselectivity metabolism of MA were firstly studied, and then human and rat long-chain 2-hydroxy-acid oxidase 2 (HAO2) were recombinantly expressed to study the metabolic profiles of S-MA and its analogues. The results indicated that HAO2 might catalyze the stereoselectivity metabolism of S-MA in rats. Human HAO2 (hHAO2) and rat HAO2 (rHAO2) isozymes β₁ and β₂ were successfully cloned and expressed with high purity and good enzyme activities. The enzyme kinetic profiles of these enzymes were different for S-MA and analogues. The order of catalytic efficiency for hHAO2 and rHAO2, however, was reverse. It might be relevance to the difference in active amino acid residues and loop 4 in human and rat L-2-hydroxy acid oxidase isozyme B crystal structures.

Structure and role for active site lid of lactate monooxygenase from Mycobacterium smegmatis

Lactate monooxygenase (LMO) catalyzes the FMN-dependent "coupled" oxidation of lactate and O₂ to acetate, carbon dioxide, and water, involving pyruvate and hydrogen peroxide as enzyme-bound intermediates. Other α-hydroxy acid oxidase family members follow an "uncoupled pathway," wherein the α-keto acid product quickly dissociates before the reduced flavin reacts with oxygen. Here, we report the structures of Mycobacterium smegmatis wild-type LMO and a wild-type-like C203A variant at 2.1 Å and 1.7 Å resolution, respectively. The overall LMO fold and active site organization, including a bound sulfate mimicking substrate, resemble those of other α-hydroxy acid oxidases. Based on structural similarity, LMO is similarly distant from lactate oxidase, glycolate oxidase, mandelate dehydrogenase, and flavocytochrome b₂ and is the first representative enzyme of its type. Comparisons with other α-hydroxy acid oxidases reveal that LMO has a longer and more compact folded active site loop (Loop 4), which is known in related flavoenzymes to undergo order/disorder transitions to allow substrate/product binding and release. We propose that LMO's Loop 4 has an enhanced stability that is responsible for the slow product release requisite for the coupled pathway. We also note electrostatic features of the LMO active site that promote substrate binding. Whereas the physiological role of LMO remains unknown, we document what can currently be assessed of LMO's distribution in nature, including its unexpected occurrence, presumably through horizontal gene transfer, in halophilic archaea and in a limited group of fungi of the genus Beauveria. BROAD STATEMENT OF IMPACT: This first crystal structure of the FMN-dependent α-hydroxy acid oxidase family member lactate monooxygenase (LMO) reveals it has a uniquely large active site lid that we hypothesize is stable enough to explain the slow dissociation of pyruvate that leads to its "coupled" oxidation of lactate and O₂ to produce acetate, carbon dioxide, and water. Also, the relatively widespread distribution of putative LMOs supports their importance and provides new motivation for their further study.

Translational misreading, amino acid misincorporation and misinterpretations. The case of the flavocytochrome b2 H373Q variant

Amino acid misincorporation during protein synthesis occurs naturally at a low level. Protein sequence errors, depending on the level and the nature of the misincorporation, can have various consequences. When site-directed mutagenesis is used as a tool for understanding the role of a side chain in enzyme catalysis, misincorporation in a variant with intrinsically low activity may lead to misinterpretations concerning the enzyme mechanism. We report here one more example of such a problem, dealing with flavocytochrome b₂ (Fcb2), a lactate dehydrogenase, member of a family of FMN-dependent L-2-hydroxy acid oxidizing enzymes. Two papers have described the properties of the Fcb2 catalytic base H373Q variant, each one using a different expression system with the same base change for the mutation. The two papers found similar apparent kinetic parameters. But the first one demonstrated the existence of a low level of histidine misincorporation, which led to an important correction of the variant residual activity (Gaume et al. (1995) Biochimie, 77, 621). The second paper did not investigate the possibility of a misincorporation (Tsai et al. (2007) Biochemistry, 46, 7844). The two papers had different mechanistic conclusions. We show here that in this case the misincorporation does not depend on the expression system. We bring the proof that Tsai et al. (2007) were led to an erroneous mechanistic conclusion for having missed the phenomenon as well as for having misinterpreted the crystal structure of the variant. This work is another illustration of the caution one should exercise when characterizing enzyme variants with low activity.

Trifluorosubstrates as mechanistic probes for an FMN-dependent l-2-hydroxy acid-oxidizing enzyme

A controversy exists with respect to the mechanism of l-2-hydroxy acid oxidation by members of a family of FMN-dependent enzymes. A so-called carbanion mechanism was initially proposed, in which the active site histidine abstracts the substrate α-hydrogen as a proton, followed by electron transfer from the carbanion to the flavin. But an alternative mechanism was not incompatible with some results, a mechanism in which the active site histidine instead picks up the substrate hydroxyl proton and a hydride transfer occurs. Even though more recent experiments ruling out such a mechanism were published (Rao & Lederer (1999) Protein Science 7, 1531-1537), a few authors have subsequently interpreted their results with variant enzymes in terms of a hydride transfer. In the present work, we analyse the reactivity of trifluorolactate, a substrate analogue, with the flavocytochrome b2 (Fcb2) flavodehydrogenase domain, compared to its reactivity with an NAD-dependent lactate dehydrogenase (LDH), for which this compound is known to be an inhibitor (Pogolotti & Rupley (1973) Biochem. Biophys. Res. Commun, 55, 1214-1219). Indeed, electron attraction by the three fluorine atoms should make difficult the removal of the α-H as a hydride. We also analyse the reactivity of trifluoropyruvate with the FMN- and NAD-dependent enzymes. The results substantiate a different effect of the fluorine substituents on the two enzymes compared to their normal substrates. In the discussion we analyse the conclusions of recent papers advocating a hydride transfer mechanism for the family of l-2-hydroxy acid oxidizing FMN-dependent enzymes.

Speeding up the product release: a second-sphere contribution from Tyr191 to the reactivity of L-lactate oxidase revealed in crystallographic and kinetic studies of site-directed variants

Among α-hydroxy acid-oxidizing flavoenzymes l-lactate oxidase (LOX) is unique in featuring a second-sphere tyrosine (Tyr191 in Aerococcus viridans LOX; avLOX) at the binding site for the substrate's carboxylate group. Y191F, Y191L and Y191A variants of avLOX were constructed to affect a hydrogen-bond network connecting Tyr191 to the carboxylate of the bound ligand via the conserved Tyr40 and to examine consequent effects on enzymatic reactivity. Kinetic studies at 20 °C and pH 6.5 revealed that release of pyruvate product was decreased 4.7-fold (Y191F), 19-fold (Y191L) and 28-fold (Y191A) compared with wild-type enzyme (~ 141 s(-1)) and thus became mainly rate limiting for l-lactate oxidation by the variants at a steady-state under air-saturated conditions. In the Y191L and the Y191A variants, but not in the Y191F variant, l-lactate binding was also affected strongly by the site-directed substitution. Reduction of the flavin cofactor by l-lactate and its reoxidation by molecular oxygen were, however, comparatively weakly affected by the replacements of Tyr191. Unlike the related lactate monooxygenase, which prevents the fast dissociation of pyruvate to promote its oxidative decarboxylation by H2 O2 into acetate, CO2 and water as final reaction products, all avLOX variants retained their native oxidase activity where catalytic turnover results in the equivalent formation of H2O2. The 1.9 Å crystal structure of the Y191F variant bound with FMN and pyruvate revealed a strictly locally disruptive effect of the site-directed substitution. Product off-rates appear to be dictated by partitioning of residues including Tyr191 from an active-site lid loop into bulk solvent and modulation of the hydrogen bond strength that links Tyr40 with the pyruvate's carboxylate group. Overall, this study emphasizes the possibly high importance of contributions from second-sphere substrate-binding residues to the fine-tuning of reactivity in α-hydroxy acid-oxidizing flavoenzymes, requiring that the catalytic steps of flavin reduction and oxidation are properly timed with the physical step of α-keto acid product release.

The Ala95-to-Gly substitution in Aerococcus viridans l-lactate oxidase revisited - structural consequences at the catalytic site and effect on reactivity with O2 and other electron acceptors

Aerococcus viridansl-lactate oxidase (avLOX) is a biotechnologically important flavoenzyme that catalyzes the conversion of L-lactate and O₂ into pyruvate and H₂O₂. The enzymatic reaction underlies different biosensor applications of avLOX for blood L-lactate determination. The ability of avLOX to replace O₂ with other electron acceptors such as 2,6-dichlorophenol-indophenol (DCIP) allows the possiblity of analytical and practical applications. The A95G variant of avLOX was previously shown to exhibit lowered reactivity with O₂ compared to wild-type enzyme and therefore was employed in a detailed investigation with respect to the specificity for different electron acceptor substrates. From stopped-flow experiments performed at 20 °C (pH 6.5), we determined that the A95G variant (fully reduced by L-lactate) was approximately three-fold more reactive towards DCIP (1.0 ± 0.1 × 10(6) M(-1) ·s(-1) ) than O₂, whereas avLOX wild-type under the same conditions was 14-fold more reactive towards O₂(1.8 ± 0.1 × 10(6) m(-1) ·s(-1)) than DCIP. Substituted 1,4-benzoquinones were up to five-fold better electron acceptors for reaction with L-lactate-reduced A95G variant than wild-type. A 1.65-Å crystal structure of oxidized A95G variant bound with pyruvate was determined and revealed that the steric volume created by removal of the methyl side chain of Ala95 and a slight additional shift in the main chain at position Gly95 together enable the accomodation of a new active-site water molecule within hydrogen-bond distance to the N5 of the FMN cofactor. The increased steric volume available in the active site allows the A95G variant to exhibit a similar trend with the related glycolate oxidase in electron acceptor substrate specificities, despite the latter containing an alanine at the analogous position.

Combining solvent isotope effects with substrate isotope effects in mechanistic studies of alcohol and amine oxidation by enzymes

Oxidation of alcohols and amines is catalyzed by multiple families of flavin- and pyridine nucleotide-dependent enzymes. Measurement of solvent isotope effects provides a unique mechanistic probe of the timing of the cleavage of the OH and NH bonds, necessary information for a complete description of the catalytic mechanism. The inherent ambiguities in interpretation of solvent isotope effects can be significantly decreased if isotope effects arising from isotopically labeled substrates are measured in combination with solvent isotope effects. The application of combined solvent and substrate (mainly deuterium) isotope effects to multiple enzymes is described here to illustrate the range of mechanistic insights that such an approach can provide. This article is part of a Special Issue entitled: Enzyme Transition States from Theory and Experiment.

MD and QM/MM studies on long-chain L-α-hydroxy acid oxidase: substrate binding features and oxidation mechanism

Long-chain L-α-hydroxy acid oxidase (LCHAO) is a flavin mononucleotide (FMN)-dependent oxidase that dehydrogenates l-α-hydroxy acids to keto acids. There were two different mechanisms, named as hydride transfer (HT) mechanism and carbanion (CA) mechanism, respectively, proposed about the catalytic process for the FMN-dependent L-α-hydroxy acid oxidases on the basis of biochemical data. However, crystallographic and kinetic studies could not provide enough evidence to prove one of the mechanisms or eliminate the alternative. In the present studies, theoretical computations were carried out to study the molecular mechanism for LCHAO-catalyzed dehydrogenation of L-lactate. Our molecular dynamics (MD) simulations indicated that L-lactate prefers to bind with LCHAO in a hydride transfer mode rather than a carbanion mode. Quantum mechanics/molecular mechanics (QM/MM) calculations were further carried out to obtain the optimized structures of reactants, transition states, and products at the level of ONIOM-EE (B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p):AMBER). Quantum chemical studies indicated that LCHAO-catalyzed dehydrogenation of L-lactate would be a stepwise catalytic reaction in a hydride transfer mechanism but not a carbanion mechanism. MD simulations, binding free energy calculations, and QM/MM computations were also implemented on the complex between L-lactate and Y129F mutant LCHAO. By comparing the Y129F mutant system with the wild-type system, it was further confirmed that the key residue Tyr129 in the active site of LCHAO would not affect L-lactate's binding to LCHAO but play an important role on the catalytic reaction process through an H-bond interaction.

Flavoprotein oxidases: classification and applications

This review provides an overview of oxidases that utilise a flavin cofactor for catalysis. This class of oxidative flavoenzymes has shown to harbour a large number of biotechnologically interesting enzymes. Applications range from their use as biocatalysts for the synthesis of pharmaceutical compounds to the integration in biosensors. Through the recent developments in genome sequencing, the number of newly discovered oxidases is steadily growing. Recent progress in the field of flavoprotein oxidase discovery and the obtained biochemical knowledge on these enzymes are reviewed. Except for a structure-based classification of known flavoprotein oxidases, also their potential in recent biotechnological applications is discussed.

Engineering of Aerococcus viridans L-lactate oxidase for site-specific PEGylation: characterization and selective bioorthogonal modification of a S218C mutant

A defined bioconjugate of Aerococcus viridans L-lactate oxidase and poly(ethylene glycol) 5000 was prepared and characterized in its structural and functional properties in comparison to the unmodified enzyme. Because the L-lactate oxidase in the native form does not contain cysteines, we introduced a new site for chemical modification via thiol chemistry by substituting the presumably surface-exposed serine-218, a nonconserved residue in the amino acid sequence, with cysteine. The resulting S218C mutant was isolated from Escherichia coli and shown in kinetic assays to be similarly (i.e., about half as) active as the native enzyme, thus validating the structure-guided design of the mutation. Using maleimide-activated methoxypoly(ethylene glycol) 5000 in about 10-fold molar excess over protein, the S218C mutant was converted in high yield (94%) into PEGylated derivative, while the native enzyme was totally unreactive under equivalent conditions. PEGylation caused only a relatively small decrease (30%) in the specific activity of the S218C mutant, and it did not change the protein stability. PEGylation went along with enhancement of the apparent size of the homotetrameric L-lactate oxidase in gel permeation chromatography, from 170 kDa to 250 kDa. The protein hydrodynamic diameter determined by dynamic light scattering increased from 11.9 nm in unmodified S218C mutant to 16.4 nm in the PEGylated form. Site-selective PEGylation of the mutated L-lactate oxidase, using orthogonal maleimide-thiol coupling, could therefore facilitate incorporation of the enzyme into biosensors currently employed for determination of blood L-lactate levels, and it could also support different applications of the enzyme in applied biocatalysis.

High resolution crystal structure of rat long chain hydroxy acid oxidase in complex with the inhibitor 4-carboxy-5-[(4-chlorophenyl)sulfanyl]-1, 2, 3-thiadiazole. Implications for inhibitor specificity and drug design

Long chain hydroxy acid oxidase (LCHAO) is responsible for the formation of methylguanidine, a toxic compound with elevated serum levels in patients with chronic renal failure. Its isozyme glycolate oxidase (GOX), has a role in the formation of oxalate, which can lead to pathological deposits of calcium oxalate, in particular in the disease primary hyperoxaluria. Inhibitors of these two enzymes may have therapeutic value. These enzymes are the only human members of the family of FMN-dependent l-2-hydroxy acid-oxidizing enzymes, with yeast flavocytochrome b(2) (Fcb2) among its well studied members. We screened a chemical library for inhibitors, using in parallel rat LCHAO, human GOX and the Fcb2 flavodehydrogenase domain (FDH). Among the hits was an inhibitor, CCPST, with an IC(50) in the micromolar range for all three enzymes. We report here the crystal structure of a complex between this compound and LCHAO at 1.3 Å resolution. In comparison with a lower resolution structure of this enzyme, binding of the inhibitor induces a conformational change in part of the TIM barrel loop 4, as well as protonation of the active site histidine. The CCPST interactions are compared with those it forms with human GOX and those formed by two other inhibitors with human GOX and spinach GOX. These compounds differ from CCPST in having the sulfur replaced with a nitrogen in the five-membered ring as well as different hydrophobic substituents. The possible reason for the ∼100-fold difference in affinity between these two series of inhibitors is discussed. The present results indicate that specificity is an issue in the quest for therapeutic inhibitors of either LCHAO or GOX, but they may give leads for this quest.

Potent and Selective Inhibitors of Long Chain l-2-Hydroxy Acid Oxidase Reduced Blood Pressure in DOCA Salt-Treated Rats

l-2-Hydroxy acid oxidase (Hao2) is a peroxisomal enzyme with predominant expression in the liver and kidney. Hao2 was recently identified as a candidate gene for blood pressure quantitative trait locus in rats. To investigate a pharmacological role of Hao2 in the management of blood pressure, selective Hao2 inhibitors were developed. Optimization of screening hits 1 and 2 led to the discovery of compounds 3 and 4 as potent and selective rat Hao2 inhibitors with pharmacokinetic properties suitable for in vivo studies in rats. Treatment with compound 3 or 4 resulted in a significant reduction or attenuation of blood pressure in an established or developing model of hypertension, deoxycorticosterone acetate-treated rats. This is the first report demonstrating a pharmacological benefit of selective Hao2 inhibitors in a relevant model of hypertension.

Dynamics of flavin semiquinone protolysis in L-alpha-hydroxyacid-oxidizing flavoenzymes--a study using nanosecond laser flash photolysis

The reactions of the flavin semiquinone generated by laser-induced stepwise two-photon excitation of reduced flavin have been studied previously (El Hanine-Lmoumene C & Lindqvist L. (1997) Photochem Photobiol 66, 591-595) using time-resolved spectroscopy. In the present work, we have used the same experimental procedure to study the flavin semiquinone in rat kidney long-chain hydroxy acid oxidase and in the flavodehydrogenase domain of flavocytochrome b(2) FDH, two homologous flavoproteins belonging to the family of FMN-dependent L-2-hydroxy acid-oxidizing enzymes. For both proteins, pulsed laser irradiation at 355 nm of the reduced enzyme generated initially the neutral semiquinone, which has rarely been observed previously for these enzymes, and hydrated electron. The radical evolved with time to the anionic semiquinone that is known to be stabilized by these enzymes at physiological pH. The deprotonation kinetics were biphasic, with durations of 1-5 micros and tens of microseconds, respectively. The fast phase rate increased with pH and Tris buffer concentration. However, this increase was about 10-fold less pronounced than that reported for the neutral semiquinone free in aqueous solution. pK(a) values close to that of the free flavin semiquinone were obtained from the transient protolytic equilibrium at the end of the fast phase. The second slow deprotonation phase may reflect a conformational relaxation in the flavoprotein, from the fully reduced to the semiquinone state. The anionic semiquinone is known to be an intermediate in the flavocytochrome b(2) catalytic cycle. In light of published kinetic studies, our results indicate that deprotonation of the flavin radical is not rate-limiting for the intramolecular electron transfer processes in this protein.

Structure of human glycolate oxidase in complex with the inhibitor 4-carboxy-5-[(4-chlorophenyl)sulfanyl]-1,2,3-thiadiazole

Glycolate oxidase, a peroxisomal flavoenzyme, generates glyoxylate at the expense of oxygen. When the normal metabolism of glyoxylate is impaired by the mutations that are responsible for the genetic diseases hyperoxaluria types 1 and 2, glyoxylate yields oxalate, which forms insoluble calcium deposits, particularly in the kidneys. Glycolate oxidase could thus be an interesting therapeutic target. The crystal structure of human glycolate oxidase (hGOX) in complex with 4-carboxy-5-[(4-chlorophenyl)sulfanyl]-1,2,3-thiadiazole (CCPST) has been determined at 2.8 A resolution. The inhibitor heteroatoms interact with five active-site residues that have been implicated in catalysis in homologous flavodehydrogenases of L-2-hydroxy acids. In addition, the chlorophenyl substituent is surrounded by nonconserved hydrophobic residues. The present study highlights the role of mobility in ligand binding by glycolate oxidase. In addition, it pinpoints several structural differences between members of the highly conserved family of flavodehydrogenases of L-2-hydroxy acids.

Oxidation of amines by flavoproteins

Many flavoproteins catalyze the oxidation of primary and secondary amines, with the transfer of a hydride equivalent from a carbon-nitrogen bond to the flavin cofactor. Most of these amine oxidases can be classified into two structural families, the D-amino acid oxidase/sarcosine oxidase family and the monoamine oxidase family. This review discusses the present understanding of the mechanisms of amine and amino acid oxidation by flavoproteins, focusing on these two structural families.

L-lactate dehydrogenation in flavocytochrome b2: a first principles molecular dynamics study

First principles molecular dynamics studies on active-site models of flavocytochrome b2 (L-lactate : cytochrome c oxidoreductase, Fcb2), in complex with the substrate, were carried out for the first time to contribute towards establishing the mechanism of the enzyme-catalyzed L-lactate oxidation reaction, a still-debated issue. In the calculated enzyme-substrate model complex, the L-lactate alpha-OH hydrogen is hydrogen bonded to the active-site base H373 Nepsilon, whereas the Halpha is directed towards flavin N5, suggesting that the reaction is initiated by alpha-OH proton abstraction. Starting from this structure, simulation of L-lactate oxidation led to formation of the reduced enzyme-pyruvate complex by transfer of a hydride from lactate to flavin mononucleotide, without intermediates, but with alpha-OH proton abstraction preceding Halpha transfer and a calculated free energy barrier (12.1 kcal mol(-1)) consistent with that determined experimentally (13.5 kcal mol(-1)). Simulation results also revealed features that are of relevance to the understanding of catalysis in Fcb2 homologs and in a number of flavoenzymes. Namely, they highlighted the role of: (a) the flavin mononucleotide-ribityl chain 2'OH group in maintaining the conserved K349 in a geometry favoring flavin reduction; (b) an active site water molecule belonging to a S371-Wat-D282-H373 hydrogen-bonded chain, conserved in the structures of Fcb2 family members, which modulates the reactivity of the key catalytic histidine; and (c) the flavin C4a-C10a locus in facilitating proton transfer from the substrate to the active-site base, favoring the initial step of the lactate dehydrogenation reaction.

Active site and loop 4 movements within human glycolate oxidase: implications for substrate specificity and drug design

Human glycolate oxidase (GO) catalyzes the FMN-dependent oxidation of glycolate to glyoxylate and glyoxylate to oxalate, a key metabolite in kidney stone formation. We report herein the structures of recombinant GO complexed with sulfate, glyoxylate, and an inhibitor, 4-carboxy-5-dodecylsulfanyl-1,2,3-triazole (CDST), determined by X-ray crystallography. In contrast to most alpha-hydroxy acid oxidases including spinach glycolate oxidase, a loop region, known as loop 4, is completely visible when the GO active site contains a small ligand. The lack of electron density for this loop in the GO-CDST complex, which mimics a large substrate, suggests that a disordered to ordered transition may occur with the binding of substrates. The conformational flexibility of Trp110 appears to be responsible for enabling GO to react with alpha-hydroxy acids of various chain lengths. Moreover, the movement of Trp110 disrupts a hydrogen-bonding network between Trp110, Leu191, Tyr134, and Tyr208. This loss of interactions is the first indication that active site movements are directly linked to changes in the conformation of loop 4. The kinetic parameters for the oxidation of glycolate, glyoxylate, and 2-hydroxy octanoate indicate that the oxidation of glycolate to glyoxylate is the primary reaction catalyzed by GO, while the oxidation of glyoxylate to oxalate is most likely not relevant under normal conditions. However, drugs that exploit the unique structural features of GO may ultimately prove to be useful for decreasing glycolate and glyoxylate levels in primary hyperoxaluria type 1 patients who have the inability to convert peroxisomal glyoxylate to glycine.

Crystallographic study on the interaction of l-lactate oxidase with pyruvate at 1.9 Å resolution

L-Lactate oxidase (LOX) from Aerococcus viridans catalyzes the oxidation of L-lactate to pyruvate by the molecular oxygen and belongs to a large family of 2-hydroxy acid-dependent flavoenzymes. To investigate the interaction of LOX with pyruvate in structural details and understand the chemical mechanism of flavin-dependent L-lactate dehydrogenation, the LOX-pyruvate complex was crystallized and the crystal structure of the complex has been solved at a resolution of 1.90 Angstrom. One pyruvate molecule bound to the active site and located near N5 position of FMN for subunits, A, B, and D in the asymmetric unit, were identified. The pyruvate molecule is stabilized by the interaction of its carboxylate group with the side-chain atoms of Tyr40, Arg181, His265, and Arg268, and of its keto-oxygen atom with the side-chain atoms of Tyr146, Tyr215, and His265. The alpha-carbon of pyruvate is found to be 3.13 Angstrom from the N5 atom of FMN at an angle of 105.4 degrees from the flavin N5-N10 axis.

Purification and characterization of recombinant human liver glycolate oxidase

Glycolate oxidase, an FMN-dependent peroxisomal oxidase, plays an important role in plants, related to photorespiration, and in animals, where it can contribute to the production of oxalate with formation of kidney stones. The best studied plant glycolate oxidase is that of spinach; it has been expressed as a recombinant enzyme, and its crystal structure is known. With respect to animals, the enzyme purified from pig liver has been characterized in detail in terms of activity and inhibition, the enzyme from human liver in less detail. We describe here the purification and initial characterization of the recombinant human glycolate oxidase. Its substrate specificity and the inhibitory effects of a number of anions are in agreement with the properties expected from previous work on glycolate oxidases from diverse sources. The recombinant enzyme presents an inhibition by excess glycolate and by excess DCIP, which has not been documented before. These inhibitions suggest that glycolate binds to the active site of the reduced enzyme, and that DCIP also has affinity for the oxidized enzyme. Glycolate oxidase belongs to a family of l-2-hydroxy-acid-oxidizing flavoenzymes, with strongly conserved active-site residues. A comparison of some of the present results with studies dealing with other family members suggests that residues outside the active site influence the binding of a number of ligands, in particular sulfite.

Mechanistic and structural studies of H373Q flavocytochrome b2: effects of mutating the active site base

His373 in flavocytochrome b2 has been proposed to act as an active site base during the oxidation of lactate to pyruvate, most likely by removing the lactate hydroxyl proton. The effects of mutating this residue to glutamine have been determined to provide further insight into its role. The kcat and kcat/Klactate values for the mutant protein are 3 to 4 orders of magnitude smaller than the wild-type values, consistent with a critical role for His373. Similar effects are seen when the mutation is incorporated into the isolated flavin domain of the enzyme, narrowing the effects to lactate oxidation rather than subsequent electron transfers. The decrease of 3500-fold in the rate constant for reduction of the enzyme-bound FMN by lactate confirms this part of the reaction as that most effected by the mutation. The primary deuterium and solvent kinetic isotope effects for the mutant enzyme are significantly smaller than the wild-type values, establishing that bond cleavage steps are less rate-limiting in H373Q flavocytochrome b2 than in the wild-type enzyme. The structure of the mutant enzyme with pyruvate bound, determined at 2.8 A, provides a rationale for these effects. The orientation of pyruvate in the active site is altered from that seen in the wild-type enzyme. In addition, the active site residues Arg289, Asp 292, and Leu 286 have altered positions in the mutant protein. The combination of an altered active site and the small kinetic isotope effects is consistent with the slowest step in turnover being a conformational change involving a conformation in which lactate is bound unproductively.

Insights into the mechanisms of flavoprotein oxidases from kinetic isotope effects

Deuterium, solvent, and (15)N kinetic isotope effects have been used to probe the mechanisms by which flavoproteins oxidize carbon-oxygen and carbon-nitrogen bonds in amines, hydroxy acids, and alcohols. For the amine oxidases d-amino acid oxidase, N-methyltryptophan oxidase, and tryptophan monooxygenase, d-serine, sarcosine, and alanine are slow substrates for which CH bond cleavage is fully rate limiting. Inverse isotope effects for each of 0.992-0.996 are consistent with a common mechanism involving hydride transfer from the uncharged amine. Computational analyses of possible mechanisms support this conclusion. Deuterium and solvent isotope effects with wild-type and mutant variants of the lactate dehydrogenase flavocytochrome b(2) show that OH and CH bond cleavage are not concerted, but become so in the Y254F enzyme. This is consistent with a highly asynchronous reaction in which OH bond cleavage precedes hydride transfer. The results of Hammett analyses and solvent and deuterium isotope effects support a similar mechanism for alcohol oxidase.

The 2.1 A structure of Aerococcus viridans L-lactate oxidase (LOX)

The crystal structure of L-lactate oxidase (LOX) from Aerococcus viridans has been determined at 2.1 A resolution. LOX catalyzes the flavin mononucleotide (FMN) dependent oxidation of lactate to pyruvate and hydrogen peroxide. LOX belongs to the alpha-hydroxy-acid oxidase flavoenzyme family; members of which bind similar substrates and to some extent have conserved catalytic properties and structural motifs. LOX crystallized as two tightly packed tetramers in the asymmetric unit, each having fourfold symmetry. The present structure shows a conserved FMN coordination, but also reveals novel residues involved in substrate binding compared with other family members.