Crystal structure of D-amino acid oxidase: a case of active site mirror-image convergent evolution with flavocytochrome b2

Theoretical analyses of the fluorescence lifetimes of the d-amino acid oxidase–benzoate complex dimer from porcine kidney: molecular dynamics simulation and photoinduced electron transfer

The mechanism of photoinduced electron transfer from benzoate and aromatic amino acids to the excited isoalloxazine in the d-amino acid oxidase–benzoate complex dimer was studied using molecular dynamics simulation and an electron transfer theory.

Characterization of human DAAO variants potentially related to an increased risk of schizophrenia

Considering the key role of d-serine in N-methyl-d-aspartate receptor-mediated neurotransmission, it is highly relevant to define the role that enzymes play in d-serine synthesis and degradation. In particular, the details of regulation of the d-serine catabolic human enzyme d-amino acid oxidase (hDAAO) are unknown although different lines of evidence have shown it to be involved in schizophrenia susceptibility. Here we investigated the effect of three single nucleotide polymorphisms and known mutations in hDAAO, i.e., D31H, R279A, and G331V. A very low amount of soluble G331V hDAAO is produced in E. coli cells: the recombinant variant enzyme is fully active. Human U87 glioblastoma cells transiently transfected for G331V hDAAO show a low viability, a significant amount of protein aggregates, and augmented apoptosis. The recombinant D31H and R279A hDAAO variants do not show alterations in tertiary and quaternary structures, thermal stability, binding affinity for inhibitors, and the modulator pLG72, whereas the kinetic efficiency and the affinity for d-serine and for FAD were higher than for the wild-type enzyme. While these effects for the substitution at position 31 cannot be structurally explained, the R279A mutation might affect the hDAAO FAD-binding affinity by altering the "structurally ambivalent" peptide V47-L51. In agreement with the observed increased activity, expression of D31H and R279A hDAAO variants in U87 cells produces a higher decrease in cellular d/(d+l) serine ratio than the wild-type counterpart. In vivo, these substitutions could affect cellular d-serine concentration and its release at synapsis and thus might be relevant for schizophrenia susceptibility.

Oligolamellar vesicles for covalent immobilization and stabilization of D-amino acid oxidase

Oligolamellar phospholipid vesicles incorporated with d-amino acid oxidase from porcine kidney (OV-DAO) were prepared by encapsulating pre-formed enzyme-bound unilamellar vesicles (UV-DAO) with bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The bilayer of UV-DAO was composed of POPC, 30 mol% of cholesterol and 15 mol% of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl) (NGPE) that was responsible for covalent linking to D-amino acid oxidase (DAO). OV-DAO and UV-DAO showed the activity to catalyze the oxidation of D-alanine as measured based on the hydrogen peroxide produced. The oligolamellar and unilamellar structure of OV-DAO and UV-DAO, respectively was elucidated based on the quenching characteristics of bilayers-incorporated fluorescent lipid 7-nitro-2,1,3-benzoxadiazol-4-yl-phosphoethanolamine (NBD-PE) and the size distribution of the vesicles measured with the dynamic light scattering method. The enzyme activity of OV-DAO and UV-DAO was significantly stabilized at 50°C compared to that of free DAO at the fixed enzyme concentration of 3.29 μg/mL. At the temperature, OV-DAO and UV-DAO showed the remaining activity of 52.7 and 29.6%, respectively at the incubation time of 20 min while free DAO was completely deactivated. Thus the dimeric form of DAO could be stabilized by its coupling to the surface of UV-DAO membrane being the inner bilayer of OV-DAO. Furthermore, the thermal denaturation of DAO and dissociation of flavin adenine dinucleotide (FAD) from the subunits of enzyme were prevented in the aqueous phase formed between the bilayers of OV-DAO.

Catalytic and structural role of a conserved active site histidine in berberine bridge enzyme

Berberine bridge enzyme (BBE) is a paradigm for the class of bicovalently flavinylated oxidases, which catalyzes the oxidative cyclization of (S)-reticuline to (S)-scoulerine. His174 was identified as an important active site residue because of its role in the stabilization of the reduced state of the flavin cofactor. It is also strictly conserved in the family of BBE-like oxidases. Here, we present a detailed biochemical and structural characterization of a His174Ala variant supporting its importance during catalysis and for the structural organization of the active site. Substantial changes in all kinetic parameters and a decrease in midpoint potential were observed for the BBE His174Ala variant protein. Moreover, the crystal structure of the BBE His174Ala variant showed significant structural rearrangements compared to wild-type enzyme. On the basis of our findings, we propose that His174 is part of a hydrogen bonding network that stabilizes the negative charge at the N1–C2=O locus via interaction with the hydroxyl group at C2′ of the ribityl side chain of the flavin cofactor. Hence, replacement of this residue with alanine reduces the stabilizing effect for the transiently formed negative charge and results in drastically decreased kinetic parameters as well as a lower midpoint redox potential.

Structure and function of ∆1-tetrahydrocannabinolic acid (THCA) synthase, the enzyme controlling the psychoactivity of Cannabis sativa

∆1-Tetrahydrocannabinolic acid (THCA) synthase catalyzes the oxidative cyclization of cannabigerolic acid (CBGA) into THCA, the precursor of the primary psychoactive agent ∆1-tetrahydrocannabinol in Cannabis sativa. The enzyme was overproduced in insect cells, purified, and crystallized in order to investigate the structure-function relationship of THCA synthase, and the tertiary structure was determined to 2.75Å resolution by X-ray crystallography (R(cryst)=19.9%). The THCA synthase enzyme is a member of the p-cresol methyl-hydroxylase superfamily, and the tertiary structure is divided into two domains (domains I and II), with a flavin adenine dinucleotide coenzyme positioned between each domain and covalently bound to His114 and Cys176 (located in domain I). The catalysis of THCA synthesis involves a hydride transfer from C3 of CBGA to N5 of flavin adenine dinucleotide and the deprotonation of O6' of CBGA. The ionized residues in the active site of THCA synthase were investigated by mutational analysis and X-ray structure. Mutational analysis indicates that the reaction does not involve the carboxyl group of Glu442 that was identified as the catalytic base in the related berberine bridge enzyme but instead involves the hydroxyl group of Tyr484. Mutations at the active-site residues His292 and Tyr417 resulted in a decrease in, but not elimination of, the enzymatic activity of THCA synthase, suggesting a key role for these residues in substrate binding and not direct catalysis.

Mechanistic and structural analyses of the role of His67 in the yeast polyamine oxidase Fms1

The flavoprotein oxidase Fms1 from Saccharomyces cerevisiae catalyzes the oxidation of spermine and N(1)-acetylspermine to spermidine and 3-aminopropanal or N-acetyl-3-aminopropanal. Within the active site of Fms1, His67 is positioned to form hydrogen bonds with the polyamine substrate. This residue is also conserved in other polyamine oxidases. The catalytic properties of H67Q, H67N, and H67A Fms1 have been characterized to evaluate the role of this residue in catalysis. With both spermine and N(1)-acetylspermine as the amine substrate, the value of the first-order rate constant for flavin reduction decreases 2-3 orders of magnitude, with the H67Q mutation having the smallest effect and H67N the largest. The k(cat)/K(O2) value changes very little upon mutation with N(1)-acetylspermine as the amine substrate and decreases only an order of magnitude with spermine. The k(cat)/K(M)-pH profiles with N(1)-acetylspermine are bell-shaped for all the mutants; the similarity to the profile of the wild-type enzyme rules out His67 as being responsible for either of the pK(a) values. The pH profiles for the rate constant for flavin reduction for all the mutant enzymes similarly show the same pK(a) as wild-type Fms1, about ∼7.4; this pK(a) is assigned to the substrate N4. The k(cat)/K(O2)-pH profiles for wild-type Fms1 and the H67A enzyme both show a pK(a) of about ∼6.9; this suggests His67 is not responsible for this pH behavior. With the H67Q, H67N, and H67A enzymes the k(cat) value decreases when a single residue is protonated, as is the case with the wild-type enzyme. The structure of H67Q Fms1 has been determined at a resolution of 2.4 Å. The structure shows that the mutation disrupts a hydrogen bond network in the active site, suggesting that His67 is important both for direct interactions with the substrate and to maintain the overall active site structure.

Crystallization and preliminary crystallographic analysis of D-aspartate oxidase from porcine kidney

D-Aspartate oxidase (DDO) from porcine kidney was crystallized by the sitting-drop vapour-diffusion method using PEG 8000 as a precipitant. The crystal belonged to space group P2(1), with unit-cell parameters a = 79.38, b = 144.0, c = 80.46 Å, β = 101.1°, and diffracted to 1.80 Å resolution. Molecular-replacement trials using the structure of human D-amino-acid oxidase, which is 42% identical in sequence to DDO, as a search model provided a satisfactory solution.

Comparative study of flavins binding with human serum albumin: a fluorometric, thermodynamic, and molecular dynamics approach

Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are derivatives of riboflavin (RF), a water-soluble vitamin, more commonly known as vitamin B(2). Flavins have attracted special attention in the last few years because of the recent discovery of a large number of flavoproteins. In this work, these flavins are used as extrinsic fluorescence markers for probing the microheterogeneous environment of a well-known transport protein, human serum albumin (HSA). Steady-state and time-resolved fluorescence experiments confirm that both FMN and FAD bind to the Sudlow's site-1 (SS1) binding pocket of HSA, where Trp214 resides. In the case of RF, a fraction of RF molecules binds at the SS1, whereas the major fraction of RF molecules remains unbound or surface bound to the protein. Moreover, flavin(s)-HSA interactions are monitored with the help of isothermal titration calorimetry, which provides free energy, enthalpy, and entropy changes of binding along with the binding constants. The molecular picture of binding interaction between flavins and HSA is well explored by docking and molecular dynamics studies.

Cloning, characterization and mutagenesis of Russell's viper venom L-amino acid oxidase: Insights into its catalytic mechanism

To investigate the structure-function relationships and geographic variations of L-amino acid oxidase (LAAO) from Daboia venoms, a single LAAO (designated as DrLAO) was purified from eastern Indian Daboia russelii venom and characterized. The purified DrLAO showed subunit molecular mass of 60-64kDa; its N-terminal sequence (1-20) was identical to those of several true viper LAAOs. Its preferred substrates were hydrophobic l-amino acids and the kinetic specificities were ordered as follows: Phe, Tyr, Met, Leu, and Trp. Enzyme assay and Western blotting showed that the venom LAAO contents of D. russelii were higher than those of Daboia siamensis. DrLAO dose-dependently inhibited ADP- and collagen-induced platelet aggregation with IC(50) values of 0.27 and 0.82μM, respectively. Apparently, DrLAO may synergize with other venom components to prolong and enhance bleeding symptoms after Daboia envenoming. The full sequence of DrLAO was deduced from its cDNA sequence and then confirmed by peptide mass fingerprinting. Molecular phylogenetic analysis revealed that SV-LAAO family members could be differentiated not only by snake taxonomy but also by the variations at position 223, and they divided into H223, S223, N223, and D223 subclasses. We have further prepared recombinant DrLAO and mutants by the Pichia expression system. Mutagenic analyses of DrLAO His223 revealed that this residue bound substrates instead of serving as an essential base in the catalytic steps. Our results suggest a direct hydride transfer from substrate to FAD as the mechanism for SV-LAAOs.

PDB-scale analysis of known and putative ligand-binding sites with structural sketches

Computational investigation of protein functions is one of the most urgent and demanding tasks in the field of structural bioinformatics. Exhaustive pairwise comparison of known and putative ligand-binding sites, across protein families and folds, is essential in elucidating the biological functions and evolutionary relationships of proteins. Given the vast amounts of data available now, existing 3D structural comparison methods are not adequate due to their computation time complexity. In this article, we propose a new bit string representation of binding sites called structural sketches, which is obtained by random projections of triplet descriptors. It allows us to use ultra-fast all-pair similarity search methods for strings with strictly controlled error rates. Exhaustive comparison of 1.2 million known and putative binding sites finished in ∼30 h on a single core to yield 88 million similar binding site pairs. Careful investigation of 3.5 million pairs verified by TM-align revealed several notable analogous sites across distinct protein families or folds. In particular, we succeeded in finding highly plausible functions of several pockets via strong structural analogies. These results indicate that our method is a promising tool for functional annotation of binding sites derived from structural genomics projects.

Revisitation of the βCl-elimination reaction of D-amino acid oxidase: new interpretation of the reaction that sparked flavoprotein dehydrogenation mechanisms

D-amino acid oxidase (DAAO) from pig has been reported to catalyze the β-elimination of Cl(-) from βCl-D-alanine via abstraction of the substrate α-H as H(+) ("carbanion mechanism") (Walsh, C. T., Schonbrunn, A., and Abeles, R. H. (1971) J. Biol. Chem. 246, 6855-6866). In view of the fundamental mechanistic importance of this reaction and of the recent reinterpretation of the DAAO dehydrogenation step as occurring via a hydride mechanism, we reinvestigated the elimination reaction using yeast DAAO. That enzyme catalyzes the same reactions as the pig enzyme but with a much higher efficiency and a substantially different kinetic behavior. The reaction is initiated by a very rapid and fully reversible dehydrogenation step. This leads to an equilibrium (k(on) ≈ k(reverse)) between the complexes of oxidized enzyme-βCl-D-alanine and reduced enzyme-βCl-iminopyruvate. In the presence of O(2) the latter complex can partition between an oxidative half-reaction and elimination of Cl(-), which proceeds at a rate of ≈50 s(-1). This step forms a complex between oxidized enzyme and enamine that is characterized by a charge transfer absorption (which describes its rates of formation and decay). A minimal scheme that lists relevant steps of the reductive and oxidative half-reactions and elimination pathways along with the estimate of the corresponding rate constants is presented. β-Elimination of Cl(-) is proposed to originate at the locus of the enzyme-βCl-iminopyruvate complex. A chemical mechanism that can account for elimination is discussed in detail.

Crystallographic snapshots of the complete reaction cycle of nicotine degradation by an amine oxidase of the monoamine oxidase (MAO) family

FAD-linked oxidases constitute a class of enzymes which catalyze dehydrogenation as a fundamental biochemical reaction, followed by reoxidation of reduced flavin. Here, we present high-resolution crystal structures showing the flavoenzyme 6-hydroxy-l-nicotine oxidase in action. This enzyme was trapped during catalytic degradation of the native substrate in a sequence of discrete reaction states corresponding to the substrate-reduced enzyme, a complex of the enzyme with the intermediate enamine product and formation of the final aminoketone product. The inactive d-stereoisomer binds in mirror symmetry with respect to the catalytic axis, revealing absolute stereospecificity of hydrogen transfer to the flavin. The structural data suggest deprotonation of the substrate when bound at the active site, an overall binary complex mechanism and oxidation by direct hydride transfer. The amine nitrogen has a critical role in the dehydrogenation step and may activate carbocation formation at the α-carbon via delocalization from the lone pair to σ* C(α)-H. Enzymatically assisted hydrolysis of the intermediate product occurs at a remote (P site) cavity. Substrate entry and product exit follow different paths. Structural and kinetic data suggest that substrate can also bind to the reduced enzyme, associated with slower reoxidation as compared to the rate of reoxidation of free enzyme. The results are of general relevance for the mechanisms of flavin amine oxidases.

A series of D-amino acid oxidase inhibitors specifically prevents and reverses formalin-induced tonic pain in rats

We have found that mutation of D-amino acid oxidase (DAO) diminished formalin-induced tonic pain. The present research further studied the analgesic effects of a series of DAO inhibitors in this model. 5-Chlorobenzo[d]isoxazol-3-ol (CBIO), 4H-thieno[3,2-b]pyrrole-5-carboxylic acid (compound 8), 5-methylpyrazole-3-carboxylic acid (AS057278), sodium benzoate, and 4-nitro-3-pyrazole carboxylic acid (NPCA) inhibited rat spinal cord-derived DAO activity in a concentration-dependent manner, with maximal inhibition of 100% and potency rank of CBIO > compound 8 > AS057278 > sodium benzoate > NPCA. In rats, intrathecal injections of CBIO, compound 8, AS057278, and sodium benzoate but not NPCA specifically prevented formalin-induced tonic pain but not acute nociception, with the same potency order as in the DAO activity assay. The highly potent analgesia of DAO inhibitors was evidenced by CBIO, which prevented 50% pain at 0.06 μg, approximately 5-fold the potency of morphine. CBIO given after formalin challenge also reversed the established pain state to the same degree as prevention. The antihyperalgesic potencies of these DAO inhibitors were highly correlated to their inhibitions of spinal DAO activity. Maximum inhibition of pain by these compounds was approximately 60%, comparable with that of the N-methyl-D-aspartic acid receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), suggesting that a larger portion of formalin-induced tonic pain is "DAO-sensitive," whereas the remaining 40% of tonic pain and acute nociception is "DAO-insensitive." These findings, combined with our previous DAO gene mutation and induction results, indicate spinal DAO mediates both induction and maintenance of formalin-induced tonic pain and further validate spinal DAO as a novel and efficacious target molecule for the treatment of chronic pain.

Metabolism of the neuromodulator D-serine

Over the past years, accumulating evidence has indicated that D-serine is the endogenous ligand for the glycine-modulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. D-Serine is synthesized in glial cells and neurons by the pyridoxal-5' phosphate-dependent enzyme serine racemase, and it is released upon activation of glutamate receptors. The cellular concentration of this novel messenger is regulated by both serine racemase isomerization and elimination reactions, as well as by its selective degradation catalyzed by the flavin adenine dinucleotide-containing flavoenzyme D-amino acid oxidase. Here, we present an overview of the current knowledge of the metabolism of D-serine in human brain at the molecular and cellular levels, with a specific emphasis on the brain localization and regulatory pathways of D-serine, serine racemase, and D-amino acid oxidase. Furthermore, we discuss how D-serine is involved with specific pathological conditions related to N-methyl-D-aspartate receptors over- or down-regulation.

Role of the active site residues arginine-216 and arginine-237 in the substrate specificity of mammalian D-aspartate oxidase

D-aspartate oxidase (DDO) and D-amino acid oxidase (DAO) are flavin adenine dinucleotide-containing flavoproteins that catalyze the oxidative deamination of D-amino acids. Unlike DAO, which acts on several neutral and basic D-amino acids, DDO is highly specific for acidic D-amino acids. Based on molecular modeling and simulated annealing docking analyses, a recombinant mouse DDO carrying two substitutions (Arg-216 to Leu and Arg-237 to Tyr) was generated (R216L-R237Y variant). This variant and two previously constructed single-point mutants of mouse DDO (R216L and R237Y variants) were characterized to investigate the role of Arg-216 and Arg-237 in the substrate specificity of mouse DDO. The R216L-R237Y and R216L variants acquired a broad specificity for several neutral and basic D-amino acids, and showed a considerable decrease in activity against acidic D-amino acids. The R237Y variant, however, did not show any additional specificity for neutral or basic D-amino acids and its activity against acidic D-amino acids was greatly reduced. The kinetic properties of these variants indicated that the Arg-216 residue is important for the catalytic activity and substrate specificity of mouse DDO. However, Arg-237 is, apparently, only marginally involved in substrate recognition, but is important for catalytic activity. Notably, the substrate specificity of the R216L-R237Y variant differed significantly from that of the R216L variant, suggesting that Arg-237 has subsidiary effects on substrate specificity. Additional experiments using several DDO and DAO inhibitors also suggested the involvement of Arg-216 in the substrate specificity and catalytic activity of mouse DDO and that Arg-237 is possibly involved in substrate recognition by this enzyme. Collectively, these results indicate that Arg-216 and Arg-237 play crucial and subsidiary role(s), respectively, in the substrate specificity of mouse DDO.

A lysine conserved in the monoamine oxidase family is involved in oxidation of the reduced flavin in mouse polyamine oxidase

Lysine 315 of mouse polyamine amine oxidase corresponds to a lysine residue that is conserved in the flavoprotein amine oxidases of the monoamine oxidase structural family. In several structures, this lysine residue forms a hydrogen bond to a water molecule that is hydrogen-bonded to the flavin N(5). Mutation of Lys315 in polyamine oxidase to methionine was previously shown to have no effect on the kinetics of the reductive half-reaction of the enzyme (M. Henderson Pozzi, V. Gawandi, P.F. Fitzpatrick, Biochemistry 48 (2009) 1508-1516). In contrast, the mutation does affect steps in the oxidative half-reaction. The k(cat) value is unaffected by the mutation; this kinetic parameter likely reflects product release. At pH 10, the k(cat)/K(m) value for oxygen is 25-fold lower in the mutant enzyme. The k(cat)/K(O2) value is pH-dependent for the wild-type enzyme, decreasing below a pK(a) of 7.0, while this kinetic parameter for the mutant enzyme is pH-independent. This is consistent with the neutral form of Lys315 being required for more rapid flavin oxidation. The solvent isotope effect on the k(cat)/K(O2) value increases from 1.4 in the wild-type enzyme to 1.9 in the mutant protein, and the solvent inventory changes from linear to bowed. The effects of the mutation can be explained by the lysine orienting the bridging water so that it can accept the proton from the flavin N(5) during flavin oxidation. In the mutant enzyme the lysine amine would be replaced by a water chain.

Mechanistic studies of human spermine oxidase: kinetic mechanism and pH effects

In mammalian cells, the flavoprotein spermine oxidase (SMO) catalyzes the oxidation of spermine to spermidine and 3-aminopropanal. Mechanistic studies have been conducted with the recombinant human enzyme. The initial velocity pattern in which the ratio between the concentrations of spermine and oxygen is kept constant establishes the steady-state kinetic pattern as ping-pong. Reduction of SMO by spermine in the absence of oxygen is biphasic. The rate constant for the rapid phase varies with the substrate concentration, with a limiting value (k(3)) of 49 s(-1) and an apparent K(d) value of 48 microM at pH 8.3. The rate constant for the slow step is independent of the spermine concentration, with a value of 5.5 s(-1), comparable to the k(cat) value of 6.6 s(-1). The kinetics of the oxidative half-reaction depend on the aging time after the spermine and enzyme are mixed in a double-mixing experiment. At an aging time of 6 s, the reaction is monophasic with a second-order rate constant of 4.2 mM(-1) s(-1). At an aging time of 0.3 s, the reaction is biphasic with two second-order constants equal to 4.0 and 40 mM(-1) s(-1). Neither is equal to the k(cat)/K(O(2)) value of 13 mM(-1) s(-1). These results establish the existence of more than one pathway for the reaction of the reduced flavin intermediate with oxygen. The k(cat)/K(M) value for spermine exhibits a bell-shaped pH profile, with an average pK(a) value of 8.3. This profile is consistent with the active form of spermine having three charged nitrogens. The pH profile for k(3) shows a pK(a) value of 7.4 for a group that must be unprotonated. The pK(i)-pH profiles for the competitive inhibitors N,N'-dibenzylbutane-1,4-diamine and spermidine show that the fully protonated forms of the inhibitors and the unprotonated form of an amino acid residue with a pK(a) of approximately 7.4 in the active site are preferred for binding.

A single Phe54Tyr substitution improves the catalytic activity and thermostability of Trigonopsis variabilis D-amino acid oxidase

The industrial importance of Trigonopsis variabilisd-amino acid oxidase (TvDAAO) is represented by its biocatalytic oxidative deamination of cephalosporin C (CPC) to yield glutaryl-7-aminocephalosporanic acid (GL-7-ACA). The process has been incorporated into a two-step bioconversion to produce 7-aminocephalosporanic acid, the crucial synthetic nucleus for several semi-synthetic cephalosporin antibiotics. A homology model of TvDAAO indicated that residue F54 is in a close proximity to the in silico docked CPC. Substitution of this F54 to Tyr (F54Y) resulted in 6-fold improvement in k(cat,app) and approximately 2.5-fold increase in K(i) of GL-7-ACA. Heat treatment (55 degrees C, 60 min) did not decrease the activity of F54Y. It is suggested that the Tyr substitution might initiate hydrogen bond formation with the amino group of CPC and facilitate deamination. Faster substrate turnover, reduced GL-7-ACA inhibition and improved thermostability of the F54Y substitution render it a useful candidate in industrial production of semi-synthetic cephems.

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.

Discovery, SAR, and pharmacokinetics of a novel 3-hydroxyquinolin-2(1H)-one series of potent D-amino acid oxidase (DAAO) inhibitors

3-Hydroxyquinolin-2(1H)-one (2) was discovered by high throughput screening in a functional assay to be a potent inhibitor of human DAAO, and its binding affinity was confirmed in a Biacore assay. Cocrystallization of 2 with the human DAAO enzyme defined the binding site and guided the design of new analogues. The SAR, pharmacokinetics, brain exposure, and effects on cerebellum D-serine are described. Subsequent evaluation against the rat DAAO enzyme revealed a divergent SAR versus the human enzyme and may explain the high exposures of drug necessary to achieve significant changes in rat or mouse cerebellum D-serine.

Interaction of D-amino acid oxidase with carbon nanotubes: implications in the design of biosensors

We have investigated the interaction of d-amino acid oxidase (DAAO) with single-walled carbon nanotubes (CNT) by spectroscopic ellipsometry. Dynamic adsorption experiments were performed at different experimental conditions. In addition, the activity of the enzyme adsorbed at different conditions was studied. Our results indicate that DAAO can be adsorbed to CNT at different pH values and concentrations by a combination of hydrophobic and electrostatic interactions. Considering that the highest enzymatic activity was obtained by adsorbing the protein at pH 5.7 and 0.1 mg x mL(-1), our results indicate that DAAO can adopt multiple orientations on the surface, which are ultimately responsible for significant differences in catalytic activity.

Synthesis and biological evaluation of D-amino acid oxidase inhibitors

D-amino acid oxidase (DAAO) catalyzes the oxidation of D-amino acids including d-serine, a full agonist at the glycine site of the NMDA receptor. A series of benzo[ d]isoxazol-3-ol derivatives were synthesized and evaluated as DAAO inhibitors. Among them, 5-chloro-benzo[ d]isoxazol-3-ol (CBIO) potently inhibited DAAO with an IC50 in the submicromolar range. Oral administration of CBIO in conjunction with d-serine enhanced the plasma and brain levels of d-serine in rats compared to the oral administration of d-serine alone.