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

Yeast D-amino acid oxidase: structural basis of its catalytic properties

The 3D structure of the flavoprotein D-amino acid oxidase (DAAO) from the yeast Rhodotorula gracilis (RgDAAO) in complex with the competitive inhibitor anthranilate was solved (resolution 1.9A) and structural features relevant for the overall conformation and for catalytic activity are described. The FAD is bound in an elongated conformation in the core of the enzyme. Two anthranilate molecules are found within the active site cavity; one is located in a funnel forming the entrance, and the second is in contact with the flavin. The anchoring of the ligand carboxylate with Arg285 and Tyr223 is found for all complexes studied. However, while the active site group Tyr238-OH interacts with the carboxylate in the case of the substrate D-alanine, of D-CF(3)-alanine, or of L-lactate, in the anthranilate complex the phenol group rotates around the C2-C3 bond thus opening the entrance of the active site, and interacts there with the second bound anthranilate. This movement serves in channeling substrate to the bottom of the active site, the locus of chemical catalysis. The absence in RgDAAO of the "lid" covering the active site, as found in mammalian DAAO, is interpreted as being at the origin of the differences in kinetic mechanism between the two enzymes. This lid has been proposed to regulate product dissociation in the latter, while the side-chain of Tyr238 might exert a similar role in RgDAAO. The more open active site architecture of RgDAAO is the origin of its much broader substrate specificity. The RgDAAO enzyme forms a homodimer with C2 symmetry that is different from that reported for mammalian D-amino acid oxidase. This different mode of aggregation probably causes the differences in stability and tightness of FAD cofactor binding between the DAAOs from different sources.

Role of tyrosine 238 in the active site of Rhodotorula gracilis D-amino acid oxidase. A site-directed mutagenesis study

Y238, one of the very few conserved residues in the active site of d-amino acid oxidases (DAAO), was mutated to phenylalanine and serine in the enzyme from the yeast Rhodotorula gracilis. The mutated proteins are catalytically competent thus eliminating Tyr238 as an active-site acid/base catalyst. Y238F and Y238S mutants exhibit a threefold slower turnover on d-alanine as substrate, which can be attributed to a slower rate of product release relative to the wild-type enzyme (a change of the rate constants for substrate binding was also evident). The Y238 DAAO mutants have spectral properties similar to those of the wild-type enzyme but the degree of stabilization of the flavin semiquinone and the redox properties in the free form of Y238S are different. The binding of the carboxylic acid competitive inhibitors and the substrate d-alanine are changed only slightly, suggesting that the overall substrate binding pocket remains intact. In agreement with data from the pH dependence of ligand binding and with the protein crystal structure, site-directed mutagenesis results emphasize the importance of residue Y238 in controlling access to the active site instead of a role in the substrate/ligand interaction.

Conversion of the dimeric D-amino acid oxidase from Rhodotorula gracilis to a monomeric form. A rational mutagenesis approach

The relevance of the dimeric state for the structure/function relationships of Rhodotorula gracilis D-amino acid oxidase (RgDAAO) holoenzyme has been investigated by rational mutagenesis. Deletion of 14 amino acids in a surface loop (connecting beta-strands 12 and 13) transforms RgDAAO from a dimeric protein into a stable monomer. The mutant enzyme is still catalytically competent and retains its binding with the FAD coenzyme. Dimerization has been used by this flavoenzyme in evolution to achieve maximal activity, a tighter interaction between the protein moiety and the coenzyme, and higher thermal stability.

Engineering the substrate specificity of D-amino-acid oxidase

The high resolution crystal structure of D-amino-acid oxidase (DAAO) from the yeast Rhodotorula gracilis provided us with the tool to engineer the substrate specificity of this flavo-oxidase. DAAO catalyzes the oxidative deamination of D-amino acids, with the exception of D-aspartate and D-glutamate (which are oxidized by D-aspartate oxidase, DASPO). Following sequence homology, molecular modeling, and simulated annealing docking analyses, the active site residue Met-213 was mutated to arginine. The mutant enzyme showed properties close to those of DASPO (e.g. the oxidation of D-aspartate and the binding of l-tartrate), and it was still active on D-alanine. The presence of an additional guanidinium group in the active site of the DAAO mutant allowed the binding (and thus the oxidation) of D-aspartate, but it was also responsible for a lower catalytic activity on D-alanine. Similar results were also obtained when two additional arginines were simultaneously introduced in the active site of DAAO (M213R/Y238R mutant, yielding an architecture of the active site more similar to that obtained for the DASPO model), but the double mutant showed very low stability in solution. The decrease in maximal activity observed with these DAAO mutants could be due to alterations in the precise orbital alignment required for efficient catalysis, although even the change in the redox properties (more evident in the DAAO-benzoate complex) could play a role. The rational design approach was successful in producing an enzymatic activity with a new, broader substrate specificity, and this approach could also be used to develop DAAO variants suitable for use in biotechnological applications.

Analysis of the roles of amino acid residues in the flavoprotein tryptophan 2-monooxygenase modified by 2-oxo-3-pentynoate: characterization of His338, Cys339, and Cys511 mutant enzymes

The flavoprotein tryptophan 2-monooxygenase catalyzes the oxidative decarboxylation of tryptophan to indoleacetamide. His338, Cys339, and Cys511 of the Pseudomonas savastanoi enzyme were previously identified as possible active-site residues by modification with 2-oxo-3-pentynoate ([G. Gadda, L.J. Dangott, W.H. Johnson Jr., C.P. Whitman, P.F. Fitzpatrick, Biochemistry 38 (1999) 5822-5828]). The H338N, C339A, and C511S enzymes have been characterized to determine the roles of these residues in catalysis. The steady-state kinetic parameters with both tryptophan and methionine decrease only slightly in the case of the H338N and C339A enzymes; the decrease in activity is greater for the C511S enzyme. Only in the case of the C511S enzyme do deuterium kinetic isotope effects on kinetic parameters indicate a significant change in catalytic rates. The structural bases for the effects of the mutations can be interpreted by identification of L-amino acid oxidase and tryptophan monooxygenase as homologous proteins.

Analysis of conserved active site residues in monoamine oxidase A and B and their three-dimensional molecular modeling

Monoamine oxidase (MAO) is a key enzyme responsible for the degradation of serotonin, norepinephrine, dopamine, and phenylethylamine. It is an outer membrane mitochondrial enzyme existing in two isoforms, A and B. We have recently generated 14 site-directed mutants of human MAO A and B, and we found that four key amino acids, Lys-305, Trp-397, Tyr-407, and Tyr-444, in MAO A and their corresponding amino acids in MAO B, Lys-296, Trp-388, Tyr-398, and Tyr-435, play important roles in MAO catalytic activity. Based on the polyamine oxidase three-dimensional crystal structure, it is suggested that Lys-305, Trp-397, and Tyr-407 in MAO A and Lys-296, Trp-388, and Tyr-398 in MAO B may be involved in the non-covalent binding to FAD. Tyr-407 and Tyr-444 in MAO A (Tyr-398 and Tyr-435 in MAO B) may form an aromatic sandwich that stabilizes the substrate binding. Asp-132 in MAO A (Asp-123 in MAO B) located at the entrance of the U-shaped substrate-binding site has no effect on MAO A nor MAO B catalytic activity. The similar impact of analogous mutants in MAO A and MAO B suggests that these amino acids have the same function in both isoenzymes. Three-dimensional modeling of MAO A and B using polyamine oxidase as template suggests that the overall tertiary structure and the active sites of MAO A and B may be similar.

Versatility and specificity in flavoenzymes: control mechanisms of flavin reactivity

Flavoenzymes are characterized by their remarkable versatility and strict specificity. The former can be grasped when flavoenzymes are treated as a whole, while the latter refers to each flavoenzyme in which the broad versatility of flavin is specifically controlled. The versatility stems from the variety of the redox, ionic, and electronic states that the flavin ring system can adopt. Versatility of flavoenzymes is reflected in their classification, which has generally been based on substrates and reactions catalyzed. A different classification is presented according to the number of electrons transferred in the reductive and oxidative half reactions. Specificity of each flavoenzyme is understood in terms of the regulatory mechanism of the broad reactive potentiality of flavin. The elements of this regulatory mechanism include hydrogen-bonding network, electrostatic effect, charge-transfer interaction, positioning between a substrate/ligand and flavin, and modulation of resonance hybridization, each of which is explained with relevant examples provided mainly by studies from the author's group.

Glycine oxidase from Bacillus subtilis. Characterization of a new flavoprotein

Glycine oxidase (GO) is a homotetrameric flavoenzyme that contains one molecule of non-covalently bound flavin adenine dinucleotide per 47 kDa protein monomer. GO is active on various amines (sarcosine, N-ethylglycine, glycine) and d-amino acids (d-alanine, d-proline). The products of GO reaction with various substrates have been determined, and it has been clearly shown that GO catalyzes the oxidative deamination of primary and secondary amines, a reaction similar to that of d-amino acid oxidase, although its sequence homology is higher with enzymes such as sarcosine oxidase and N-methyltryptophane oxidase. GO shows properties that are characteristic of the oxidase class of flavoproteins: it stabilizes the anionic flavin semiquinone and forms a reversible covalent flavin-sulfite complex. The approximately 300 mV separation between the two FAD redox potentials is in accordance with the high amount of the anionic semiquinone formed on photoreduction. GO can be distinguished from d-amino acid oxidase by its low catalytic efficiency and high apparent K(m) value for d-alanine. A number of active site ligands have been identified; the tightest binding is observed with glycolate, which acts as a competitive inhibitor with respect to sarcosine. The presence of a carboxylic group and an amino group on the substrate molecule is not mandatory for binding and catalysis.

Overexpression of a recombinant wild-type and His-tagged Bacillus subtilis glycine oxidase in Escherichia coli

We have cloned the gene coding for the Bacillus subtilis glycine oxidase (GO), a new flavoprotein that oxidizes glycine and sarcosine to the corresponding alpha-keto acid, ammonia and hydrogen peroxide. By inserting the DNA encoding for GO into the multiple cloning site of the expression vector pT7.7 we produced a recombinant plasmid (pT7-GO). The pT7-GO encodes a fully active fusion protein with six additional residues at the N-terminus of GO (MARIRA). In BL21(DE3)pLysS Escherichia coli cells, and under optimal isopropyl thio-beta-D-galactoside induction conditions, soluble and active chimeric GO was expressed up to 1.14 U g(-1) of cell (and a fermentation yield of 3.82 U x L(-1) of fermentation broth). An N-terminal His-tagged protein (HisGO) was also successfully expressed in E. coli as a soluble protein and a fully active holoenzyme. HisGO represents approximately 3.9% of the total soluble protein content of the cell. The His-tagged GO was purified in a single step by nickel-chelate chromatography to a specific activity of 1.06 U x mg(-1) protein at 25 degrees C and with a yield of 98%. The characterization of the purified enzyme showed that GO is a homotetramer of approximately 180 kDa with the spectral properties typical of flavoproteins. GO exhibits good thermal stability, with a Tm of 46 degrees C after 30 min incubation; its stability is maximal in the 7.0-8.5 pH range. A comparison of amino-acid sequence and substrate specificity indicates that GO has similarities to other flavoenzymes acting on primary amines and on D-amino acids.

Identification and role of ionizing functional groups at the active center of Rhodotorula gracilis D-amino acid oxidase

D-Amino acid oxidase (DAAO) is a flavoprotein oxidase that catalyzes the oxidation of amino acids and produces ketoacids and H(2)O(2). The rate of product release from reduced DAAO from Rhodotorula gracilis is pH dependent and reflects a pK(a) of approximately 9.3. Binding of benzoate and 3,3,3-trifluoro-D-alanine to wild-type and Y238F-DAAO is also pH dependent (pK(a)=9.8+/-0.1 and 9.05+/-0.1, respectively for benzoate binding). However, binding of benzoate to Y223F-DAAO is pH independent, indicating the pK(a) is due to Y223-OH. This latter residue is thus involved in substrate binding, and probably is the group that governs product release. In contrast to this, the second active site tyrosine, Y238, has little influence on ligand binding.

pH and kinetic isotope effects in d-amino acid oxidase catalysis

The effects of pH, solvent isotope, and primary isotope replacement on substrate dehydrogenation by Rhodotorula gracilis d-amino acid oxidase were investigated. The rate constant for enzyme-FAD reduction by d-alanine increases approximately fourfold with pH, reflecting apparent pKa values of approximately 6 and approximately 8, and reaches plateaus at high and low pH. Such profiles are observed in all presteady-state and steady-state kinetic experiments, using both d-alanine and d-asparagine as substrates, and are inconsistent with the operation of a base essential to catalysis. A solvent deuterium isotope effect of 3.1 +/- 1.1 is observed on the reaction with d-alanine at pH 6; it decreases to 1.2 +/- 0.2 at pH 10. The primary substrate isotope effect on the reduction rate with [2-D]d-alanine is 9.1 +/- 1.5 at low and 2.3 +/- 0.3 at high pH. At pH 6.0, the solvent isotope effect is 2.9 +/- 0.8 with [2-D]d-alanine, and the primary isotope effect is 8.4 +/- 2.4 in D2O. Thus, primary and solvent kinetic isotope effects (KIEs) are independent of the presence of the other isotope, i.e. the 'double' kinetic isotope effect is the product of the individual KIEs, consistent with a transition state in which rupture of the two bonds of the substrate to hydrogen is concerted. These results support a hydride transfer mechanism for the dehydrogenation reaction in d-amino acid oxidase and argue against the occurrence of any intermediates in the process. A pKa,app of approximately 8 is interpreted to arise from the microscopic ionization of the substrate amino acid alpha-amino group, but also includes contributions from kinetic parameters.

X-ray structure of 12-oxophytodienoate reductase 1 provides structural insight into substrate binding and specificity within the family of OYE

BACKGROUND

12-Oxophytodienoate reductase (OPR) is a flavin mononucleotide (FMN)-dependent oxidoreductase in plants that belongs to the family of Old Yellow Enzyme (OYE). It was initially characterized as an enzyme involved in the biosynthesis of the plant hormone jasmonic acid, where it catalyzes the reduction of the cyclic fatty acid derivative 9S,13S-12-oxophytodienoate (9S,13S-OPDA) to 1S,2S-3-oxo-2(2'[Z]-pentenyl)-cyclopentane-1-octanoate. Several isozymes of OPR are now known that show different stereoselectivities with regard to the four stereoisomers of OPDA.

RESULTS

Here, we report the high-resolution crystal structure of OPR1 from Lycopersicon esculentum and its complex structures with the substrate 9R,13R-OPDA and with polyethylene glycol 400. OPR1 crystallizes as a monomer and folds into a (betaalpha)(8) barrel with an overall structure similar to OYE. The cyclopentenone ring of 9R,13R-OPDA is stacked above the flavin and activated by two hydrogen bonds to His187 and His190. The olefinic bond is properly positioned for hydride transfer from the FMN N(5) and proton transfer from Tyr192 to Cbeta and Calpha, respectively. Comparison of the OPR1 and OYE structures reveals striking differences in the loops responsible for binding 9R,13R-OPDA in OPR1.

CONCLUSIONS

Despite extensive biochemical characterization, the physiological function of OYE still remains unknown. The similar catalytic cavity structures and the substrate binding mode in OPR1 strongly support the assumption that alpha,beta-unsaturated carbonyl compounds are physiological substrates of the OYE family. The specific binding of 9R,13R-OPDA by OPR1 explains the experimentally observed stereoselectivity and argues in favor of 9R,13R-OPDA or a structurally related oxylipin as natural substrate of OPR1.

Evidence for an essential arginine in the flavoprotein nitroalkane oxidase

The flavoprotein nitroalkane oxidase from the fungus Fusarium oxysporum catalyzes the oxidative denitrification of primary or secondary nitroalkanes to yield the respective aldehydes or ketones, hydrogen peroxide and nitrite. The enzyme is inactivated in a time-dependent fashion upon treatment with the arginine-directed reagents phenylglyoxal, 2,3-butanedione, and cyclohexanedione. The inactivation shows first order kinetics with all reagents. Valerate, a competitive inhibitor of the enzyme, fully protects the enzyme from inactivation, indicating that modification is active site directed. The most rapid inactivation is seen with phenylglyoxal, with a k(inact) of 14.3 +/- 1.1 M(-1) min(-1) in phosphate buffer at pH 7.3 and 30 degrees C. The lack of increase in the enzymatic activity of the phenylglyoxal-inactivated enzyme after removing the unreacted reagent by gel filtration is consistent with inactivation being due to covalent modification of the enzyme. A possible role for an active site arginine in substrate binding is discussed.

Artificial flavin receptors: effects of hydrogen bonding on redox properties of a flavin mimic

This review describes the roles of hydrogen bonding on the redox properties of a flavin mimic by using artificial flavin receptors. The receptors exploited are melamine derivatives bearing guanidinium ion(s) that strongly bind 6-azaflavin through five or seven hydrogen bonds involving N(1), C(2)=O, N(3)-H, C(4)=O, N(5), and N(6) positions in CHCl3 and CHCl3-acetonitrile. It has been shown that receptors are quite useful for examination of the hydrogen bonding effects on the redox potential, stability of the anionic semiquinone radical, and the oxidation activity of 6-azaflavin. The functionalized receptors have a substrate- or a metal-binding site have been shown to facilitate the reactions by forming the noncovalent assemblies. A possibility as an apoprotein model of the receptors has been mentioned also.

D-Aspartate oxidase and D-amino acid oxidase are localised in the peroxisomes of terrestrial gastropods

D-Aspartate oxidase and D-amino acid oxidase were found in high activity in the tissues of representative species of terrestrial gastropods. Analytical subcellular fractionation demonstrated that both of these oxidases co-localised with the peroxisome markers, acyl-CoA oxidase and catalase, in the digestive gland homogenate. Electron microscopy of peak peroxisome fractions showed particles of uniform size with generally well preserved variably electron-dense matrices bounded by an apparently single limiting membrane. Many of the particles exhibited a core region of enhanced electron density. Catalase cytochemistry of peak fractions confirmed the peroxisome identity of the organelles. Peroxisome-enriched subcellular fractions were used to investigate the properties of gastropod D-aspartate oxidase and D-amino acid oxidase activities. The substrate and inhibitor specificities of the two activities demonstrated that two distinct enzymes were present analogous to, but not identical to, the equivalent mammalian peroxisomal enzymes.

The catalytic role of tyrosine 254 in flavocytochrome b2 (L-lactate dehydrogenase from baker's yeast). Comparison between the Y254F and Y254L mutant proteins

Flavocytochrome b2 catalyses the oxidation of L-lactate to pyruvate in yeast mitochondrial intermembrane space. Its flavoprotein domain is a member of a family of FMN-dependent 2-hydroxy-acid-oxidizing enzymes. Numerous solution studies suggest that the first step of the reaction consists of proton abstraction from lactate C2, leading to a carbanion that subsequently yields electrons to FMN. The crystal structure suggests that the enzyme base is His373, and that Tyr254 may be hydrogen bonded to the substrate hydroxyl. Studies carried out with the Y254F mutant [Dubois, J., Chapman, S.K., Mathews, F.S., Reid, G.A. & Lederer, F. (1990) Biochemistry 29, 6393-6400] showed that Tyr254 does not act as a base but stabilizes the transition state. As the mutation did not induce any change in substrate affinity, the question of the existence of the hydrogen bond in the Michaelis complex remained open. Similar results with glycolate oxidase, mutated at the same position, led to the suggestion that these enzymes actually operate via a hydride transfer mechanism [Macheroux, P., Kieweg, V., Massey, V., Soderlind, E., Stenberg, K. & Lindqvist, Y. (1993) Eur. J. Biochem. 213, 1047-1054]. In the present work, we have re-investigated the matter by analysing the properties of a Y254L mutant flavocytochrome b2, as well as the behaviour of the Y254F enzyme with two substrates other than lactate, and a series of inhibitors. The Y254L protein is less efficient with L-lactate than the wild-type enzyme by a factor of 500, but the substrate affinity is unchanged. In contrast, L-phenyllactate and mandelate, poor substrates (the latter acting more as an inhibitor), exhibit an increased affinity. In addition, the Y254L mutant enzyme is more efficient with phenyllactate than lactate as a substrate. In order to rationalize these observations, we have modelled phenyllactate and mandelate in the active site, using previously described modelling experiments with lactate as a starting point. The results indicate that mandelate cannot bind in an orientation allowing proton abstraction by His373, due to steric interference by the side chains of Ala198 and Leu230. It might possibly adopt a binding mode as proposed previously for lactate, which leads to a hydride transfer and with which the 198 and 230 side chains do not interfere. However, other researchers [Sinclair, R., Reid, G.A. & Chapman, S.K. (1998) Biochem. J. 333, 117-120] showed that A198G, L230A and A198G/L230A mutant enzymes exhibit a strongly improved mandelate dehydrogenase activity. These results indicate that relief of the steric crowding facilitates catalysis by enabling a better mandelate orientation at the active site, suggesting that its productive binding mode is similar to that proposed for lactate in the carbanion mechanism. The modelling studies therefore support the hypothesis of a carbanion mechanism for all substrates. In addition, we present the effect of the two mutations at position 254 on the binding of a number of competitive inhibitors (such as sulfite, D-lactate, propionate) and of inhibitors that are known to bind at the active site both when the flavin is oxidized and when it is in the semiquinone state (propionate, oxalate and L-lactate at high concentrations). Unexpectedly, the results indicate that the integrity of Tyr254 is necessary for the binding of these inhibitors at the semiquinone stage.

Deflavination of flavo-oxidases by nucleophilic reagents

Using spectroscopic techniques we studied the effect of the nucleophilic reagents cyanide, cyanate and thiocyanate on three flavo-oxidases namely alcohol oxidase (AO), glucose oxidase (GOX) and D-amino acid oxidase (DAOX). All three ions, added at concentrations in the mM range, caused release of the flavin adenine dinucleotide (FAD) co-factors from the enzyme molecules. In the case of AO this was accompanied by significant conformational perturbations, which was not observed for GOX and DAOX. As suggested from fluorescence, absorption and circular dichroism spectral changes at least one phenolic hydroxyl group became ionized upon FAD release from AO and a new class of Trp residues, fluorescent only in apo-AO protein, was demasked.

Substrate dehydrogenation by flavoproteins

Enzymes with tightly bound FMN or FAD as cofactor catalyze the oxidation of a wide range of substrates. The chemical versatility of the isoalloxazine ring provides these enzymes with a range of potential mechanisms. Recent progress in elucidating the mechanisms of oxidation of organic substrates by flavoenzymes is described, focusing on the oxidation of alcohols, amino and hydroxy acids, amines, and nitroalkanes. With each family of enzymes, an attempt is made to integrate mechanistic, structural, and biomimetic data into a common catalytic mechanism.

Identification, purification, and characterization of iminodiacetate oxidase from the EDTA-degrading bacterium BNC1

Microbial degradation of synthetic chelating agents, such as EDTA and nitrilotriacetate (NTA), may help immobilizing radionuclides and heavy metals in the environment. The EDTA- and NTA-degrading bacterium BNC1 uses EDTA monooxygenase to oxidize NTA to iminodiacetate (IDA) and EDTA to ethylenediaminediacetate (EDDA). IDA- and EDDA-degrading enzymes have not been purified and characterized to date. In this report, an IDA oxidase was purified to apparent homogeneity from strain BNC1 by using a combination of eight purification steps. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single protein band of 40 kDa, and by using size exclusion chromatography, we estimated the native enzyme to be a homodimer. Flavin adenine dinucleotide was determined as its prosthetic group. The purified enzyme oxidized IDA to glycine and glyoxylate with the consumption of O2. The temperature and pH optima for IDA oxidation were 35 degrees C and 8, respectively. The apparent Km for IDA was 4.0 mM with a kcat of 5.3 s(-1). When the N-terminal amino acid sequence was determined, it matched exactly with that encoded by a previously sequenced hypothetical oxidase gene of BNC1. The gene was expressed in Escherichia coli, and the gene product as a C-terminal fusion with a His tag was purified by a one-step nickel affinity chromatography. The purified fusion protein had essentially the same enzymatic activity and properties as the native IDA oxidase. IDA oxidase also oxidized EDDA to ethylenediamine and glyoxylate. Thus, IDA oxidase is likely the second enzyme in both NTA and EDTA degradation pathways in strain BNC1.

The x-ray structure of D-amino acid oxidase at very high resolution identifies the chemical mechanism of flavin-dependent substrate dehydrogenation

Flavin is one of the most versatile redox cofactors in nature and is used by many enzymes to perform a multitude of chemical reactions. d-Amino acid oxidase (DAAO), a member of the flavoprotein oxidase family, is regarded as a key enzyme for the understanding of the mechanism underlying flavin catalysis. The very high-resolution structures of yeast DAAO complexed with d-alanine, d-trifluoroalanine, and l-lactate (1.20, 1.47, and 1.72 A) provide strong evidence for hydride transfer as the mechanism of dehydrogenation. This is inconsistent with the alternative carbanion mechanism originally favored for this type of enzymatic reaction. The step of hydride transfer can proceed without involvement of amino acid functional groups. These structures, together with results from site-directed mutagenesis, point to orbital orientation/steering as the major factor in catalysis. A diatomic species, proposed to be a peroxide, is found at the active center and on the Re-side of the flavin. These results are of general relevance for the mechanisms of flavoproteins and lead to the proposal of a common dehydrogenation mechanism for oxidases and dehydrogenases.

Redox potentials and their pH dependence of D-amino-acid oxidase of Rhodotorula gracilis and Trigonopsis variabilis

The redox potentials and pH characteristics of D-amino-acid oxidase (EC 1.4.3.3; DAAO) from the yeast Rhodotorula gracilis and Trigonopsis variabilis were measured in the pH range 6.5-8.5 at 15 degrees C. In the free enzyme form, the anionic red semiquinone is quantitatively formed in both DAAOs, indicating that a two single-electron transfer mechanism is active. The semiquinone species is also thermodynamically stable, as indicated by the large separation of the single-electron transfer potentials. The first electron potential is pH-independent, while the second electron transfer is pH-dependent exhibiting a approximately -60 mV/pH unit slope, consistent with a one-electron/one-proton transfer. In the presence of the substrate analogue benzoate, the two-electron transfer is the thermodynamically favoured process for both DAAOs, with only a quantitative difference in the stabilization of the anionic semiquinone. Clearly binding of the substrate (or substrate analogue) modulates the redox properties of the two enzymes. In both cases, in the presence and absence of benzoate, the slope of Em vs. pH (-30 mV/pH unit) corresponds to an overall two-electron/one-proton transfer in the reduction to yield the anionic reduced flavin. This behaviour is similar to that reported for DAAO from pig kidney. The differences in potentials and the stability of the semiquinone intermediate measured for the three DAAOs probably stem from different isoalloxazine environments. In the case of R. gracilis DAAO, the low stability of the semiquinone form in the DAAO-benzoate complex can be explained by the shift in position of the side chain of Arg285 following substrate analogue binding.

Expression of Trigonopsis variabilis D-amino acid oxidase gene in Escherichia coli and characterization of its inactive mutants

The D-amino acid oxidase cDNA gene (daao) of Trigonopsis variabilis was prepared by reverse transcriptase-polymerase chain reaction (PCR) and cloned into Escherichia coli expression vector, pTrc99A, under the control of tac promoter. Expression of daao gene significantly affected the growth and morphology of E. coli. The highest D-amino acid oxidase (DAAO) activity was 705 U (mg of protein)(-)(1), which was about 12-fold higher than that of D-alanine-induced T. variabilis. The DAAO protein exhibited activity on native-PAGE and had a M(r)value of 39.3 kDa. We also constructed an expression plasmid, pKm-DAAO, in which kanamycin instead of ampicillin was used as the selective marker. High-performance liquid chromatography (HPLC) analysis demonstrated that cephalosporin C could be converted to 7-glutarylcephalosporanic acid by cell-free extract of E. coli harboring pKm-DAAO. Four inactive DAAO mutants were obtained by error-prone PCR. Sequence analysis of these four DAAO mutants indicated the occurrence of mutations at Val-167, Pro-291, Pro-309, and Ala-343 residues. The His(6)-tagged DAAOs were expressed in E. coli and purified by nickel ion affinity chromatography. The results showed that all DAAO mutants lost their enzymatic activities and characteristic adsorption spectra for flavoenzyme. Based on the crystal structure of a homologous protein, pig DAAO, it is suggested that these four residues may play essential structural roles in DAAO conformation, thereby influencing DAAO's catalytic activity.

Aryl-alcohol oxidase protein sequence: a comparison with glucose oxidase and other FAD oxidoreductases

Aryl-alcohol oxidase (AAO), an FAD-dependent enzyme involved in lignin degradation, has been cloned from Pleurotus eryngii. The AAO protein is composed of 593 amino acids, 27 of which form a signal peptide. It shows 33% sequence identity with glucose oxidase from Aspergillus niger and lower homology with other oxidoreductases. The predicted secondary structures of both enzymes are very similar. For AAO, it is predicted to contain 13 putative alpha-helices and two major beta-sheets, each of the putative beta-sheets formed by six beta-strands. The ADP binding site and the signature-2 consensus sequence of the glucose-methanol-choline (GMC) oxidoreductases were also present. Moreover, residues potentially involved in catalysis and substrate binding were identified in the vicinity of the flavin ring. They include two histidines (H502 and H546) and several aromatic residues (Y78, Y92 and F501), as reported in other FAD oxidoreductases.

The structure of L-amino acid oxidase reveals the substrate trajectory into an enantiomerically conserved active site

The structure of L-amino acid oxidase (LAAO) from Calloselasma rhodostoma has been determined to 2.0 A resolution in the presence of two ligands: citrate and o-aminobenzoate (AB). The protomer consists of three domains: an FAD-binding domain, a substrate-binding domain and a helical domain. The interface between the substrate-binding and helical domains forms a 25 A long funnel, which provides access to the active site. Three AB molecules are visible within the funnel of the LAAO-AB complex; their orientations suggest the trajectory of the substrate to the active site. The innermost AB molecule makes hydrogen bond contacts with the active site residues, Arg90 and Gly464, and the aromatic portion of the ligand is situated in a hydrophobic pocket. These contacts are proposed to mimic those of the natural substrate. Comparison of LAAO with the structure of mammalian D-amino acid oxidase reveals significant differences in their modes of substrate entry. Furthermore, a mirror-symmetrical relationship between the two substrate-binding sites is observed which facilitates enantiomeric selectivity while preserving a common arrangement of the atoms involved in catalysis.

Guinea pig D-amino-acid oxidase cDNA and phylogenetic position

The nucleotide sequence of cDNA that encodes guinea pig D-amino-acid oxidase (DAO) was determined. The cDNA consisted of 1,399 nucleotides and a poly(A) tail. The cDNA encodes 347 amino acid residues. In contrast to the hamster, rat, and mouse DAOs, guinea pig DAO had the 25th amino acid residue. The homology in amino acid sequences between the guinea pig DAO and the rodent DAOs was not high in comparison to the homology in amino acid sequences between the guinea pig DAO and DAOs of humans, pigs and rabbits. The phylogenetic position of the guinea pig varied depending on the source of sequences (amino acids or nucleotides) and the methods of phylogenetic tree construction. These results suggest that the guinea pig is not a simple rodent.

Role of arginine 285 in the active site of Rhodotorula gracilis D-amino acid oxidase. A site-directed mutagenesis study

Arg(285), one of the very few conserved residues in the active site of d-amino acid oxidases, has been mutated to lysine, glutamine, aspartate, and alanine in the enzyme from the yeast Rhodotorula gracilis (RgDAAO). The mutated proteins are all catalytically competent. Mutations of Arg(285) result in an increase ( approximately 300-fold) of K(m) for the d-amino acid and in a large decrease ( approximately 500-fold) of turnover number. Stopped-flow analysis shows that the decrease in turnover is paralleled by a similar decrease in the rate of flavin reduction (k(2)), the latter still being the rate-limiting step of the reaction. In agreement with data from the protein crystal structure, loss of the guanidinium group of Arg(285) in the mutated DAAOs drastically reduces the binding of several carboxylic acids (e.g. benzoate). These results highlight the importance of this active site residue in the precise substrate orientation, a main factor in this redox reaction. Furthermore, Arg(285) DAAO mutants have spectral properties similar to those of the wild-type enzyme, but show a low degree of stabilization of the flavin semiquinone and a change in the redox properties of the free enzyme. From this, we can unexpectedly conclude that Arg(285) in the free enzyme form is involved in the stabilization of the negative charge on the N(1)-C(2)=O locus of the isoalloxazine ring of the flavin. We also suggest that the residue undergoes a conformational change in order to bind the carboxylate portion of the substrate/ligand in the complexed enzyme.

Effect of hydrogen peroxide on d-amino acid oxidase from Rhodotorula gracilis

D-amino acid oxidase from Rhodotorula gracilis is a FAD-containing enzyme that belongs to the oxidase class that is characterized by the ability of the reduced flavin to react quickly with oxygen, yielding hydrogen peroxide and the oxidized cofactor. Hydrogen peroxide, necessary for the production of glutaryl-7-ACA from cephalosporin C had a deleterious effect on the enzyme. H(2)O(2) induced the oxidation of tryptophan and cysteine residues of the protein that could be involved in the dimerization process, required for the attainment of a fully competent enzyme. H(2)O(2) had also a kinetic effect on the reaction catalyzed by D-amino acid oxidase. It was a pure noncompetitive inhibitor; the corresponding inhibition constants were K(is) = 0.52 mM and K(ii) = 0.70 mM.

Design and properties of human D-amino acid oxidase with covalently attached flavin

An "artificial flavinylation" approach was developed to replace a native noncovalent flavin prosthetic group with a covalently attached flavin analogue in recombinant human d-amino acid oxidase. The protein residue Gly-281 was replaced with Cys by site-directed mutagenesis, followed by reaction between mutated apoenzyme and the thiol-reactive flavin analogue, 8-methylsulfonyl FAD. The stoichiometric process of flavin attachment was accompanied by gain in enzymatic activity, reaching up to 26% activity of the recombinant native enzyme. The steady-state kinetic data together with the results of limited proteolysis and benzoate-binding studies suggest that, although mutation perturbs protein structural and catalytic properties, the flavinylation alone does not have any negative impact. We conclude that, despite the implemented restraints on its mobility, the covalently attached flavin is properly positioned within the protein active site and acts efficiently during d-amino acid oxidase catalytic turnover.

Design and properties of human D-amino acid oxidase with covalently attached flavin

An "artificial flavinylation" approach was developed to replace a native noncovalent flavin prosthetic group with a covalently attached flavin analogue in recombinant human d-amino acid oxidase. The protein residue Gly-281 was replaced with Cys by site-directed mutagenesis, followed by reaction between mutated apoenzyme and the thiol-reactive flavin analogue, 8-methylsulfonyl FAD. The stoichiometric process of flavin attachment was accompanied by gain in enzymatic activity, reaching up to 26% activity of the recombinant native enzyme. The steady-state kinetic data together with the results of limited proteolysis and benzoate-binding studies suggest that, although mutation perturbs protein structural and catalytic properties, the flavinylation alone does not have any negative impact. We conclude that, despite the implemented restraints on its mobility, the covalently attached flavin is properly positioned within the protein active site and acts efficiently during d-amino acid oxidase catalytic turnover.

On the interpretation of quantitative structure-function activity relationship data for lactate oxidase

The native flavin, FMN, has been removed from the l-lactate oxidase of Aerococcus viridans, and the apoprotein reconstituted with 12 FMN derivatives with various substituents at the flavin 6- and 8-positions. Impressive linear relationships are exhibited between the sum of the Hammett final sigma(para) and final sigma(ortho) parameters and the redox potentials of the free flavins, and between the redox potentials of the free and enzyme-bound flavins. Rapid reaction kinetics studies of the reconstituted enzymes with the substrates l-lactate and l-mandelate show an increase in the reduction rate constant with increasing redox potential, except that, with lactate, a limiting rate constant of approximately 700 s(-1) is obtained with flavins of high potential. Similar breakpoints are found in plots of the rate constants for flavin N5-sulfite adduct formation and for the reaction of the reduced enzymes with molecular oxygen. These results are interpreted in terms of a two-step equilibrium preceding the chemical reaction step, in which the second equilibrium step provides an upper limit to the rate with which the particular substrate or ligand is positioned with the flavin in the correct fashion for the observed chemical reaction to occur. The relationship of rate constants for flavin reduction and N5-sulfite adduct formation with flavin redox potential below the observed breakpoint indicate development of significant negative charge in the transition states of the reactions. In the case of reduction by substrate, the results are consistent either with a hydride transfer mechanism or with the so called "carbanion" mechanism, in which the substrate alpha-proton is abstracted by an enzyme base protected from exchange with solvent. These conclusions are supported by substrate alpha-deuterium isotope effects and by solvent viscosity effects on sulfite binding.

Flavoenzymes: diverse catalysts with recurrent features

Many biochemical processes exploit the extraordinary versatility of flavoenzymes and their flavin cofactors. Flavoproteins are now known to have a variety of folding topologies but a careful examination of their structures suggests that there are recurrent features in their catalytic apparatus. The flavoenzymes that catalyse dehydrogenation reactions share a few invariant features in the hydrogen-bond interactions between their protein and flavin constituents. Similarly, the positioning of the reactive part of the substrate with respect to the cofactor is generally conserved. Modulation of substrate and cofactor reactivity and exact positioning of the substrate are key elements in the mode of action of these enzymes.

Studies on the reaction mechanism of Rhodotorula gracilis D-amino-acid oxidase. Role of the highly conserved Tyr-223 on substrate binding and catalysis

We have studied D-amino-acid oxidase from Rhodotorula gracilis by site-directed mutagenesis for the purpose of determining the presence or absence of residues having a possible role in acid/base catalysis. Tyr-223, one of the very few conserved residues among D-amino-acid oxidases, has been mutated to phenylalanine and to serine. Both mutants are active catalysts in turnover with D-alanine, and they are reduced by D-alanine slightly faster than wild-type enzyme. The Tyr-223 --> Phe mutant is virtually identical to the wild-type enzyme, whereas the Tyr-223 --> Ser mutant exhibits 60-fold slower substrate binding and at least 800-fold slower rate of product release relative to wild-type. These data eliminate Tyr-223 as an active-site acid/base catalyst. These results underline the importance of Tyr-223 for substrate binding and exemplify the importance of steric interactions in RgDAAO catalysis.

Identification of a novel mycobacterial histone H1 homologue (HupB) as an antigenic target of pANCA monoclonal antibody and serum immunoglobulin A from patients with Crohn's disease

pANCA is a marker antibody associated with inflammatory bowel disease (IBD), including most patients with ulcerative colitis and a subset with Crohn's disease. This study addressed the hypothesis that pANCA reacts with an antigen(s) of microbial agents potentially relevant to IBD pathogenesis. Using a pANCA monoclonal antibody, we have previously identified the C-terminal basic random-coil domain of histone H1 as a pANCA autoantigen. BLAST analysis of the peptide databases revealed H1 epitope homologues in open reading frames of the Mycobacterium tuberculosis genome. Western analysis of extracts from six mycobacterial species directly demonstrated reactivity to a single, conserved approximately 32-kDa protein. Direct protein sequencing, followed by gene cloning, revealed a novel 214-amino-acid protein, an iron-regulated protein recently termed HupB. Sequence analysis demonstrated its homology with the mammalian histone H1 gene family, and recombinant protein expression confirmed its reactivity with the 5-3 pANCA monoclonal antibody. Binding activity of patient serum immunoglobulin G (IgG) to HupB did not correlate with reactivity to histone H1 or pANCA, indicating the complex character of the pANCA antigen. However, anti-HupB IgA was strongly associated with Crohn's disease (P < 0.001). These findings indicate that the 5-3 pANCA monoclonal antibody detects a structural domain recurrent among mycobacteria and cross-reactive with a DNA-binding domain of histone H1. The association of HupB-binding serum IgA with IBD provides new evidence for the association of a mycobacterial species with Crohn's disease.

D-aspartate oxidase is present in ovaries, eggs and embryos but not in testis of Xenopus laevis

D-aspartate oxidase (DASPO) is an FAD dependent flavoprotein which catalyzes the oxidative deamination of D-aspartate using oxygen as electron acceptor. D-aspartate and DASPO are supposed to be involved in the regulation of the central nervous system and in the animal development. This manuscript describes for the first time the presence of DASPO in Xenopus laevis fertilized eggs and embryos and suggests a different tissue distribution of this enzyme in adult male and female animals. In particular, by means of 2D-electrophoresis and affinity purified specific anti-DASPO antibodies, the enzyme was localized in fertilized eggs of X. laevis and in ovaries of adult animals but it was shown to be absent in the testis suggesting a gender specific expression. The protein from Xenopus ovaries has been purified by means of immunoprecipitation and it has M(r) of 30 kDa and pI of 8.1.

Evolution of protein function, from a structural perspective

The recent growth in structural data, and ensuing analyses, have revealed the structural and functional versatility of protein families. With respect to enzymes, local active-site mutations, variations in surface loops and recruitment of additional domains accommodate the diverse substrate specificities and catalytic activities observed within several superfamilies. Conversely, some functions have more than one structural solution, having evolved independently several times during evolution. Combined with the existence of multi-functional genes, which have arisen by gene recruitment, these phenomena must be considered in the process of genome annotation.

Isolation and structural characterization of a cytotoxic L-amino acid oxidase from Agkistrodon contortrix laticinctus snake venom: preliminary crystallographic data

We have purified a cytotoxic L-amino acid oxidase (LAO) from Agkistrodon contortrix laticinctus snake venom by means of Superdex-200 gel filtration, followed by phenyl-Sepharose CL-4B chromatography. The purified enzyme (ACL LAO) is a dimer on gel filtration, with a M(r) of 60,000 for the monomer as estimated by SDS-PAGE. LAO activity was tested against 15 amino acids, but only 9 were oxidized by the enzyme, suggesting that it presents some degree of specificity. ACL LAO has apoptosis-inducing activity in an HL-60 cell culture assay. After 24 h treatment with 25 micrograms/ml of ACL LAO, the typical DNA fragmentation pattern of apoptotic cells was observed on agarose gel electrophoresis. NMR analysis showed the presence of a flavin mononucleotide prosthetic group. To solve its 3-D structure, crystals of the purified protein were grown in 0.1 M Tris-HCl, pH 8.5, and 2 M (NH(4))(2)SO(4). Diffraction data collected to 3.5 A showed that the protein crystallized in the tetragonal system, with unit cell a = b = 103.22 A, c = 183.45 A. This is the first report of preliminary crystallization data for a snake venom L-amino acid oxidase.

Production of a new D-amino acid oxidase from the fungus Fusarium oxysporum

The fungus Fusarium oxysporum produced a D-amino acid oxidase (EC 1. 4.3.3) in a medium containing glucose as the carbon and energy source and ammonium sulfate as the nitrogen source. The specific D-amino acid oxidase activity was increased up to 12.5-fold with various D-amino acids or their corresponding derivatives as inducers. The best inducers were D-alanine (2.7 microkat/g of dry biomass) and D-3-aminobutyric acid (2.6 microkat/g of dry biomass). The addition of zinc ions was necessary to permit the induction of peroxisomal D-amino acid oxidase. Bioreactor cultivations were performed on a 50-liter scale, yielding a volumetric D-amino acid oxidase activity of 17 microkat liter(-1) with D-alanine as an inducer. Under oxygen limitation, the volumetric activity was increased threefold to 54 microkat liter(-1) (3,240 U liter(-1)).

Covalent flavinylation of L-aspartate oxidase from Escherichia coli using N6-(6-carboxyhexyl)-FAD succinimidoester

L-Aspartate oxidase is a flavoprotein catalyzing the first step in the de novo biosynthesis of pyridine nucleotides in E. coli. Binding of FAD to L-aspartate oxidase is relatively weak (K(d) 6.7 x 10(-7) M), resulting in partial loss of the coenzyme under many experimental conditions. Only the three-dimensional structure of the apo-enzyme has been obtained so far. In order to probe the flavin-binding site of the enzyme, apo-L-aspartate oxidase has been reacted with N6-(6-carboxyhexyl)-FAD succinimidoester. The structural characterization of the resulting N6-(6-carbamoylxyhexyl)FAD-L-aspartate oxidase shows the covalent incorporation of 1 FAD-analog/monomer. Residue Lys38 was identified as the target of the covalent modification. N6-(6-carbamoylxyhexyl)-FAD-L-aspartate oxidase shows only 2% catalytic activity as compared to the native enzyme. Comparison of some properties of the flavinylated and native enzymes suggests that, although the flavin is covalently bound to the former in the region predicted from molecular modeling studies, the microenvironment around the isoallossazine is different in the two forms.

Structure of L-aspartate oxidase: implications for the succinate dehydrogenase/fumarate reductase oxidoreductase family

BACKGROUND

Given the vital role of NAD+ in cell metabolism, the enzymes involved in bacterial de novo NAD+ biosynthesis are possible targets for drug design against pathogenic bacteria. The first reaction in the pathway is catalysed by L-aspartate oxidase (LASPO), a flavoenzyme that converts aspartate to iminoaspartate using either molecular oxygen or fumarate as electron acceptors. LASPO has considerable sequence homology with the flavoprotein subunits of succinate dehydrogenase (SDH) and fumarate reductase (FRD).

RESULTS

The crystal structure of the apoform of LASPO from Escherichia coli has been determined to 2.2 A resolution. The enzyme shows a novel fold for an FAD-dependent protein, comprising a three-domain structure: an FAD-binding domain with the dinucleotide-binding fold, a C-terminal three-helical bundle domain, and an alpha + beta capping domain, which is topologically similar to the small subunit of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase. The interface between the FAD-binding and capping domains defines a cleft in which the active site is located.

CONCLUSIONS

A number of strictly conserved residues present in all three domains indicate that LASPO, SDH and FRD share the same overall folding topology. Many of these conserved residues are in the FAD-binding site and active centre, suggesting a similar catalytic mechanism. Thus, LASPO, SDH and FRD form a class of functionally and structurally related oxidoreductases that are all able to reduce fumarate and to oxidise a dicarboxylate substrate.

The role of a conserved histidine residue, His324, in Trigonopsis variabilis D-amino acid oxidase

To investigate the functional role of an invariant histidine residue in Trigonopsis variabilis D-amino acid oxidase (DAAO), a set of mutant enzymes with replacement of the histidine residue at position 324 was constructed and their enzymatic properties were examined. Wild-type and mutant enzymes have been purified to homogeneity using the His-bound column and the molecular masses were determined to be 39.2 kDa. Western blot analysis revealed that the in vivo synthesized mutant enzymes are immuno-identical with that of the wild-type DAAO. The His324Asn and His324Gln mutants displayed comparable enzymatic activity to that of the wild-type enzyme, while the other mutant DAAOs showed markedly decreased or no detectable activity. The mutants, His324/Asn/Gln/Ala/Tyr/Glu, exhibited 38-181% increase in Km and a 2-10-fold reduction in kcat/Km. Based on the crystal structure of a homologous protein, pig kidney DAAO, it is suggested that His324 might play a structural role for proper catalytic function of T. variabilis DAAO.

Reductive half-reaction of the H172Q mutant of trimethylamine dehydrogenase: evidence against a carbanion mechanism and assignment of kinetically influential ionizations in the enzyme-substrate complex

The reactions of wild-type trimethylamine dehydrogenase (TMADH) and of a His-172-->Gln (H172Q) mutant were studied by rapid-mixing stopped-flow spectroscopy over the pH range 6.0-10.5, to address the potential role of His-172 in abstracting a proton from the substrate in a 'carbanion' mechanism for C-H bond cleavage. The pH-dependence of the limiting rate for flavin reduction (klim) was studied as a function of pH for the wild-type enzyme with perdeuterated trimethylamine as substrate. The use of perdeuterated trimethylamine facilitated the unequivocal identification of two kinetically influential ionizations in the enzyme-substrate complex, with macroscopic pKa values of 6.5+/-0.2 and 8.4+/-0.1. A plot of klim/Kd revealed a bell-shaped curve and two kinetically influential ionizations with macroscopic pKa values of 9.4+/-0.1 and 10.5+/-0.1. Mutagenesis of His-172, a potential active-site base and a component of a novel Tyr-His-Asp triad in the active site of TMADH, revealed that the pKa of 8.4+/-0.1 for the wild-type enzyme-substrate complex represents ionization of the imidazolium side-chain of His-172. H172Q TMADH retains catalytic competence throughout the pH range investigated. At pH 10.5, and in contrast with the wild-type enzyme, flavin reduction in H172Q TMADH is biphasic. The fast phase is dependent on the trimethylamine concentration and exhibits a kinetic isotope effect of about 3; C-H bond cleavage is thus partially rate-limiting. In contrast, the slow phase does not show hyperbolic dependence on substrate concentration, and the observed rate shows no dependence on isotope, revealing that C-H bond cleavage is not rate-limiting. The analysis of H172Q TMADH, together with data recently acquired for the Y169F mutant of TMADH, reveals that C-H bond breakage is not initiated via abstraction of a proton from the substrate by an active-site base. The transfer of reducing equivalents to flavin via a carbanion mechanism is therefore unlikely.

A 30-angstrom-long U-shaped catalytic tunnel in the crystal structure of polyamine oxidase

BACKGROUND

Polyamines are essential for cell growth and differentiation; compounds interfering with their metabolism are potential anticancer agents. Polyamine oxidase (PAO) plays a central role in polyamine homeostasis. The enzyme utilises an FAD cofactor to catalyse the oxidation of the secondary amino groups of spermine and spermidine.

RESULTS

The first crystal structure of a polyamine oxidase has been determined to a resolution of 1.9 Angstroms. PAO from Zea mays contains two domains, which define a remarkable 30 Angstrom long U-shaped catalytic tunnel at their interface. The structure of PAO in complex with the inhibitor MDL72527 reveals the residues forming the catalytic machinery and unusual enzyme-inhibitor CH.O H bonds. A ring of glutamate and aspartate residues surrounding one of the two tunnel openings contributes to the steering of the substrate towards the inside of the tunnel.

CONCLUSIONS

PAO specifically oxidizes substrates that have both primary and secondary amino groups. The complex with MDL72527 shows that the primary amino groups are essential for the proper alignment of the substrate with respect to the flavin. Conservation of an N-terminal sequence motif indicates that PAO is member of a novel family of flavoenzymes. Among these, monoamine oxidase displays significant sequence homology with PAO, suggesting a similar overall folding topology.

Monomeric sarcosine oxidase: structure of a covalently flavinylated amine oxidizing enzyme

BACKGROUND

Monomeric sarcosine oxidases (MSOXs) are among the simplest members of a recently recognized family of eukaryotic and prokaryotic enzymes that catalyze similar oxidative reactions with various secondary or tertiary amino acids and contain covalently bound flavins. Other members of this family include heterotetrameric sarcosine oxidase, N-methyltryptophan oxidase and pipecolate oxidase. Mammalian sarcosine dehydrogenase and dimethylglycine dehydrogenase may be more distantly related family members.

RESULTS

The X-ray crystal structure of MSOX from Bacillus sp. B-0618, expressed in Escherichia coli, has been solved at 2.0 A resolution by multiwavelength anomalous dispersion (MAD) from crystals of the selenomethionine-substituted enzyme. Fourteen selenium sites, belonging to two MSOX molecules in the asymmetric unit, were used for MAD phasing and to define the local twofold symmetry axis for electron-density averaging. The structures of the native enzyme and of two enzyme-inhibitor complexes were also determined.

CONCLUSIONS

MSOX is a two-domain protein with an overall topology most similar to that of D-amino acid oxidase, with which it shares 14% sequence identity. The flavin ring is located in a very basic environment, making contact with sidechains of arginine, lysine, histidine and the N-terminal end of a helix dipole. The flavin is covalently attached through an 8alpha-S-cysteinyl linkage to Cys315 of the catalytic domain. Covalent attachment is probably self-catalyzed through interactions with the positive sidechains and the helix dipole. Substrate binding is probably stabilized by hydrogen bonds between the substrate carboxylate and two basic sidechains, Arg52 and Lys348, located above the re face of the flavin ring.

Substrate specificity of a nitroalkane-oxidizing enzyme

The flavoprotein nitroalkane oxidase from Fusarium oxysporum catalyzes the oxidation of nitroalkanes to aldehydes with production of hydrogen peroxide and nitrite. The substrate specificity of the FAD-containing enzyme has been determined as a probe of the active site structure. Nitroalkane oxidase is active on primary and secondary nitroalkanes, with a marked preference for unbranched primary nitroalkanes. The V/K values for primary nitroalkanes increase with increasing length of the alkyl chain, reaching a maximum with 1-nitrobutane, suggesting a hydrophobic binding site sufficient to accommodate a four carbon chain. Each methylene group of the substrate contributes approximately 2.6 kcal mol-1 in binding energy. The V/K values for substrates containing a hydroxyl group are two orders of magnitude smaller than those of the corresponding nitroalkanes, also consistent with a hydrophobic binding site. 3-Nitro-1-propionate is a competitive inhibitor with a Kis value of 3.1 +/- 0.2 mM.