Microspectrofluorometry as a tool for investigation of non-calcium interactions of Indo-1.

Indo-1 is a fluorescent calcium probe used to measure intracellular free calcium concentrations. These measurements are often performed by comparing the fluorescence intensities of Indo-1-treated cells at two selected wavelengths corresponding to the maxima of the fluorescence spectra of the calcium-bound and calcium-free forms. In this study, we used an optical multichannel analyser to numerise the fluorescence emitted by a single cell. A computerised resolution of numerised spectra was used on intracellular Indo-1 fluorescence. Calculation of numerical and graphic estimators allows us to evaluate the fit of the resolution. Different sets of characteristic spectra were compared using this method. It appeared that no linear combination of the two known forms of Indo-1 and of the cell autofluorescence can fit with spectra of Indo-1-treated cells. In addition, a study of the physico-chemical properties of Indo-1 shows the existence of two other forms of the molecule: a protonated form (maximum emission at 455 nm) and a form in interaction with proteins (maximum emission at 438 nm). Taking into account the contribution of these two new forms leads to an improved spectral resolution of the fluorescence of Indo-1-treated living cells and, therefore, improves calcium measurements. Moreover, quantification of the amount of the protonated form of Indo-1 allows a measurement of intracellular pH at the same time as calcium determination.     

Flavins of NADPH-cytochrome P-450 reductase: evidence for structural alteration of flavins in their one-electron-reduced semiquinone states from resonance Raman spectroscopy

The mechanism of electron transfer from NADPH to cytochrome P-450 through FAD and FMN of the reductase is largely unknown. In this paper, we report the resonance Raman spectral properties of the oxidized and the semiquinonoid states of the flavins in the holoenzyme and the FMN-depleted forms, respectively, of detergent-solubilized rabbit liver microsomal NADPH-cytochrome P-450 reductase. The resonance Raman spectra of the oxidized forms [FAD; FMN] and [FAD;-] were essentially identical, indicating similar binding interactions of these flavins with the protein. To the contrary, the spectra of the semiquinonoid FADH. and FMNH. forms revealed significant spectral differences. Both O2-unstable species, characterized as [FADH.; FMNH2] and [FADH.;-] excited at 568.2 nm, have dominant spectral peaks at approximately 1611, 1539-1543, 1377, 1305, 1263, and 1226 cm-1. However, in the O2-stable [FAD; FMNH.] species, resonance Raman bands were located at 1611, 1532, 1388, 1304, 1268, and 1227 cm-1 when excited at the same wavelength. The approximately 10-cm-1 shifts of the 1532- and 1388-cm-1 bands suggest that the environments surrounding rings II and III of the isoalloxazines change upon reduction to semiquinonoid forms. It is proposed that N1 of FADH. (as a hydrogen-bond acceptor) and N5 of FMNH. (as donor) provide the distinguishing flavin-protein interactions in the semiquinonoid states. Furthermore, the resonance Raman spectra of the semiquinonoid species appear to be missing a number of bands assigned to ring I vibrations in the spectra of the oxidized flavins.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reaction of morphinone reductase with 2-cyclohexen-1-one and 1-nitrocyclohexene: proton donation, ligand binding, and the role of residues Histidine 186 and Asparagine 189

Morphinone reductase (MR) catalyzes the NADH-dependent reduction of alpha/beta unsaturated carbonyl compounds in a reaction similar to that catalyzed by Old Yellow Enzyme (OYE1). The two enzymes are related at the sequence and structural levels, but key differences in active site architecture exist which have major implications for the reaction mechanism. We report detailed kinetic and solution NMR data for wild-type MR and two mutant forms in which residues His-186 and Asn-189 have been exchanged for alanine residues. We show that both residues are involved in the binding of the reducing nicotinamide coenzyme NADH and also the binding of the oxidizing substrates 2-cyclohexen-1-one and 1-nitrocyclohexene. Reduction of 2-cyclohexen-1-one by FMNH(2) is concerted with proton transfer from an unknown proton donor in the active site. NMR spectroscopy and flavin reoxidation studies with 2-cyclohexen-1-one are consistent with His-186 being unprotonated in oxidized, reduced, and ligand-bound MR, suggesting that His-186 is not the key proton donor required for the reduction of 2-cyclohexen-1-one. Hydride transfer is decoupled from proton transfer with 1-nitrocyclohexene as oxidizing substrate, and unlike with OYE1 the intermediate nitronate species produced after hydride transfer from FMNH(2) is not converted to 1-nitrocyclohexane. The work highlights key mechanistic differences in the reactions catalyzed by MR and OYE1 and emphasizes the need for caution in inferring mechanistic similarities in structurally related proteins.     

Catalytic importance of the substrate binding order for the FMNH2-dependent alkanesulfonate monooxygenase enzyme

The two-component alkanesulfonate monooxygenase system from Escherichia coli includes an FMN reductase (SsuE) and an FMNH2-dependent alkanesulfonate monooxygenase (SsuD) involved in the acquisition of sulfur from alkanesulfonates during sulfur starvation. The SsuD enzyme directly catalyzes the oxidation of alkanesulfonate to aldehyde and sulfite in the presence of O2 and FMNH2. The goal of these studies was to investigate the kinetic mechanism of SsuD through rapid reaction kinetics and substrate binding studies. The SsuD enzyme shows a clear preference for FMNH2 (Kd, 0.32 +/- 0.15 microM) compared to FMN (Kd, 10.2 +/- 0.4 microM) with a 1:1 binding stoichiometry for each form of the flavin. The kinetic trace of premixed SsuD and FMNH2 mixed with oxygenated buffer was best fit to a double exponential with no observed formation of the C4a-(hydro)peroxyflavin. However, when FMNH2 was mixed with SsuD and oxygenated buffer an initial fast phase (kobs, 12.9 s-1) was observed, suggesting that the mixing order is critical for the accumulation of the C4a-(hydro)peroxyflavin. Results from fluorimetric titrations with octanesulfonate imply that reduced flavin must bind first to promote octanesulfonate binding. When octanesulfonate was included in the kinetic studies the C4a-(hydro)peroxyflavin was observed at 370 nm when FMNH2 was not premixed with SsuD, which correlated with an increase in octanal product. There was a clear hyperbolic dependence on octanesulfonate binding, indicating that octanesulfonate binds in rapid equilibrium, and further results indicated there was a second isomerization step following binding. These results suggest that an ordered substrate binding mechanism is important in the desulfonation reaction by SsuD with reduced flavin binding first followed by either O2 or octanesulfonate.     

Site-directed mutagenesis of potential catalytic residues in 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase, and hypothesis on the catalytic mechanism of 2,4-dioxygenolytic ring cleavage

1H-3-Hydroxy-4-oxoquinoline 2,4-dioxygenase (Qdo) is a cofactor-free dioxygenase proposed to belong to the alpha/beta hydrolase fold superfamily of enzymes. Alpha/beta Hydrolases contain a highly conserved catalytic triad (nucleophile-acidic residue-histidine). We previously identified a corresponding catalytically essential histidine residue in Qdo. However, as shown by amino acid replacements through site-directed mutagenesis, nucleophilic and acidic residues of Qdo considered as possible triad residues were not absolutely required for activity. This suggests that Qdo does not contain the canonical catalytic triad of the alpha/beta hydrolase fold enzymes. Some radical trapping agents affected the Qdo-catalyzed reaction. A hypothetical mechanism of Qdo-catalyzed dioxygenation of 1H-3-hydroxy-4-oxoquinoline is compared with the dioxygenation of FMNH2 catalyzed by bacterial luciferase, which also uses a histidine residue as catalytic base.     

Activity coupling and complex formation between bacterial luciferase and flavin reductases.

Luminous bacteria contain several species of flavin reductases, which catalyze the reduction of FMN using NADH and/or NADPH as a reductant. The reduced FMN (i.e. FMNH(2)) so generated is utilized along with a long-chain aliphatic aldehyde and molecular oxygen by luciferase as substrates for the bioluminescence reaction. In this report, the general properties of luciferases and reductases from luminous bacteria are briefly summarized. Earlier and more recent studies demonstrating the direct transfer of FMNH(2) from reductases to luciferase are surveyed. Using reductases and luciferases from Vibrio harveyi and Vibrio fischeri, two mechanisms were uncovered for the direct transfer of reduced flavin cofactor and reduced flavin product of reductase to luciferase. A complex of an NADPH-specific reductase (FRP(Vh)) and luciferase from V. harveyi has been detected in vitro and in vivo. Both constituent enzymes in such a complex are catalytically active. The reduction of FRP(Vh)-bound FMN cofactor by NADPH is reversible, allowing the cellular contents of NADP(+) and NADPH as a factor for the regulation of the production of FMNH(2) by FRP(Vh) for luciferase bioluminescence. Other regulations of the activity coupling between reductase and luciferase are also discussed.     

Reaction of o-phthalaldehyde and thiols with primary amines: Fluorescence properties of 1-alkyl(and aryl)thio-2-alkylisoindoles

The fluorescence properties of 1-alkyl(and aryl)thio-2-alkylisoindoles, formed by the reaction of o-phthalaldehyde (OPTA) and thiols with primary amines, are reported. Variations in thiol and amine substituents and solvent polarity have large effects on the isoindole fluorescence spectra. These parameters, in addition to 3-thiol substitution of the isoindoles, pH, and the use of phosphate vs borate aqueous buffers, were found to have dramatic effects on the corrected relative fluorescence intensity. Low concentrations and nonaqueous solvents apparently stabilized most adducts while aqueous solutions, especially at low pH, caused pseudo-first-order decomposition, probably via hydrolysis to the corresponding 2,3-dihydro-1H-isoindole-1-one. However, 3.3 × 10⁻⁸ solutions of the more intensely fluorescent adducts (total adduct 5 pmol) were readily detected if the fluorescence was determined shortly after adding the isoindole to pH 9.2 borate buffer. The adduct formed using ethanethiol and n-propylamine possessed spectral properties which were the most responsive to changes in solvent polarity and was the most stable under the various conditions employed. Finally, arguments are presented that these isoindoles are the products in several other fluorogenic assays using OPTA.     

Influence of the protein environment on the redox potentials of flavodoxins from Clostridium beijerinckii

The flavin mononucleotide (FMN) quinones in flavodoxin have two characteristic redox potentials, namely, Em(FMNH./FMNH-) for the one-electron reduction of the protonated FMN (E1) and Em(FMN/FMNH.) for the proton-coupled one-electron reduction (E2). These redox potentials in native and mutant flavodoxins obtained from Clostridium beijerinckii were calculated by considering the protonation states of all titratable sites as well as the energy contributed at the pKa value of FMN during protonation at the N5 nitrogen (pKa(N5)). E1 is sensitive to the subtle differences in the protein environments in the proximity of FMN. The protein dielectric volume that prevents the solvation of charged FMN quinones is responsible for the downshift of 130-160 mV of the E1 values with respect to that in an aqueous solution. The influence of the negatively charged 5'-phosphate group of FMN quinone on E1 could result in a maximum shift of 90 mV. A dramatic difference of 130 mV in the calculated E2 values of FMN quinone of the native and G57T mutant flavodoxins is due to the difference in the pKa(N5) values. This is due to the difference in the influence exerted by the carbonyl group of the protein backbone at residue 57.     

Stopped-flow kinetic analysis of the bacterial luciferase reaction.

The kinetics of the reaction catalyzed by bacterial luciferase have been measured by stopped-flow spectrophotometry at pH 7 and 25 degrees C. Luciferase catalyzes the formation of visible light, FMN, and a carboxylic acid from FMNH2, O2, and the corresponding aldehyde. The time courses for the formation and decay of the various intermediates have been followed by monitoring the absorbance changes at 380 and 445 nm along with the emission of visible light using n-decanal as the alkyl aldehyde. The synthesis of the 4a-hydroperoxyflavin intermediate (FMNOOH) was monitored at 380 nm after various concentrations of luciferase, O2, and FMNH2 were mixed. The second-order rate constant for the formation of FMNOOH from the luciferase-FMNH2 complex was found to be 2.4 x 10(6) M-1 s-1. In the absence of n-decanal, this complex decays to FMN and H2O2 with a rate constant of 0.10 s-1. The enzyme-FMNH2 complex was found to isomerize prior to reaction with oxygen. The production of visible light reaches a maximum intensity within 1 s and then decays exponentially over the next 10 s. The formation of FMN from the intermediate pseudobase (FMNOH) was monitored at 445 nm. This step of the reaction mechanism was inhibited by high levels of n-decanal which indicated that a dead-end luciferase-FMNOH-decanal could form. The time courses for these optical changes have been incorporated into a comprehensive kinetic model. Estimates for 15 individual rate constants have been obtained for this model by numeric simulations of the various time courses.     

Dynamic fluorescence properties of bacterial luciferase intermediates

Three fluorescent species produced by the reaction of bacterial luciferase from Vibrio harveyi with its substrates have the same dynamic fluorescence properties, namely, a dominant fluorescence decay of lifetime of 10 ns and a rotational correlation time of 100 ns at 2 degrees C. These three species are the metastable intermediate formed with the two substrates FMNH2 and O2, both in its low-fluorescence form and in its high-fluorescence form following light irradiation, and the fluorescent transient formed on including the final substrate tetradecanal. For native luciferase, the rotational correlation time is 62 or 74 ns (2 degrees C) derived from the decay of the anisotropy of the intrinsic fluorescence at 340 nm or the fluorescence of bound 8-anilino-1-naphthalenesulfonic acid (470 nm), respectively. The steady-state anisotropy of the fluorescent intermediates is 0.34, and the fundamental anisotropy from a Perrin plot is 0.385. The high-fluorescence intermediate has a fluorescence maximum at 500 nm, and its emission spectrum is distinct from the bioluminescence spectrum. The fluorescence quantum yield is 0.3 but decreases on dilution with a quadratic dependence on protein concentration. This, and the large value of the rotational correlation time, would be explained by protein complex formation in the fluorescent intermediate states, but no increase in protein molecular weight is observed by gel filtration or ultracentrifugation. The results instead favor a proposal that, in these intermediate states, the luciferase undergoes a conformational change in which its axial ratio increases by 50%.     

Multiple structures for the 2-aminopurine-cytosine mispair

The base pair formed between 2-aminopurine (2AP) and cytosine (C) is an intermediate in transition mutations generated by 2AP. To date, several structures have been proposed for the 2AP-C mispair, including those involving a rare tautomer, a protonated base pair, and a neutral wobble structure. In this paper, we describe a series of UV, fluorescence, and NMR studies which demonstrate that an equilibrium exists between the neutral wobble and the protonated Watson-Crick structures. The apparent pK value for the transition between the structures is 5.9-6.0. Formation of a Watson-Crick base pair is accomplished predominantly by protonation of the 2AP residue as indicated by UV spectral changes, fluorescence quenching, and changes in proton chemical shifts. Rapid transfer of the shared proton between the 2AP and cytosine residues is indicated by the rapid exchange of the cytosine amino protons of the protonated Watson-Crick configuration. The relative contribution of the neutral wobble and protonated Watson-Crick configurations to 2AP-induced transition mutations is discussed.     

Utilization of dihydroflavin mononucleotide and superoxide anion for the decyclization of L-tryptophan by murine epididymal indoleamine 2,3-dioxygenase

Dihydroflavin mononucleotide (FMNH2) together with a regenerating enzyme system effectively supported L-tryptophan decyclization by indoleamine 2,3-dioxygenase isolated from murine epididymis. The native murine dioxygenase was a monomeric protein with Mr 40,000 +/- 1000, an apparent pI of 4.9 +/- 0.1, and an optimum pH within the range of 7 to 8. Using FMNH2 with FMN oxidoreductase, the enzyme attained significantly higher activity than the apparent maximal activity obtained by using the other electron donor systems examined (e.g., riboflavin, FAD, tetrahydrobiopterin, methylene blue). A kinetic study with the FMNH2 cofactor suggested the occurrence of a complex reaction (L-tryptophan-FMNH2 interdependency) and a theoretical K'm of 14 microM or a Km of 13 microM was estimated for the substrate. L-Tryptophan 2,3-dioxygenation was competitively inhibited by L-5-hydroxytryptophan with a Ki of 1 microM. The reaction rate was reduced to less than 50% of that of the control in the presence of superoxide dismutase and was decreased to 3% of the control in the absence of catalase. Thus, superoxide anion does not appear to be the only form of O2 participating in the reaction. However, these data indicate that the activation of molecular oxygen is an essential factor for an optimum catalysis and a mechanism of FMNH2-dependent oxygenation of L-tryptophan by murine indoleamine 2,3-dioxygenase.     

Crystal structure of the plant PPC decarboxylase AtHAL3a complexed with an ene-thiol reaction intermediate

The Arabidopsis thaliana protein AtHAL3a decarboxylates 4'-phosphopantothenoylcysteine to 4'-phosphopantetheine, a step in coenzyme A biosynthesis. Surprisingly, this decarboxylation reaction is carried out as an FMN-dependent redox reaction. In the first half-reaction, the side-chain of the cysteine residue of 4'-phosphopantothenoylcysteine is oxidised and the thioaldehyde intermediate decarboxylates spontaneously to the 4'-phosphopantothenoyl-aminoethenethiol intermediate. In the second half-reaction this compound is reduced to 4'-phosphopantetheine and the FMNH(2) cofactor is re-oxidised. The active site mutant C175S is unable to perform this reductive half-reaction. Here, we present the crystal structure of the AtHAL3a mutant C175S in complex with the reaction intermediate pantothenoyl-aminoethenethiol and FMNH(2). The geometry of binding suggests that reduction of the C(alpha)=C(beta) double bond of the intermediate can be performed by direct hydride-transfer from N5 of FMNH(2) to C(beta) of the aminoethenethiol-moiety supported by a protonation of C(alpha) by Cys175. The binding mode of the substrate is very similar to that previously observed for a pentapeptide to the homologous enzyme EpiD that introduces the aminoethenethiol-moiety as final reaction product at the C terminus of peptidyl-cysteine residues. This finding further supports our view that these homologous enzymes form a protein family of homo-oligomeric flavin-containing cysteine decarboxylases, which we have termed HFCD family.     

A fluorescence study of A23187 interaction with phospholipid vesicles

The fluorescence of the ionophore A23187 has been monitored in suspensions of egg yolk phosphatidylcholine (EYPC) and dipalmitoyl phosphatidylcholine (DPPC) vesicles. Both the protonated form of A23187 and the Ca²⁺ complex exhibit fluorescence enhancement when extracted into a hydrophobic environment. Measurements of fluorescence intensity versus lipid concentration were thus used to establish lower limits to the lipid/ water partition coefficients. Values obtained in this way were ? 50 ml water/mg phosphatidylcholine. Quenching of A23187 fluorescence by the spin labels 5NMS (methyl ester of 5-nitroxyl stearate), 12NMS, 16NMS, and TEMPO stearamide in EYPC and DPPC vesicles was also investigated. In EYPC all the labels yielded fairly linear Stern-Volmer plots, with TEMPO stearamide quenching about half as strong as the other probes. Quenching in DPPC was generally much stronger than in EYPC, but 12 NMS and 16NMS gave hyperbolic Stern-Volmer plots, apparently due to clustering of the labels. In all the cases the protonated form of A23187 was quenched approximately twice as efficiently as the Ca²⁺ complex, possibly due to a longer fluorescence lifetime for the former. Calculations based on measured spectral properties were performed which indicate that the Förster transfer mechanism extends the nitroxides' quenching range to ～- 10 Å.     

Fast protonation of adenosine and of its radical anion formed by hydrated electron attack; a nanosecond optical and dc-conductivity pulse radiolysis study

The study of the reaction of the hydrated electron with adenosine by optical and dc-conductivity pulse radiolysis on nano- and microsecond timescales has been carried out in an attempt to answer the question whether the electron adduct radical becomes protonated or not. The following conclusions have been reached: (1) the reaction of the hydrated electron with adenosine is followed by a water-mediated protonation, which must be complete with 5 ns; (2) no spectral indication of a further protonation of the protonated electron adduct of adenosine of 2'-deoxyadenosine has been found between 40 and 5000 ns; (3) the equilibrium reaction between radiation produced H3O+ and adenosine with a pKa of 3.5 plays an important role in the kinetics of the conductivity transients.     

One- and two-photon-induced fluorescence from recombinant green fluorescent protein

The fluorescence spectral properties of recombinant green fluorescent protein (rGFP) were examined with one- and two-photon excitations using femtosecond pulses from a Ti:sapphire laser. Intensity-dependent properties of the two-photon-induced fluorescence from rGFP excited by an 800-nm, 100-fs laser beam were reported, and the two-photon excitation cross section of rGFP was measured at 800 nm as about 160 x 10(-50) cm(4)s/photon. The possible excited-state proton transfer between two electronic states at about 400 nm in protonated (RH) species and 478 nm in deprotonated (R(-)) species in rGFP was confirmed by fluorescence and fluorescence excitation anisotropy spectra. A subelectronic state (or vibronic progression) at about 420 nm in RH species was identified, which was relatively stable and not involved in the excited state proton transfer in rGFP upon irradiation.     

Ultrafast excited state dynamics of the protonated Schiff base of all-trans retinal in solvents

We present a comparative study of the ultrafast photophysics of all-trans retinal in the protonated Schiff base form in solvents with different polarities and viscosities. Steady-state spectra of retinal in the protonated Schiff base form show large absorption-emission Stokes shifts (6500-8100 cm(-1)) for both polar and nonpolar solvents. Using a broadband fluorescence up-conversion experiment, the relaxation kinetics of fluorescence is investigated with 120 fs time resolution. The time-zero spectra already exhibit a Stokes-shift of approximately 6000 cm(-1), indicating depopulation of the Franck-Condon region in < or =100 fs. We attribute it to relaxation along skeletal stretching. A dramatic spectral narrowing is observed on a 150 fs timescale, which we assign to relaxation from the S(2) to the S(1) state. Along with the direct excitation of S(1), this relaxation populates different quasistationary states in S(1), as suggested from the existence of three distinct fluorescence decay times with different decay associated spectra. A 0.5-0.65 ps decay component is observed, which may reflect the direct repopulation of the ground state, in line with the small isomerization yield in solvents. Two longer decay components are observed and are attributed to torsional motion leading to photo-isomerization. The various decay channels show little or no dependence with respect to the viscosity or dielectric constant of the solvents. This suggests that in the protein, the bond selectivity of isomerization is mainly governed by steric effects.     

Critical role of Glu175 on stability and folding of bacterial luciferase: stopped-flow fluorescence study

Bacterial luciferase is a heterodimeric enzyme, which catalyzes the light emission reaction, utilizing reduced FMN (FMNH2), a long chain aliphatic aldehyde and O(2), to produce green-blue light. This enzyme can be readily classed as slow or fast decay based on their rate of luminescence decay in a single turnover. Mutation of Glu175 in alpha subunit to Gly converted slow decay Xenorhabdus Luminescence luciferase to fast decay one. The following studies revealed that changing the luciferase flexibility and lake of Glu-flavin interactions are responsible for the unusual kinetic properties of mutant enzyme. Optical and thermodynamics studies have caused a decrease in free energy and anisotropy of mutant enzyme. Moreover, the role of Glu175 in transition state of folding pathway by use of stopped-flow fluorescence technique has been studied which suggesting that Glu175 is not involved in transition state of folding and appears as surface residue of the nucleus or as a member of one of a few alternative folding nuclei. These results suggest that mutation of Glu175 to Gly extended the structure of Xenorhabdus Luminescence luciferase, locally.     

Characterization of a two-component alkanesulfonate monooxygenase from Escherichia coli.

The Escherichia coli ssuEADCB gene cluster is required for the utilization of alkanesulfonates as sulfur sources, and is expressed under conditions of sulfate or cysteine starvation. The SsuD and SsuE proteins were overexpressed and characterized. SsuE was purified to homogeneity as an N-terminal histidine-tagged fusion protein. Native SsuE was a homodimeric enzyme of M(r) 58,400, which catalyzed an NAD(P)H-dependent reduction of FMN, but it was also able to reduce FAD or riboflavin. The SsuD protein was purified to >98% purity using cation exchange, anion exchange, and hydrophobic interaction chromatography. The pure enzyme catalyzed the conversion of pentanesulfonic acid to sulfite and pentaldehyde and was able to desulfonate a wide range of sulfonated substrates including C-2 to C-10 unsubstituted linear alkanesulfonates, substituted ethanesulfonic acids and sulfonated buffers. SsuD catalysis was absolutely dependent on FMNH(2) and oxygen, and was maximal for SsuE/SsuD molar ratios of 2.1 to 4.2 in 10 mM Tris-HCl, pH 9.1. Native SsuD was a homotetrameric enzyme of M(r) 181,000. These results demonstrate that SsuD is a broad range FMNH(2)-dependent monooxygenase catalyzing the oxygenolytic conversion of alkanesulfonates to sulfite and the corresponding aldehydes. SsuE is the FMN reducing enzyme providing SsuD with FMNH(2).     

Crystal structures of NADH:FMN oxidoreductase (EmoB) at different stages of catalysis

EDTA has become a major organic pollutant in the environment because of its extreme usage and resistance to biodegradation. Recently, two critical enzymes, EDTA monooxygenase (EmoA) and NADH:FMN oxidoreductase (EmoB), belonging to the newly established two-component flavin-diffusible monooxygenase family, were identified in the EDTA degradation pathway in Mesorhizobium sp. BNC1. EmoA is an FMNH2-dependent enzyme that requires EmoB to provide FMNH2 for the conversion of EDTA to ethylenediaminediacetate. To understand the molecular basis of this FMN-mediated reaction, the crystal structures of the apo-form, FMN.FMN complex, and FMN.NADH complex of EmoB were determined at 2.5 angstroms resolution. The structure of EmoB is a homotetramer consisting of four alpha/beta-single-domain monomers of five parallel beta-strands flanked by five alpha-helices, which is quite different from those of other known two-component flavin-diffusible monooxygenase family members, such as PheA2 and HpaC, in terms of both tertiary and quaternary structures. For the first time, the crystal structures of both the FMN.FMN and FMN.NADH complexes of an NADH:FMN oxidoreductase were determined. Two stacked isoalloxazine rings and nicotinamide/isoalloxazine rings were at a proper distance for hydride transfer. The structures indicated a ping-pong reaction mechanism, which was confirmed by activity assays. Thus, the structural data offer detailed mechanistic information for hydride transfer between NADH to an enzyme-bound FMN and between the bound FMNH2 and a diffusible FMN.