Effect of flavin structure and redox state on catalysis by and flavin-pterin energy transfer in Escherichia coli DNA photolyase

5-DeazaFAD bound to a hydrophobic site in apophotolyase and formed a stable reconstituted enzyme, similar to that observed with FAD. Although stoichiometric incorporation was observed, the flavin ring modification in 1-deazaFAD interfered with normal binding, decreased protein stability, and prevented formation of a stable flavin radical, unlike that observed with FAD. The results suggest that an important hydrogen bond is formed between the protein and N (1) in FAD, but not N (5), and that there is sufficient space at the normal flavin binding site near N (5) to accommodate an additional hydrogen but not near N (1). Catalytic activity was observed with enzyme containing 5-deazaFADH2 (42% of native enzyme) or 1-deazaFADH2 (11% of native enzyme) as its only chromophore, but no activity was observed with the corresponding oxidized flavins, similar to that observed with FAD and consistent with a mechanism where dimer cleavage is initiated by electron donation from excited reduced flavin to substrate. The protein environment in photolyase selectively enhanced photochemical reactivity in the fully reduced state, as evidenced by comparison with results obtained in model studies with the corresponding free flavins. Phosphorescence was observed with free or photolyase-bound 5-deazaFADH2, providing the first example of a flavin that exhibits phosphorescence in the fully reduced state. Formation of an enzyme-substrate complex resulted in a nearly identical extent of quenching of 5-deazaFADH2 phosphorescence (85.1%) and fluorescence (87.5%). The data are consistent with a mechanism involving exclusive reaction of substrate with the excited singlet state of 5-deazaFADH2, analogous to that proposed for FADH2 in native enzyme. Direct evidence for singlet-singlet energy transfer from enzyme-bound 5-deazaFADH2 to 5,10-CH(+)-H4folate was provided by the fact that pterin fluorescence was observed upon excitation of 5-deazaFADH2, accompanied by a decrease in 5-deazaFADH2 fluorescence. On the other hand, the fluorescence of enzyme-bound pterin was quenched by 5-deazaFADox, consistent with energy transfer from pterin to 5-deazaFADox. In each case, the spectral properties of the chromophores were consistent with the observed direction of energy transfer and indicated that transfer in the opposite direction was energetically unlikely. Unlike 5-deazaFAD, energy transfer from pterin to FAD is energetically feasible with FADH2 or FADox. The results indicate that the direction of flavin-pterin energy transfer at the active site of photolyase can be manipulated by changes in the flavin ring or redox state which alter the energy level of the flavin singlet.     

Reduction of substituted flavins by .CO2- and cyclic disulphide anions

5-Deazariboflavin was observed to undergo one electron-equivalent reduction on reaction with .CO2-, DS2-. or a mixture of .CO2- and eaq-. This is attributed to formation of .FlH, followed by dimerization: 2 5-DRFlH.----(5-DRFlH)2 a process not previously confirmed. In contrast, 3-deazariboflavin, 1-deazariboflavin 7-chlororiboflavin, 2-thioriboflavin, lumiflavin, flavin adenine dinucleotide and riboflavin all underwent two electron-equivalent reductions to FlH2 or FlH-species via the disproportionation: 2.FlH----FlH2+Fl Spectra of the reduced species are reported for 3-deazariboflavin, 1-deazarboflavin and 2-thioriboflavin. The anion radicals of dithiothreitol (DS2-.) and lipoamide (LS2-.) did not effect reduction beyond the two electron equivalence point. However, .CO2- was seen to effect further reactions with the dihydroforms of the higher redox potential flavins 7-chlororiboflavin and 2-thioriboflavin at pH 3 to 10, but for the other flavins only at or below pH 2. Except for the sulphur-containing 2-thioriboflavin, the protonated DS2.H and LS2.H radicals were unable to reduce any of the flavins at pH less than or equal to 3. This cause a 'falloff' in the yield of the reduced flavin which is in accord with the reaction rate constants and radical pK values. Despite their differences in absorbance, the reactions of DS2.H and LS2.H towards flavins were similar in all aspects examined. Several new rate constants are reported.     

Partial purification of a plasma membrane flavoprotein and NAD(P)H-oxidoreductase activity

Plasma membrane flavins and pterins are considered to mediate important physiological functions such as blue light photoperception and redox activity. Therefore, the presence of flavins and pterins in the plasma membrane of higher plants was studied together with NAD(P)H-dependent redox activities. Plasma membranes were isolated from the apical hooks of etiolated bean seedlings (Phaseolus vulgaris L. cv. Limburgse Vroege) by aqueous two-phase partitioning. Fluorescence spectroscopy revealed the presence of two chromophores. The first showed excitation maxima at 370 and 460 nm and an emission peak at 520 nm and was identified as a flavin. The second chromophore was probably a pterin molecule with excitation peaks at 290 and 350 nm and emission at 440 nm. Both pigments are considered intrinsic to the plasma membrane since they could not be removed by treatment with hypotonic media containing high salt and low detergent concentrations. The flavin concentration was estimated at about 500 pmol mg^?1 protein. However difficulties were encountered in quantifying the pterin concentrations. Protease treatments indicated that the flavins were non-covalently bound to the proteins. Separation of the plasma membrane proteins after solubilisation by octylglucoside, on an ion exchange system (HPLC, Mono Q), resulted in a distinct protein fraction showing flavin and pterin fluorescence and NADH oxidoreductase activity. The flavin of this fraction was identified as flavin mononucleotide (FMN) by HPLC analysis. Other minor peaks of NADH:acceptor reductase activity were resolved on the column. The presence of distinct NAD(P)H oxidases at the plasma membrane was supported by nucleotide specificity and latency studies using intact vesicles. Our work demonstrates the presence of plasma membrane flavins as intrinsic chromophores, that may function in NAD(P)H-oxidoreductase activity and suggests the presence of plasma membrane bound pterins.     

Hydrogen-bonding modulation of excited-state properties of flavins in a model of aqueous confined environment

The singlet and triplet excited states properties of lumiflavin (LF), riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) in reversed micelles (RM) of sodium docusate (AOT) in n-hexane solutions were evaluated as a function of the water to surfactant molar ratio, w(0) = [H(2)O]/[AOT], by both steady-state and time-resolved absorption and fluorescence spectroscopy. The results indicated that hydrogen-bonding interactions between the isoalloxazine ring of the flavins with the water molecules of the micellar interior play a crucial role on the modulation of the excited state properties of the flavins. Fluorescence dynamic experiments in the RM, allowed the calculation of similar values for both the internal rotational time of the flavins (θ(i)) and the hydrogen-bonding relaxation time (τ(HB)), e.g.≈ 7 and 1.5 ns at w(0) = 1 and 20, respectively. In turn, the triplet state lifetimes of the flavins were also enlarged in RM solutions at low w(0), without modifications of their quantum yields. A hydrogen bonding relaxation model is proposed to explain the singlet excited state properties of the flavins, while the changes of the triplet state decays of the flavins were related with the global composition and strength of the hydrogen bonding network inside of the RM.     

Photosensitization of singlet oxygen formation by pterins and flavins. Time-resolved studies of oxygen phosphorescence under laser excitation

To elucidate the biochemical roles of singlet molecular oxygen (1(O2)) in the light-dependent reactions photosensitized by biological blue-light photoreceptors, time-resolved measurements of photosensitized 1O2 phosphorescence (1270 nm) were performed in air-saturated aqueous ((D2)O) solutions of pterins (2-amino-4-hydroxy-6,7-dimethylpteridine (DMP) and 2-amino-4-hydroxy-6-tetrahydroxybutyl-(D-arabo)pteridine (TOP)) and flavins (riboflavin and flavin mononucleotide (FMN)) under excitation with nitrogen laser (337.1 nm) pulses. The 1(O2) quantum yields were found to be 0.16, 0.20, 0.50, and 0.50 for DMP, TOP, riboflavin, and FMN, respectively. The data indicate that pterins and flavins are rather efficient photosensitizers of 1(O2) production that might be important for their photobiological functions.     

Limited rotational motion of amphiphilic flavins in dipalmitoylphosphatidylcholine vesicles

The rotational motion of amphiphilic flavins in dipalmitoyl phospholipid bilayers was investigated with fluorescence anisotropy decay measurements. At temperatures between 10 and 50°C the rotation proved to be anisotropic, which indicated composite motion of both the aliphatic side-chain and the isoalloxazine moiety of the octadecyllumiflavin derivatives. Above the phase transition temperature (crystalline→liquid-crystalline state) the depolarization is complete within the average flavin fluorescence lifetime, implicating unrestricted motion and resulting in a non-ordered microenvironment. In the gel or crystalline state the flavin motion can best be characterized as a limited rotation or librational motion. The fluorescence decay of the flavins is heterogeneous at temperatures between 10 and 50°C, which is explained by assuming nanosecond relaxation of the polar phosphatidyl head-groups around the excited flavin. The lack of a significant cholesterol effect suggests that the isoalloxazine is located at the interphase of the bilayer and not in the hydrocarbon region. The microstructure is fluid-like, not in agreement with a preferred static localization of the flavins in the bilayer.     

Identification of the covalently bound flavin prosthetic group of cholesterol oxidase.

Highly purified preparations of cholesterol oxidase from Schizophyllum commune contain a covalently bound flavin component. A flavin peptide has been obtained by digestion with trypsin-chymotrypsin and purification on a column of phosphocellulose. Digestion with nucleotide pyrophosphatase results in increased fluorescence at pH 3.4 and release of 5'-adenylate, showing that the flavin is in the dinucleotide form. The absorption spectrum of the flavin peptide shows the hypsochromic shift of the second absorption band characteristic of 8 alpha-substituted flavins. The fluorescence at pH 7 is extensively quenched even in the mononucleotide form, with a pKa at pH 5.8 in the flavin peptide and at 5.05 following acid hydrolysis to the aminoacyl flavin level. This suggests that histidine is the amino acid substituted at the 8 alpha position of the flavin and that N(1) of the imidazole ring is the site of attachment. These data, the reduction of the flavin by borohydride, and comparison of the mobilities in high voltage electrophoresis at two pH values with N(1)- and N(3)-histidyl riboflavin and their 2',5'-anhydro forms shows that the prosthetic group of cholesterol oxidase is 8 alpha-[N(1)-histidyl]-FAD.     

Reaction of reduced flavins and flavoproteins with diphenyliodonium chloride

The reaction of diphenyliodonium chloride with free reduced flavins has been studied by stopped flow spectrophotometry under anaerobic conditions, and second order rate constants were determined as a function of pH. The reactive flavin species was identified as the reduced anion, based on an observed reaction pK of 6.7. The product mixture was independent of the initial concentration of reactant and contained approximately 20% oxidized flavin. The results can be modeled quantitatively on a modification of the mechanism proposed by Tew (Tew, D. G. (1993) Biochemistry 32, 10209-10215). The composition of the complex reaction mixture has been analyzed, and four flavin-phenyl adducts with distinctive absorbance and fluorescence characteristics have been identified, involving substitution at the flavin C4a, N5, and C8 positions. Inactivation of flavoprotein enzymes by diphenyliodonium has also been studied, and several examples were found where inactivation occurs readily, despite noninvolvement of radical intermediates in their reaction mechanisms. It can be concluded that inactivation by phenyliodonium species is not a valid indicator of catalytic mechanism involving radical intermediates. One of the several factors determining inactivation is maintenance of the enzyme flavin in the reduced form in the steady state of catalysis, the other factors being redox potential and accessibility of the inhibitor to the flavin active site.     

Inactivation of C30A trimethylamine dehydrogenase by N-cyclopropyl-alpha-methylbenzylamine, 1-phenylcyclopropylamine, and phenylhydrazine.

Trimethylamine dehydrogenase (TMADH) from the bacterium Methylophilus methylotrophus (sp. W(3)A(1)) and its C30A mutant were inactivated with three known inactivators of monoamine oxidase, namely, phenylhydrazine, N-cyclopropyl-alpha-methylbenzylamine, and 1-phenylcyclopropylamine. All three compounds irreversibly inactivated both the wild-type and C30A mutant enzymes, although phenylhydrazine was 10 times more potent than N-cyclopropyl-alpha-methylbenzylamine, which was much more potent than 1-phenylcyclopropylamine. The change in the UV--visible absorption spectra upon modification indicated that the flavin was modified. In the case of the C30A mutant, the absence of a covalent attachment of the flavin to the polypeptide has permitted LC-electrospray mass spectrometry of the reaction product to be undertaken, demonstrating new mass peaks corresponding to various chemically modified forms of the flavin cofactor. In the case of N-cyclopropyl-alpha-methylbenzylamine, masses corresponding to hydroxy-FMN and hydroxyriboflavin were detected. 1-Phenylcyclopropylamine inactivation of the C30A mutant produced three modified flavins, as evidenced by the electrospray mass spectrum: hydroxy-FMN, FMN plus C(6)H(5)COCH(2)CH(2), and hydroxy-FMN plus C(6)H(5)COCH(2)CH(2). Phenylhydrazine inactivation of the C30A mutant gave at least seven different modified flavins: hydroxyriboflavin, hydroxy-FMN, two apparently isomeric compounds corresponding to hydroxy-FMN plus one phenyl group, two apparently isomeric compounds corresponding to FMN plus one phenyl group, and FMN plus two phenyl groups. Covalent flavin adduct formation appears to be the only modification because dialysis of the inactive enzyme followed by reconstitution with FMN restores the enzyme activity to that of a noninactivated control.     

Flavocytochrome b2: simulation studies of the electron-transfer reactions among the prosthetic groups.

Simulation studies by digital computer were undertaken in order to test and clarify the interpretations deduced from experimental data concerning the electron transfer mechanism from L-lactate to flavocytochrome b2, which were presented in a preceding paper in this journal. The reaction scheme proposed as the "best" one is composed of 7 steps. It allows the best fitting of the time courses established for the oxidized flavin (Flox), the flavin semiquinone (Flsq), the fully reduced flavin (Flred), and the reduced haem (Hred); it can be extended to 1 s. This scheme also allows a good simulation of the general shape of preequilibrium titration curves obtained at a 200-ms reaction time for Hred and Flsq, and a valuable simulation of the reduction electron paramagnetic resonance time course established for Hred and Flsq at low lactate concentration. The agreement between experimental and simulated curves led to an estimation of some rate constants experimentally unknown, relative (in particular) to the electron exchange between flavin and haem and between couples of flavins. Another interest of these stimulation studies was to point out the obligatory involvement of a slow final step to perform the flavocytochrome b2 full reduction; this step could be controlled by some conformational change of the protein.     

Comparison of the dynamical structures of lipoamide dehydrogenase and glutathione reductase by time-resolved polarized flavin fluorescence.

Time-resolved polarized fluorescence spectroscopy has been applied to the bound FAD in the structurally related flavoproteins lipoamide dehydrogenase from Azotobacter vinelandii (LipDH-AV) and glutathione reductase (GR) from human erythrocytes. The fluorescence parameters as obtained from the maximum entropy analysis differ considerably in both enzymes, reflecting the unique properties of the flavin microenvironment. Three conformational substates are revealed in LipDH-AV and five in GR. Almost 90% of the population of GR molecules has a fluorescence lifetime in the order of 30 ps which originates from efficient exciplex formation with Tyr197. Equilibrium fluctuations between conformational substates are observed for LipDH-AV on a nanosecond time scale in the temperature range 277-313 K. Interconversion between conformational substates in GR is slow, indicating that large activation barriers exist between the states. In agreement with these results, a model is postulated which ascribes a role in catalysis to equilibrium fluctuations between conformational substates in GR and LipDH-AV. From time-resolved fluorescence anisotropy as a function of temperature, distinction can be made between flavin reorientational motion and interflavin energy transfer. In both proteins intersubunit energy transfer between the prosthetic groups is observed. Furthermore, it is revealed that only the flavin in glutathione reductase exhibits rapid restricted reorientational motion. Geometric information concerning the relative orientation and distance of the flavins can be extracted from the parameters describing the energy-transfer process. The obtained spatial arrangement of the flavins is in excellent agreement with crystallographic data.     

Substitution of the flavin chromophore with lipophilic side chains: A novel membrane redox label

Summary The synthesis of amphiphilic flavins substituted with C₁₈-hydrocarbon sidechains in positions 3, 5, 7, 8 and 10 is described. 3-, 7-, and 10-amphiflavins were obtained by new total syntheses. Furthermore, 3-amphiflavin was obtained by C₁₈-alkylation of natural flavin in the oxidized state, whereas 5-amphi(dihydro)flavin was obtained by alkylation under reducing conditions.In the course of these studies, a novel, selective oxidation reaction was found taking place with the 8-methyl group of natural flavins. In this way lumiflavin and riboflavin derivatives could be converted directly to flavin-8-nor-8-carboxylic acids or the corresponding alkyl esters.The new flavin derivatives lend themselves for incorporation into lipid vesicles, thus yielding the basis for model studies of anisotropic flavin chemistry and redox transfer through membranes, as described in the concomitant paper (Schmidt, W., Hemmerich, P. 1981).J. Membrane Biol. 59:129. The new flavins are characterized by means of absorption, fluorescence, and proton nuclear magnetic resonance spectroscopy.     

Visible light-induced photooxidation of glucose sensitized by riboflavin

We conducted this study to evaluate the oxidation of glucose induced by visible light in the presence of sensitizers such as methylene blue and flavins (i.e., flavin mononucleotide and riboflavin). The concentration of the sensitizers was similar to that of flavin in parenteral nutrients. The photooxidation of glucose sensitized by flavin mononucleotide or riboflavin was greater than that which was observed in the presence of methylene blue, whereas the isotopic effect of deuterium oxide (D(2)O) was enhanced more substantially in the presence of methylene blue than in the presence of flavins. These results show that methylene blue exerts its action through singlet oxygen and that at a high substrate concentration (as was used in this work) flavin mononucleotide and riboflavin act preferentially as type I sensitizers. In the flavin photosensitized processes, the presence of hydrogen peroxide, superoxide anion, and hydroxyl radical was demonstrated. The photooxidation of glucose is favored by an increase in pH, and it also depends on the energy absorbed by the system. By using a specific reagent for glucose (i.e., o-toluidine), it was possible to quantify the photoconversion of glucose. The results obtained in this work should be considered in the management of glucose-containing parenteral nutrients that are exposed to visible light in the presence of a multivitamin complex containing flavin mononucleotide.     

NADPH-flavin reductase in human erythrocytes and the reduction of methemoglobin through flavin by the enzyme

A NADPH-dehydrogenase of human erythrocytes was exhaustively purified to a homogeneous protein judging from the electrophoresis on a polyacrylamide gel in the presence of sodium dodecyl sulfate. Studies on the specificity for the electron acceptor of this enzyme suggest that flavins serve as the natural and direct electron acceptor. The enzyme showed a broad specificty for flavins and the Michaelis constants for flavins were estimated to be 5 × 10^?5 M for both FMN and riboflavin. Rapid reduction of methemoglobin by the enzyme in the presence of flavin was demonstrated, and the reduction was explained by the reduction of flavin by the enzyme, and subsequent non-enzymatic reduction of methemoglobin by the reduced flavin. The therapeutic significance of flavins was discussed with reference to the flavin reductase activity in hereditary methemoglobinemia.     

Differences in protein structure of xanthine dehydrogenase and xanthine oxidase revealed by reconstitution with flavin active site probes

The native flavin, FAD, was removed from chicken liver xanthine dehydrogenase and milk xanthine oxidase by incubation with CaCl2. The deflavoenzymes, still retaining their molybdopterin and iron-sulfur prosthetic groups, were reconstituted with a series of FAD derivatives containing chemically reactive or environmentally sensitive substituents in the isoalloxazine ring system. The reconstituted enzymes containing these artificial flavins were all catalytically active. With both the chicken liver dehydrogenase and the milk oxidase, the flavin 8-position was found to be freely accessible to solvent. The flavin 6-position was also freely accessible to solvent in milk xanthine oxidase, but was significantly less exposed to solvent in the chicken liver dehydrogenase. Pronounced differences in protein structure surrounding the bound flavin were indicated by the spectral properties of the two enzymes reconstituted with flavins containing ionizable -OH or -SH substituents at the flavin 6- or 8-positions. Milk xanthine oxidase either displayed no preference for binding of the neutral or anionic flavin (8-OH-FAD) or a slight preference for the anionic form of the flavin (6-hydroxy-FAD, 6-mercapto-FAD, and possibly 8-mercapto-FAD). On the other hand, the chicken liver dehydrogenase had a dramatic preference for binding the neutral (protonated) forms of all four flavins, perturbing the pK of the ionizable substituent greater than or equal to 4 pH units. These results imply the existence of a strong negative charge in the flavin binding site of the dehydrogenase, which is absent in the oxidase.     

Fluorescence polarization in a planar array of pigment molecules: theoretical treatment and application to flavins incorporated into artificial membranes.

A quantitative fluorescence polarization theory of molecules bound to two-dimensional plane layers has been developed when the electronic transition moments of absorption and emission are parallel within the fluorescent molecules. The transition moments are assumed to be in preferred orientation with respect to the normal to the plane and to be randomly oriented within the plane (rotational symmetry with the normal as axis of symmetry). Three basic model distributions of transition moments are investigated quantitatively. These model distributions represent a simplification but in most cases may be expected to describe reality with sufficient accuracy. For all distributions, two cases of different mobility of molecules are treated: (a) the lifetime of fluorescence is small compared with the characteristic relaxation time of the distribution, and (b) the lifetime of fluorescence is long, so that a complete reorientation of transition moments during the excited state can take place. From the quantitative calculations four characteristic quantities are derived, which are appropriate for the analysis of experimental data. Experiments are carried out with phosphatidylcholine bilayer membranes which contain three differently substituted amphiphilic flavins. All three flavins yield similar data. Their analyses predict free and fast mobility of the flavin chromophore.     

Interactions of flavins with melanin. Studies on equilibrium binding of riboflavin to dopa-melanin and some spectroscopic characteristics of flavin-melanin complex

Natural melanins are photoprotective pigments that in mammals are principally found in the skin, hair, and eyes. Although the molecular mechanism of photoprotection of pigmented cells has not yet been established, several hypotheses have been proposed with melanin acting as a light filter, free radical scavenger, and quencher of electronically excited states of reactive intermediates. It can be expected that the detoxicating efficiency of melanin should be enhanced if the melanin and potentially cytotoxic species are brought close together. Such a situation may occur for a number of photosensitizing dyes that have the ability to bind to melanin. The interaction of melanin with flavins has been studied under strictly controlled experimental conditions. The equilibrium dialysis method has been employed to determine dissociation constants and the number of binding sites in melanin at pH 5-9. The data reveal that synthetic DOPA-melanin has two different classes of binding sites with dissociation constants of 10(-6) and 10(-5) M, respectively. The overall binding capacity of melanin, at pH 7, is 250 nmol RF/mg melanin. The amount of bound-to-melanin RF increases with pH. The absorption spectra of melanin complexes with RF and lumiflavin indicate that hydrophobic interaction may be involved in the binding of these flavins by melanin. No changes in flavin fluorescence have been detected after binding of flavin to melanin. It appears that, contrary to cationic photosensitizing dyes, the singlet excited state of flavin molecules is not quenched by melanin.     

Oxidation of 2-thioflavins by peroxides. Formation of flavin 2-S-oxides

The reaction of 2-thioriboflavin (sulfur replacing the oxygen substituent at position 2 alpha) with hydrogen peroxide at pH approximately 10 leads to a blue flavin (lambda max = 565 nm) which was purified in stable, homogeneous form. Titrations of 2-thioflavins with m-chloroperoxybenzoic acid also yield the same blue flavins with consumption of 1 eq of peracid. Anaerobic reduction of the blue flavin by sodium dithionite requires 4e- eq, and leads to formation of 1,5-dihydro-2-thioflavin. Oxidation of the latter with O2 restores the original 2-thioflavin. pH titration of the blue flavin shows two pKa values of 2.4 and 6.6, with no apparent ionization in the pH range 8-11. These results suggest that the blue flavin is a flavin 2-S-oxide. The visible absorption spectra of flavin 2-S-oxides show a pronounced dependence on solvent polarity. This property suggests that these flavin analogs may be useful hydrophilic/hydrophobic probes of flavoprotein active sites. Flavin 2-S-oxides can be oxidized further to the 2-sulfinate and 2-sulfonate analogs, some properties of which are described.