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F Lederer

Publications and source records attributed to F Lederer.

At least 37 records · Page 2Linked to original sources
PubMed2000

Stable solitons of quadratic ginzburg-landau equations

We present a physical model based on coupled Ginzburg-Landau equations that supports stable temporal solitary-wave pulses. The system consists of two parallel-coupled cores, one having a quadratic nonlinearity, the other one being effectively linear. The former core is active, with bandwidth-limited amplification built into it, while the latter core has only losses. Parameters of the model can be easily selected so that the zero background is stable. The model has nongeneric exact analytical solutions in the form of solitary pulses ("dissipative solitons"). Direct numerical simulations, using these exact solutions as initial configurations, show that they are unstable; however, the evolution initiated by the exact unstable solitons ends up with nontrivial stable localized pulses, which are very robust attractors. Direct simulations also demonstrate that the presence of group-velocity mismatch (walkoff) between the two harmonics in the active core makes the pulses move at a constant velocity, but does not destabilize them.

Journal Article↗
PubMed2000

Three-dimensional walking spatiotemporal solitons in quadratic media

Two-parameter families of chirped stationary three-dimensional spatiotemporal solitons in dispersive quadratically nonlinear optical media featuring type-I second-harmonic generation are constructed in the presence of temporal walk-off. Basic features of these walking spatiotemporal solitons, including their dynamical stability, are investigated in the general case of unequal group-velocity dispersions at the fundamental and second-harmonic frequencies. In the cases when the solitons are unstable, the growth rate of a dominant perturbation eigenmode is found as a function of the soliton wave number shift. The findings are in full agreement with the stability predictions made on the basis of a marginal linear-stability curve. It is found that the walking three-dimensional spatiotemporal solitons are dynamically stable in most cases; hence in principle they may be experimentally generated in quadratically nonlinear media.

Journal Article↗
PubMed2000

Azimuthal instability of spinning spatiotemporal solitons

We find one-parameter families of three-dimensional spatiotemporal bright vortex solitons (doughnuts, or spinning light bullets), in dispersive quadratically nonlinear media. We show that they are subject to a strong instability against azimuthal perturbations, similarly to the previously studied (2+1)-dimensional bright spatial vortex solitons. The instability breaks the spinning soliton into several fragments, each being a stable nonspinning light bullet.

Journal Article↗
PubMed1999

Exact solitary-wave solutions of chi(2) Ginzburg-Landau equations.

A family of exact temporal solitary-wave solutions (dissipative solitons) to the equations governing second-harmonic generation in quadratically nonlinear optical waveguides, in the presence of linear bandwidth-limited gain at the fundamental harmonic and linear loss at the second harmonic, is found, and the existence domain for the solutions is delineated. Direct numerical simulations of the solitons demonstrate that, as well as the classical pulse solutions to the cubic Ginzburg-Landau equation, the dissipative solitons can propagate robustly over a considerable distance before the model's intrinsic instability leads to onset of "turbulence." Two-soliton bound states are also predicted and then found in the direct simulations. We estimate real values of the physical parameters necessary for the existence of the solitons predicted, and conclude that they can be observed experimentally. A promising application for the solitons is their use in closed-loop cavities.

Journal Article↗
PubMed1999

Soliton generation in optical fibers for a dual-frequency input.

We analyze scenarios of soliton generation in an ideal fiber for an input that consists of either two in-phase or out-of-phase solitonlike optical pulses at different frequencies. In both cases the relationship between the structure of the emerging solitons and the frequency separation of the initial solitons is studied both analytically and numerically. Depending on the value of the frequency detuning, if the two initial solitons are in phase (symmetric input), two bound solitons with equal amplitudes (breather), a single soliton, or a pair of solitons, which have equal amplitudes and exhibit opposite velocities, can be generated. When the two initial solitons are out-of-phase (antisymmetric input), only the last scenario takes place. Also, we calculated the threshold values of the frequency separation at which the structure of the emerging solitons changes. Moreover, we demonstrated that two of these critical frequencies correspond to cusplike maxima of the energy density of the radiative modes. Finally, we show that these analytical results are entirely verified by numerical simulations.

Journal Article↗
PubMed1998

Probing intramolecular electron transfer within flavocytochrome b2 with a monoclonal antibody.

Flavocytochrome b2 or L-lactate dehydrogenase from yeast is a tetrameric enzyme which oxidizes lactate at the expense of cytochrome c or artificial electron acceptors. The prosthetic group FMN is reduced by the substrate and then transfers sequentially the reducing equivalents to heme b2 in the same subunit. The latter is reoxidized by cytochrome c. The crystal structure of the enzyme indicates that each subunit is composed of a flavodehydrogenase domain (FDH) and a cytochrome b2 domain; the latter, which encompasses the first 99 residues of the peptide chain, is mobile relative to the tetrameric FDH assembly. We describe here the properties of a monoclonal antibody elicited against the holoenzyme. It only recognizes the heme-binding domain, with a Kd lower than 10(-7) M, and its epitope is conformational. In the enzyme-IgG complex, flavin is reduced normally and can be reoxidized by ferricyanide, but no longer by heme b2. Stopped-flow experiments in the absence of electron acceptors give no indication of flavin to heme electron transfer in the enzyme-antibody complex. In other words, the two domains are functionally uncoupled. The binding stoichiometry is 1/1 for the Fab fragment with respect to the isolated, monomeric, heme-binding domain, but 2/4 with respect to the enzyme tetramer; furthermore, binding of two Fab fragments per tetramer is sufficient to cause inhibition of intra-subunit flavin to heme electron transfer in all four subunits. Altogether these results can only be rationalized by considering that mobility of the cytochrome domain with respect to the FDH is an essential component of the catalytic cycle. The first experiment designed to locate the epitope shows it does not encompass the interdomain peptide linker (so-called hinge region, centered on residues 99-100).

Animals↗
PubMed1998

About the pKa of the active-site histidine in flavocytochrome b2 (yeast L-lactate dehydrogenase).

Flavocytochrome b2 or L-lactate dehydrogenase from yeasts catalyzes the oxidation of L-lactate at the expense of monoelectronic acceptors such as cytochrome c, its physiological partner. When incubated in the presence of both L-lactate and a keto acid, the enzyme catalyzes a transhydrogenation reaction wherein only the flavin is involved. During this reaction, the substrate alpha-hydrogen is transferred not only to the solvent but also in part to the keto acid, which acts as reverse substrate. Thus, when bound to the reduced enzyme, this hydrogen is sticky. In the context of a carbanion mechanism, it resides on Nepsilon of His373, the active site base. We have shown before that a correlation between the amount of intermolecular hydrogen transfer from [2-3H] lactate and the keto acid reverse substrate concentration enables the determination of the first-order rate constant, kHe, for exchange of the substrate-derived protein-bound hydrogen with bulk solvent (Urban P, Lederer F, 1985, J Biol Chem 260:11115-11122). In this work, we show that the exchange with the solvent appears to be independent of the phosphate buffer concentration in the range from 40 to 500 mM. It is thus probable that exchange occurs directly with water molecules. The second-order rate constant for exchange is then 0.16 (+/-0.03) M(-1) s(-1). Using the Eigen equation, this figure yields a pKa of 9.1+/-0.1 for His373 in the reduced enzyme, compared to a probable value of 6.0 or less in the oxidized enzyme (Suzuki H, Ogura YC, 1970, J Biochem 67:291-295). The mechanistic significance of these results is discussed.

Binding Sites↗
PubMed1997

Molecular interpretation of inhibition by excess substrate in flavocytochrome b2: a study with wild-type and Y143F mutant enzymes.

The crystal structure of flavocytochrome b2 (L-lactate dehydrogenase) from Saccharomyces cerevisiae suggests that Tyr143 plays a dual role at the active site: it contributes to substrate binding and, most importantly, makes a hydrogen bond to a heme propionate, which could facilitate communication between the domains. Previous work on the Y143F mutant enzyme provided support for these hypotheses [Miles, C. S., Rouvière-Fourmy, N., Lederer, F., Mathews, F. S., Reid, G. A., Black, M. T., & Chapman, S. K. (1992) Biochem. J. 285, 187-192; Rouvière-Fourmy, N., Capeillère-Blandin, C., & Lederer, F. (1994) Biochemistry 33, 798-806]. In the course of kinetic comparisons between the wild-type (WT) enzyme and the Y143F mutant protein, we observed for the latter signs of inhibition by excess substrate at much lower concentrations than observed for the former. A detailed investigation of the phenomenon has shown that, for the wild-type and Y143F forms, lactate at high concentrations inhibits both cytochrome c and ferricyanide reduction. In these cases, inhibition appears to be a specific effect, since acetate at identical concentrations exerts an inhibitory effect that is markedly weaker than that of lactate. In the pre-steady-state, in the absence of acceptor, flavin and heme reduction are unaffected by high substrate concentrations in the WT enzyme case. For the Y143F mutant, flavin reduction is similarly unaffected, but heme reduction is inhibited to nearly the same extent by high lactate and acetate concentrations. In this case, inhibition can probably be ascribed to ionic strength effects. The combination of stopped-flow and steady-state results suggests that lactate binds with weak affinity at the active site when the flavin is in the semiquinone state, preventing electron transfer to heme b2 and hence to acceptors. This phenomenon is analogous to the inhibition exerted by pyruvate when bound to the enzyme at the semiquinone stage [Tegoni, M., Janot, J. M., & Labeyrie, F. (1990) Eur. J. Biochem. 190, 329-342]. We suggest that the substrate carboxylate and the heme propionate of the mobile heme-binding domain compete for the Tyr143 hydroxyl group, hence for approach to the flavin. In the Y143F mutant enzyme, in which the interdomain interaction is impaired, competition would play in favor of the substrate, resulting in the inhibition at lower lactate concentrations than observed for the wild-type enzyme.

Acetic Acid↗
PubMed1996

Functional properties of the histidine-aspartate ion pair of flavocytochrome b2 (L-lactate dehydrogenase): substitution of Asp282 with asparagine.

The FMN prosthetic group of flavocytochrome b2 or L-lactate dehydrogenase oxidizes lactate to pyruvate. The reducing equivalents are then transferred one by one, intramolecularly, to heme b2 and then to external acceptors. Substrate oxidation is thought to begin with abstraction of the substrate alpha-hydrogen as a proton by an enzyme base. It has been proposed that this role is played by His373, which lies close to the flavin in the crystal structure and interacts with Asp282. It has also been shown before, using hydrogen exchange measurements, that the pKa of His373 is substantially increased in the wild-type reduced enzyme compared to that in the oxidized state. We report here the enzymatic properties of the D282N mutant flavocytochrome b2. Steady-state rate measurements with [2-1H]lactate and [2-2H]-lactate indicate that, as predicted, the Michaelis complex stability is hardly affected, whereas the transition state for proton abstraction increases in energy by 2.8 kcal/mol. Steady-state inhibition studies were conducted with a number of active-site ligands: sulfite, D-lactate, pyruvate, and oxalate. Binding was found to be most affected for oxalate, but kinetic patterns indicated oxalate and pyruvate were still capable of binding to the enzyme both at the oxidized and semiquinone stages, whereas inhibition by excess substrate, due to lactate binding at the semiquinone stage, was lost. Finally, analysis of the intermolecular hydrogen transfer catalyzed by the enzyme between [2-3H]lactate and fluoropyruvate indicated that the substitution with asparagine facilitates exchange of the histidine-bound proton and hence induces a decrease in the pKa value of H373 in the reduced enzyme of about 1.4 pH units. Nevertheless, the rate constant value for exchange with the solvent of the enzyme-bound substrate alpha-proton indicates that H373 is still protonated in the reduced mutant enzyme at neutral pH. Thus, the D282N mutation destabilizes the transition state for proton abstraction and decreases the pKa of H373 in the reduced enzyme but is insufficient to bring it back to a normal value.

Asparagine↗
PubMed1996

The role of a beta barrel loop 4 extension in modulating the physical and functional properties of long-chain 2-hydroxy-acid oxidase isozymes.

Peroxisomal long-chain 2-hydroxy-acid oxidase, an FMN-dependent enzyme, catalyzes the oxidation of a variety of L-2-hydroxy acids into keto acids at the expense of oxygen. We recently reported the cloning and sequencing of its CDNA and the existence of a weakly expressed isozyme [Belmouden, A., Le, K. H. D., Lederer, F. & Garchon, H. J. (1993) Eur. J. Biochem. 214, 17-251. This isozyme, beta 2 differs from the major one in having a three-residue insertion, -VRK-, in loop 4 of the beta 8 alpha 8 barrel. In the crystal structures of homologous flavocytochrome beta 2, and glycolate oxidase, the corresponding region of loop 4 is disordered. We now report on the constitutive high-level expression of isozymes beta 1, and beta 2 in Escherichia coli under control of the lambda pL promoter, and on the influence of the E. coli genetic background and the growth medium on the expression level. We describe the properties of isozyme beta 2 and compare them with those of pure isoform beta 1. The visible spectra of the purified enzymes differ in the position of the near-ultraviolet band of the prosthetic group. pH titration studies indicate that the FMN ionizes at N3 at a lower pH than free flavin and that there is a small pKa difference between the isozymes. To our knowledge, the only other known case of a lowered pKa for the protein-bound flavin is that of glycolate oxidase. In the CD spectra of the FMN region, a marked difference between isozymes in the 270-300-nm region appears to be related to the pKa difference for the N3-H bond. Kinetic parameters for a number of substrates and inhibitors are indistinguishable within the limits of experimental error, with the exception of values for kcat for mandelate (the most active substrate), Km for hydroxyhippurate (a new substrate), Ki for cinnamate and oxalate, and Kd for sulfite. The differences are no larger than twofold. The foregoing comparison between isozymes beta 1 and beta 2 shows that the naturally engineered insertion in loop 4 exerts some influence on the flavin spectral properties and the active-site reactivity. Since the corresponding loop 4 regions in the three-dimensional structures of flavocytochrome 2 and glycolate oxidase are 1.5-2.0 nm removed from the flavin, it would appear either that loop 4 has a very different conformation in hydroxy-acid oxidase, or that it may interact with the active site due to mobility.

Absorption↗