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Biomedical subjects

I Pecht

Publications and source records attributed to I Pecht.

At least 199 records · Page 11Linked to original sources
PubMed1976

Conformational equilibria accompanying the electron transfer between cytochrome c (P551) and azurin from Pseudomonas aeruginosa.

The redox reaction between cytochrome c (Cyt c) (P-551) and the blue copper protein azurin, both from Pseudomonas aeruginosa, was studied using the temperature-jump technique. Two relaxation times were observed in a mechanism assumed to involve three equilibria. The fast relaxation time (0.4 less than tau less than 8 ms) was ascribed to the electron exchange step. The slow relaxation time (tau congruent to 37 ms) was assigned to a conformational equilibrium of the reduced azurin that was coupled through the electron exchange step to a faster conformational equilibrium of the oxidized Cyt c (P551). But because the Cyt c (P551) isomerization, being very rapid, was uncoupled from the two slower equilibria, and was assumed to involve no spectral change, the amplitude of its relaxation time (tau congruent to 0.1 ms) would be zero. At 25 degrees C and pH 7.0 the rate constants for the oxidation and reduction of Cyt c (P551) by azurin were 6.1 X 10(6) and 7.8 X 10(6) M-1 s-1, respectively; for the formation and disappearance of the reactive conformational isomer of azurin they were 12 and 17 s-1, respectively. The rates for the Cyt c (P551) isomerization could only be estimated at approximately 10(4) s-1. The thermodynamic parameters of each reaction step were evaluated from the amplitudes of the relaxations and from Eyring plots of the rate constants. Measurements of the overall equilibrium constant showed it to be temperature independent (5-35 degrees C), i.e. deltaHtot = 0. This zero enthalpy change was found to be compatible with the enthalpies calculated for the individual steps. In the electron exchange equilibrium, the values of the activation enthalpies were two to three times higher than the values published for various low molecular weight reagents in their electron exchange with copper proteins, yet the rate of exchange between Cyt c (P551) and azurin was some hundreds of times faster. This was explained in terms of the measured positive or zero entropies of activation that could result from a high level of specificity between the proteins particularly in areas of complementary charges. The mechanism of electron transfer was considered as essentially an outer sphere reaction, of which the rate could be approximated by the Marcus theory.

Azurin↗
PubMed1976

Kinetic evidence for hapten-induced conformational transition in immunoglobin MOPC 460.

The kinetics of hapten binding to the homogeneous immunoglobulin A secreted by the murine plasmacytoma MOPC 460 was investigated by the chemical relaxation method. Two distinct relaxation times were observed in the binding equilibrium with three different haptens. A detailed concentration dependence analysis of relaxation times and amplitudes was performed with the hapten epsilon-N(2,4-dinitrophenyl)-lysine (Dnp-Lys). The results support a mechanism in which two interconvertible conformational states of the protein bind the hapten with different association constants. Hapten binding shifts the equilibrium towards the better binding state. These observations form kinetic evidence for a conformational transition induced in the immunoglobulin by ligand binding to its antigen binding site, and are in line with the allosteric hypothesis for the initiation of physiological functions by antigen-antibody association.

Allosteric Regulation↗
PubMed1975

Circular dichroism and fluorescence studies of homogeneous antibodies to type III pneumococcal polysaccharide.

The near-ultraviolet circular dichroism (CD) of three homogeneous anti-type III pneumococcal antibodies in the absence and the presence of the specific hexasaccharide ligand was studied. In addition recombinations and hybridizations of H and L chains derived from two of these antibodies were carried out and the CD spectra of bound and free reconstituted IgG molecules were measured. The results indicate that the CD spectra of the native antibodies in the 260-310-nm range are very similar in shape and sign and exhibit a positive band at 285 nm. The homologous reconstituted antibody molecules exhibited CD spectra very similar in shape and sign to those of the native antibody molecules although recombinant molecules are no longer stabilized by interchain disulfide bonds. Upon addition of the hexasaccharide ligand, a significant decrease in amplitude of the CD spectra (18-21%) occurred in all three native antibodies and their Fab fragments as well as in the homologous recombinant molecules. No CD spectral changes could be detected upon interaction of the hapten ligand with the heterologous recombinants. All homogeneous antibodies studied exhibited fluorescence quenching upon oligosaccharide binding and a blue shift of the emission maximum. This property allowed the determination of the binding constant of one selected antibody to be made. Taken together, CD and fluorescence spectroscopic data suggest that oligosaccharide ligands induced detectable conformational changes in the Fab fragment of the antibody.

Antibodies, Bacterial↗
PubMed1975

Conformational changes induced in a homogeneous anti-type III pneumococcal antibody by oligosaccharides of increasing size.

The circular polarization of luminescence (CPL) emitted by tryptophan residues was used as a sensitive probe for measuring ligand-induced structural changes in a homogeneous type III pneumococcal antibody. A series of oligosaccharide ligands of increasing size derived from type III polysaccharide by partial acid hydrolysis was assayed. Ligand-induced changes in the circular polarization of fluorescence of the antibody were observed for all antigens tested, including tetra-, hexa-, and octasaccharides and a 16-residue oligomer, the largest changes being recorded upon interaction with the intact soluble type III pneumococcal (SIII) polysaccharide. When Fab' or F(ab')2 fragments were used instead of the antibody IgG for binding of SIII polysaccharide, the extent of conformational changes was decreased. This suggests interactions between Fab and Fc portions in the IgG molecule and subsequent conformational changes in Fc part upon antigen binding. Reduction of interchain disulfide bonds abolished the additional spectral changes attributed to the Fc part but not the changes observed in the Fab part, thus suggesting that the presence of the interchain disulfide bond in the hinge region is required for maximal CPL changes to occur. Small monovalent ligands, i.e., the tetra-, hexa-, and octasaccharides, were capable of inducing CPL changes in the Fab part of the antibody molecule as well as CPL changes attributed to the Fc portion. A multivalent ligand containing about 16 sugar residues appears to be the minimal antigenic size required for triggering conformational changes attributed to the Fc part, similar to those seen in the interaction with the whole polysaccharide antigen.

Antibodies, Bacterial↗
PubMed1975

Homogeneity and variability in the structure of azurin molecules studied by fluorescence decay and circular polarization.

The fluorescence decay of apoazurin derived from Pseudomonas aeruginosa is monoexponential. By this criterion the population of molecules of apoazurin is homogeneous. The emission anisotropy factor and the absorption anisotropy factor at the red edge of the absorption band assume similar values, showing that the tryptophan residue in apoazurin has the same asymmetric environment both in the ground and excited states. This finding suggests tight packing of the protein at the tryptophan environment. Native azurin does not decay monoexponentially. Moreover, comparison between the quantum yield calculated from the decay kinetics and the one measured directly shows that the majority of the azurin molecules are not fluorescent. There is thus variability in the structure of azurin molecules with an equilibration time that is longer than the fluorescence lifetime. Different asymmetric environment was found for the tryptophan residue in oxidized and reduced holoprotein and in apoazurin, as studied by the circular polarization of the fluorescence. D(2)O increases the fluorescence lifetime of apoazurin by 6 percent, compared to the lifetime in H(2)O solution; therefore water molecules may have access to the tryptophan residue, though the latter is situated in a hydrophobic environment.

Amides↗
PubMed1975

Effect of electric fields on light-scattering and fluorescence of chromaffin granules.

Electric field pulses (in the 5 to 25 kV/cm range) were found to cause a transient increase in the intensity of scattered light (wavelength equals 369 nm, scattering angle equals 90 degrees) from aqueous sucrose suspensions of chromaffin granules. Similar observations were made with the membranes of osmotically lysed chromaffin granules. Under the same experimental conditions the degree of polarization of the scattered light changed only very slightly. The fluorescence of the hydrophobic probe diphenyl-hexatriene, incorporated into the membrane of intact chromaffin granules, showed similar transient changes in the intensity. The calculated relaxation times for these changes in optical properties were approximately 150 musec for the rising phase, and approximately 1 msec for the early stage of the decay. A further relaxation time of about 30 msec was also observed by using this probe. Essentially, all of these signals originated from the granule membrane, and could be attributed to rather small changes in particle size, membrane thickness or refractive index. Moreover, these signals were found to be completely reversible. Catecholamine release from intact granules, pulsed at voltages of 25 kV/cm, occurs already during the first few milliseconds of the transient membrane change.

Animals↗
PubMed1975

Antigen-induced conformational changes in antibodies and their Fab fragments studied by circular polarization of fluorescence.

Conformational changes induced in antibody molecules and in their Fab fragments by binding of antigen were investigated by the circular polarization of the fluorescence emitted by the tryptophan residues. This property of the fluorescence is related to the asymmetry, and thus to the conformation and environment, of the emitting chromophore. Changes in the circular polarization of the fluorescence of the antibody were observed upon binding of RNase to anti-RNase, of poly(DL-alanyl)-poly(L-lysine) to antipoly(D-alanine), and of the "loop" of lysozyme, a monovalent antigenic determinant, to anti"loop." The spectral changes were observed at different antigen-antibody ratios, including high antigen excess, indicating that they are due to antigen binding and not to aggregation. The circular polarization of fluorescence also detects changes in conformation of the different Fab fragments upon binding of the corresponding antigens. These changes in conformation were, however, markedly different from those observed for the whole antibody molecules, and indicated an interaction between the Fc and Fab fragments in the antibody molecule, and probably a change in the conformation of Fc upon binding of antigen to the antibody. In contrast, the small hapten, phosphorylcholine, did not induce a change in the circular polarization of the fluorescence of its antibody or corresponding Fab fragments. Reduction of the interchain disulfide bonds of the antibodies abolished the antigen-induced spectral changes due to the presence of the Fc portion in the molecule, but not the changes observed in Fab, suggesting that the disulfide bonds at the hinge region of the antibody are required for the transmission of the conformational change from the Fab to the Fc.

Animals↗
PubMed1974

Fluorescence enhancement of laccase induced by reduction of Cu(II) sites.

The intrinsic fluorescence of laccase (p-diphenol:O(2) oxidoreductase, EC 1.10.3.2), emitted by its tyrosinyl and tryptophanyl residues, underwent significant enhancement upon reduction of the enzyme redox sites. The increase in quantum yield reached its maximum after the addition of approximately four reduction equivalents and depended on the wavelength of excitation:54% increase for lambda(ex) = 250 nm and 76% for lambda(ex) = 305 nm.A linear correlation between this enhancement and the reduction of the type 1 Cu(II) was observed both by direct measurement and by calculation on the basis of added reductant. The implications of the fluorescence enhancement and its correlation with the reduction of the type 1 copper are discussed in terms of the possible quenching mechanisms. The possibility of a redox-induced structural transition in the protein is suggested.

Catechol Oxidase↗