Characterization of transferrin metal-binding sites by diffusion-enhanced energy transfer

Nicotinic acetylcholine receptor channel electrostatics determined by diffusion-enhanced luminescence energy transfer

The electrostatic potentials within the pore of the nicotinic acetylcholine receptor (nAChR) were determined using lanthanide-based diffusion-enhanced fluorescence energy transfer experiments. Freely diffusing Tb3+ -chelates of varying charge constituted a set of energy transfer donors to the acceptor, crystal violet, a noncompetitive antagonist of the nAChR. Energy transfer from a neutral Tb3+ -chelate to nAChR-bound crystal violet was reduced 95% relative to the energy transfer to free crystal violet. This result indicated that crystal violet was strongly shielded from solvent when bound to the nAChR. Comparison of energy transfer from positively and negatively charged chelates indicate negative electrostatic potentials of -25 mV in the channel, measured in low ionic strength, and -10 mV measured in physiological ionic strength. Debye-Hückel analyses of potentials determined at various ionic strengths were consistent with 1-2 negative charges within 8 A of the crystal violet binding site. To complement the energy transfer experiments, the influence of pH and ionic strength on the binding of [3H]phencyclidine were determined. The ionic strength dependence of binding affinity was consistent with -3.3 charges within 8 A of the binding site, according to Debye-Hückel analysis. The pH dependence of binding had an apparent pKa of 7.2, a value indicative of a potential near -170 mV if the titratable residues are constituted of aspartates and glutamates. It is concluded that long-range potentials are small and likely contribute little to selectivity or conductance whereas close interactions are more likely to contribute to electrostatic stabilization of ions and binding of noncompetitive antagonists within the channel.

Site-to-site diffusion in proteins as observed by energy transfer and frequency-domain fluorometry

We report measurements of the site-to-site diffusion coefficients in proteins and model compounds, which were measured using time-dependent energy transfer and frequency-domain fluorometry. The possibility of measuring these diffusion coefficients were shown from simulations, which demonstrate that donor (D)-to-acceptor (A) diffusion alters the donor frequency response, and that this effect is observable in the presence of a distribution of donor-to-acceptor distances. For decay times typical of tryptophan fluorescence, the simulations indicate that D-A diffusion coefficients can be measured ranging from 10(-7) to 10(-5) cm2/s. This possibility was verified by studies of a methylene-chain linked D-A pair in solutions of varying viscosity. The D-A diffusion was also measured for two labeled peptides and two proteins, melittin and troponin I. In most cases we used global analysis of data sets obtained with varying amounts of collisional quenchers to vary the donor decay time. Unfolding of troponin I results in more rapid D-A diffusion, whereas for melittin more rapid diffusion was observed in the alpha-helical state but over a limited range of distances.

Consideration of dipole orientation angles yields accurate rate equations for energy transfer in the rapid diffusion limit

Dipole-dipole energy transfer between suitable donor and acceptor chromophores is an important luminescence quenching mechanism and has been shown to be useful for distance determination at the molecular level. In the rapid diffusion limit, where the excited-state lifetime of the donor is long enough to allow the donor and acceptor to diffuse many times their average separation before deexcitation, it is usually assumed that the relative dipolar orientation is completely averaged due to rotational Brownian motion. Under this simplifying assumption, analytical expressions have been derived earlier for the energy transfer rate between donor and acceptor characterized by different geometries. Most such expressions, however, are only approximate because complete angular averaging is permitted only in a geometry that possesses spherical symmetry surrounding each chromophore. In this paper analytical expressions that correctly account for incomplete angle averaging due to steric hindrance are presented for several geometries. Each of the equations reveals a dependence of the energy transfer rate on chromophore orientation. It is shown that correctly accounting for this effect can lead to improvements in estimates of the distance of closest approach from measured quenching rates based on energy transfer experiments.

Fluorescence and kinetic properties of Ru(III) (NH3)5 modified transferrin

Diferric transferrin was modified using aquopentaammine ruthenium(II), a reagent for surface-accessible uncoordinated histidines. Introduction of the cationic Ru(III) (NH3)3 + 5 group on the imidazole of only 5.5 of the 17 uncoordinated histidines enhances the rates of pyrophosphate-assisted iron removal from the N-terminal and C-terminal binding sites by 16- and 2-fold, respectively. This differential effect on the kinetics of the two sites may partially explain why in the native protein the N-terminal site is more labile than the C-terminal site in acidic solutions where histidine residues become positively charged through protonation. The distance between the metal site and nearby uncoordinated histidines was estimated from fluorescence energy transfer measurements using Tb (III) as the donor and pentaammine ruthenium(III)-labeled imidazole of histidine as the acceptor chromophore. A Tsou Chen-Lu statistical analysis of the fluorescence quenching data suggest that two residues in each lobe of the protein are involved in quenching the fluorescence. By using estimates for the index of refraction and the quantum yield and assuming the energy transfer follows parallel first-order kinetics, an upper limit for the donor-acceptor distance of about 1.4 nm was obtained, assuming two uncoordinated histidine residues equidistant from the metal. His-207 and His-242 in the N-terminal lobe of transferrin and His-535 and His-577 in the C-terminal lobe are within this distance, based on information from the lactoferrin crystal structure. It is postulated that His-207 in the N-terminal lobe and His-535 in the C-terminal lobe are the uncoordinated residues that, when protonated or modified with Ru(III) (NH3)3 + 5, lead to accelerated loss of iron from the two binding sites of the protein.

Labelling of histone H5 and its interaction with DNA. 2. Cooperative binding of histone H5 to DNA as probed by steady-state fluorescence and diffusion-enhanced energy transfer

The interaction of histone H5 labelled with fluorescein isothiocyanate (FITC) with DNA has been studied by fluorescence titration, and diffusion-enhanced fluorescence energy transfer (DEFET) measurements with Tb(III) lanthanide chelates as donors. Analysis of the binding data by the model of Schwarz and Watanabe (J.Mol.Biol. 163, 467-484 (1983)) yielded a mean stoichiometry of 60 nucleotides per H5 molecule, independently of ionic strength, in the range of 3 to 300 mM NaCl, at very low DNA concentration (6 microM in mononucleotide). It ensues an approximate electroneutrality of the saturated complexes. Histone H5 molecules appeared to be clustered along the DNA lattice in clusters containing on average 3 to 4 H5 molecules separated by about 79 base pairs, at mid-saturation of the binding sites. The interaction process was found highly cooperative but the cooperativity parameter was also insensitive to ionic strength in the above range. DEFET experiments indicated an important decrease of accessibility of the FITC label to the TbHED3A and TbEDTA- chelates with ionic strength in the 0 to 100 mM NaCl range. In the presence of DNA, H5 appears already folded at low ionic strength so that the FITC probe is also not accessible to the donor chelate. The present study constitutes an indispensable preliminary step to further studies on the localization of histone H5 in condensed chromatin structures.

The transverse location of the retinal chromophore in the purple membrane by diffusion-enhanced energy transfer

We have used fluorescence energy transfer in the rapid-diffusion limit (RDL) to estimate the trans-membrane depth of retinal in the purple membrane (PM). Chelates of Tb(III) are excellent energy donors for the retinal chromophore of PM, having a maximum Ro value for Förster energy transfer of approximately 62 A (assuming a donor quantum yield of 1). Energy transfer rates were measured from the time-resolved emission kinetics of the donor. The distance of closest approach between chelates and the chromophore was estimated by simulating RDL energy-transfer rate constants according to geometric models of either PM sheets or membrane vesicles. The apparent rate constant for RDL energy transfer between Tb(III)HED3A and retinal in PM sheets is 1.5(+/- 0.1) x 10(6) M-1 s-1, corresponding to a depth of approximately 10 +/- 2 A for the retinal chromophore. Cell envelope vesicles (CEVs) from Halobacterium halobium were studied by using RDL energy transfer to assess the proximity of retinal to either the extracellular or intracellular face of the PM. The estimated depth of retinal from the extravesicular face of the PM is 10 +/- 3 A, based on the RDL energy-transfer rate constant. Energy-transfer levels to retinal in the PM were estimated by an indirect method with energy donors trapped in the inner-aqueous space of CEVs. The rate constants derived for this arrangement are too low to be consistent with the shortest depth of retinal deduced for PM sheets. Thus, the intravesticular face of CEVs, corresponding to the cytoplasmic face of cells, is the more distant surface from the chromophore of bacteriorhodopsin.

Interactions of adriamycin with a calcium binding site

Terbium (Tb3+) luminescence has been used to investigate the interactions of adriamycin with a specific calcium binding protein, in the plasma membrane of GH3/B6 pituitary tumor cells. The luminescence intensity and lifetime of the Tb3+-GH3/B6 complex was quenched in the presence of adriamycin. According to Stern-Volmer analysis, the quenching of Tb3+-GH3/B6 luminescence was by both membrane bound adriamycin (Ka = 3.7 x 10(5) M-1) and free adriamycin (kq = 7.3 x 10(7) M-1 s-1). The data suggests that, the calcium binding site at the outer surface of the membrane is collisionally accessible to freely diffusing adriamycin; and, that the toxin receptor site is located near the bound metal ion.

The effect of iron binding on the conformation of transferrin. A small angle x-ray scattering study

Distance distribution functions, p(r), radii of gyration, Rg, and radii of gyration of cross section, Rq, of apotransferrin, monoferric transferrin, and diferric transferrin have been compared. The alteration of Rg and Rq upon iron binding has been determined by a difference method. An unusual feature of the stepwise structural changes of transferrin upon iron saturation is that binding of the first ferric ion is responsible for more than half of the whole change in Rq, whereas Rg alters significantly only after the binding of the second ferric ion.

Theory of phase-modulation fluorescence spectroscopy for excited-state processes

Theory is presented for the analysis of excited-state reactions by fluorescence phase shift and demodulation methods. Initially, a two-state model with spectral overlap is considered to illustrate most simply the effects of excited-state reactions on the expected phase and modulation values. Secondly, a multistate model is described to illustrate the probable effects of a fluorophore interacting with several solvent molecules. We note the following unique features of phase-modulation data expected from a fluorophore whose emission spectrum shifts during the lifetime of the excited state: (1) The modulation frequency dependence of the apparent phase (tau p) and modulation (tau m lifetimes of the reacted species is opposite to that of a heterogeneous population of fluorophores. (2) For the reacted species tau p greater than tau m. For a heterogeneous sample tau p less than tau m. (3) The phase angle of the reacted species can exceed 90 degrees. For a heterogeneous sample phase angles are always less than 90 degrees. Thus, phase and modulation measurements can distinguish between time-dependent processes and spectral heterogeneity by observation of any feature described above. Additionally: (4) The lifetime of the product species can be measured directly. (5) Reverse relaxation can be identified, and the reverse relaxation rates calculated. (6) The wavelength-dependent phase and modulation data can be used to resolve the individual spectra from a two-state reaction. (7) And finally, under favorable conditions, a two-state excited-state process can be distinguished from a continuous multiple-state process. In each instance, model calculations are presented to illustrate the unique potentials of phase-modulation fluorometry for investigations of excited-state processes.

Lanthanide ion luminescence probes. Characterization of metal ion binding sites and intermetal energy transfer distance measurements in calcium-binding proteins. 1. Parvalbumin

Eu(III) laser excitation spectroscopy of the 7F0 leads to 5D0 transition reveals spectral features characteristic of the occupation of the CD and EF Ca(II)-binding sites of parvalbumin by Eu(III) ions. In addition, at pH 6.5, a signal attributable to binding at a third site is observed. This feature is abolished upon lowering the pH to 3.8. At pH 6.5, the appearance of this feature correlates with the decrease in luminescence intensity during titrations of parvalbumin with Eu(III) or Tb(III) after more than approximately 1.8 equiv of either of these ions has been added. Eu(III) ions in the primary sites coordinate zero to two H2O molecules while the third site involves about three coordinated H2O molecules. Parvalbumins in which mixed pairs of lanthanide ions, Ln(III), occupy the CD and EF sites were prepared. Nonradiative energy transfer between Eu(III) and Tb(III) acting as luminescent donors and various other Ln(III) ions serving as acceptors was observed by monitoring the excited-state lifetimes of the donor ions using a pulsed dye laser apparatus. With the assumption of a Förster-type dipole--dipole mechanism, inter-binding-site distance estimates were made from our measurements and are in reasonable agreement with the distance (11.8 A) obtained by X-ray crystallography, especially when Eu(III) is the donor. R0 values (critical distances for 50% energy transfer) in H2O solution range from 9.2 A for the Tb(III)--Ho(III) donor-acceptor pair down to 5.7 A for the Eu(III)--Ho(III) pair.