Rates of energy transfer between tryptophans and hemes in hemoglobin, assuming that the heme is a planar oscillator

Sphingomyelin-induced structural modification of native human hemoglobin and its chemically and thermally disrupted secondary structure: A photophysical exploration

Sphingomyelin-induced structural modification of Human Hemoglobin (Hb) has been investigated in its native and unfolded conformers that are partially denatured in presence of ∼ 4 M urea, completely denatured in ∼ 8 M urea and thermally disrupted (at ∼ 65 °C) state. The absorption studies unveil ground state complexation between Hb and SM. From steady-state fluorescence and quenching studies alteration of the micro-environments around Trp residues of Hb in above mentioned different cases has been determined. Moreover, lesser exposure of Trp residues to SM in thermally disrupted Hb can be accounted for the exceptionally interesting outcomes in other experiments. The alterations in the time-resolved decay profiles of native Hb, partially and totally chemically denatured as well as thermally disrupted Hb with gradual addition of SM also affirm the amendment of the proteinous micro-environment surrounding Trp residues in a view of FRET between Trp residues and heme group. Wavelength-sensitive emission spectral studies reveal that the protein shows red edge effect in its different conformations in presence and absence of SM. Interestingly, the wavelength-responsive time-resolved study at a constant excitation wavelength demonstrates that with addition of lipid the increment of the average fluorescence lifetime signifies a considerable modulation of solvation dynamics of the fluorescent Trp residues in their excited state being greatest in case of thermally disrupted Hb. Nevertheless, the loss of α-helicity of Hb at its various conformers with addition of SM has been portrayed thoroughly by means of far-UV CD spectral studies in a view of disruption of secondary structure of the protein.

Contrasting effects of pH on the modulation of the structural integrity of hemoglobin induced by sodium deoxycholate

Bile salt-mediated conformational modification of hemoglobin (Hb) was examined at three different pHs i.e., 3.2, 7.4 and 9.0. The added bile salt, sodium deoxycholate (NaDC), decreases the α-helicity in Hb (α-helix: 71.3% → 61.7% in the presence of 9.6 mM NaDC, and 83.2% → 66.2% in the presence of 14 mM NaDC, at pH 7.4 and 9.0, respectively), while a reverse pattern of modification in the Circular Dichroism (CD) spectra of Hb is found at pH 3.2. The acid-induced denatured Hb (pH 3.2) regains its structural integrity by changing conformation from a random coil to an α-helix rich secondary structure upon addition of NaDC (α-helix: 10.4% → 53.4%, β-sheet: 31.0% → 18.5% and random coil: 58.6% → 28.1%, in the presence of 0.65 mM NaDC). Also, a step-wise binding interaction pattern of Hb with NaDC was revealed at pH 7.4 and 9.0 upon variation of steady-state fluorescence intensity and average lifetime of Hb. From the fluorescence lifetime decay pattern, the decrement of energy transfer from Trp to a heme group was found upon the addition of NaDC at pH 7.4 and 9.0. However, at pH 3.2, the modification of the time-resolved fluorescence decay behavior of Hb within NaDC is typically reversed, where the energy transfer from Trp to heme is restored to some extent. Thermodynamic analysis suggests that the Hb-NaDC binding interaction is characterized by a dominant entropic contribution interpreted on the basis of release of ordered water molecules to the bulk aqueous phase.

Extending Förster resonance energy transfer measurements beyond 100 Å using common organic fluorophores: enhanced transfer in the presence of multiple acceptors

Using commercially available organic fluorophores, the current applications of Förster (fluorescence) resonance energy transfer (FRET) are limited to about 80 Å. However, many essential activities in cells are spatially and/or temporally dependent on the assembly/disassembly of transient complexes consisting of large-size macromolecules that are frequently separated by distances greater than 100 Å. Expanding the accessible range for FRET to 150 Å would open up many cellular interactions to fluorescence and fluorescence-lifetime imaging. Here, we demonstrate that the use of multiple randomly distributed acceptors on proteins/antibodies, rather than the use of a single localized acceptor, makes it possible to significantly enhance FRET and detect interactions between the donor fluorophore and the acceptor-labeled protein at distances greater than 100 Å. A simple theoretical model for spherical bodies that have been randomly labeled with acceptors has been developed. To test the theoretical predictions of this system, we carried out FRET studies using a 30-mer oligonucleotide-avidin system that was labeled with the acceptors DyLight649 or Dylight750. The opposite 5'-end of the oligonucleotide was labeled with the Alexa568 donor. We observed significantly enhanced energy transfer due to presence of multiple acceptors on the avidin protein. The results and simulation indicate that use of a nanosized body that has been randomly labeled with multiple acceptors allows FRET measurements to be extended to over 150 Å when using commercially available probes and established protein-labeling protocols.

Ultrafast dynamics of nonequilibrium resonance energy transfer and probing globular protein flexibility of myoglobin

Protein structural plasticity is critical to many biological activities and accurate determination of its temporal and spatial fluctuations is challenging and difficult. Here, we report our extensive characterization of global flexibility of a globular heme protein of myoglobin using resonance energy transfer as a molecular ruler. With site-directed mutagenesis, we use a tryptophan scan to examine local structural fluctuations from B to H helices utilizing 10 tryptophan-heme energy transfer pairs with femtosecond resolution. We observed ultrafast resonance energy transfer dynamics by following a nearly single exponential behavior in 10-100 ps, strongly indicating that the globular structure of myoglobin is relatively rigid, with no observable static or slow dynamic conformational heterogeneity. The observation is against our molecular dynamics simulations, which show large local fluctuations and give multiple exponential energy transfer behaviors, suggesting too flexible of the global structure and thus raising a serious issue of the force fields used in simulations. Finally, these ultrafast energy transfer dynamics all occur on the similar time scales of local environmental relaxations (solvation), leading to nonexponential processes caused by energy relaxations, not structural fluctuations. Our analyses of such processes reveal an intrinsic compressed- and/or stretched-exponential behaviors and elucidate the nature of inherent nonequilibrium of ultrafast resonance energy transfer in proteins. This new concept of compressed nonequilibrium transfer dynamics should be applied to all protein studies by time-resolved Förster resonance energy transfer (FRET).

Allosteric Effectors Influence the Tetramer Stability of Both R- and T-states of Hemoglobin A

The contribution of heterotropic effectors to hemoglobin allostery is still not completely understood. With the recently proposed global allostery model, this question acquires crucial significance, because it relates tertiary conformational changes to effector binding in both the R- and T-states. In this context, an important question is how far the induced conformational changes propagate from the binding site(s) of the allosteric effectors. We present a study in which we monitored the interdimeric interface when the effectors such as Cl-, 2,3-diphosphoglycerate, inositol hexaphosphate, and bezafibrate were bound. We studied oxy-Hb and a hybrid form (alphaFeO2)2-(betaZn)2 as the T-state analogue by monitoring heme absorption and Trp intrinsic fluorescence under hydrostatic pressure. We observed a pressure-dependent change in the intrinsic fluorescence, which we attribute to a pressure-induced tetramer to dimer transition with characteristic pressures in the 70-200-megapascal range. The transition is sensitive to the binding of allosteric effectors. We fitted the data with a simple model for the tetramer-dimer transition and determined the dissociation constants at atmospheric pressure. In the R-state, we observed a stabilizing effect by the allosteric effectors, although in the T-analogue a stronger destabilizing effect was seen. The order of efficiency was the same in both states, but with the opposite trend as inositol hexaphosphate > 2,3-diphosphoglycerate > Cl-. We detected intrinsic fluorescence from bound bezafibrate that introduced uncertainty in the comparison with other effectors. The results support the global allostery model by showing that conformational changes propagate from the effector binding site to the interdimeric interfaces in both quaternary states.

Analysis of resonance energy transfer in model membranes: role of orientational effects

The model of resonance energy transfer (RET) in membrane systems containing donors randomly distributed over two parallel planes separated by fixed distance and acceptors confined to a single plane is presented. Factors determining energy transfer rate are considered with special attention being given to the contribution from orientational heterogeneity of the donor emission and acceptor absorption transition dipoles. Analysis of simulated data suggests that RET in membranes, as compared to intramolecular energy transfer, is substantially less sensitive to the degree of reorientational freedom of chromophores due to averaging over multiple donor-acceptor pairs. The uncertainties in the distance estimation resulting from the unknown mutual orientation of the donor and acceptor are analyzed.

Energy transfer method in membrane studies: some theoretical and practical aspects

Some applications of resonance energy transfer (RET) method to distance estimation in membrane systems are considered. The model of energy transfer between donors and acceptors randomly distributed over parallel planes localized at the outer and inner membrane leaflets is presented. It is demonstrated that RET method can provide evidence for specific orientation of the fluorophore relative to the lipid-water interface. An approach to estimating the depth of the protein penetration in lipid bilayer is suggested.

Fluorescence properties of tryptophan residues in the monomeric d-chain of Glossoscolex paulistus hemoglobin: an interpretation based on a comparative molecular model

The primary structure of the 142 residue Glossoscolex paulistus d-chain hemoglobin has been determined from Edman degradation data of 11 endo-Glu-C peptides and 11 endo-Lys-C peptides, plus the results of Edman degradation of the intact globin. Tryptophan occupies positions 15, 33 and 129. Homology modeling allowed us to assign the positions of these Trp residues relative to the heme and its environment. The reference coordinates of the indole rings (average coordinates of the C(varepsilon2) and C(delta2) atoms) for W15 and W129 were 16.8 and 18.5 A, respectively, from the geometric center of the heme, and W33 was located in close proximity to the heme group at a distance which was approximately half of that for W15 and W129. It was possible to identify three rotamers of W33 on the basis of electrostatic and Van der Waals energy criteria. The calculated distances from the center of the heme were 8.3, 8.4 and 9.1 A for Rot1, Rot2 and Rot3, respectively. Radiationless energy transfer from the excited indole to the heme was calculated on the basis of Förster theory. For W33, the distance was more important than the orientation factor, kappa(2), due to its proximity to the heme. However, based on kappa(2), Rot2 (kappa(2)=0.945) was more favorable for the energy transfer than Rot1 (kappa(2)=0.433) or Rot3 (kappa(2)=0.125). In contrast, despite its greater distance from the heme, the kappa(2) of W129 (2.903) established it as a candidate to be more efficiently quenched by the heme than W15 (kappa(2)=0.191). Although the Förster approach is powerful for the evaluation of the relative efficiency of quenching, it can only explain pico- and sub-nanosecond lifetimes. With the average lifetime, <tau>=3 ns, measured for the apomonomer as the reference, the lifetimes calculated for each emitter were: W33-1 (1 ps), W33-2 (2 ps), W33-3 (18 ps), W129 (100 ps), and W15 (600 ps). Experimentally, there are four components for oxymonomers at pH 7: two long ones of 4.6 and 2.1 ns, which contribute approximately 90% of the total fluorescence, one of 300 ps (4%), and the last one of 33 ps (7.4%). It is clear that the equilibrium structure resulting from homology modeling explains the sub-nanosecond fluorescence lifetimes, while the nanosecond range lifetimes require more information about the protein in solution, since there is a significant contribution of lifetimes that resemble the apo molecule.

Investigation of the binding of isoform-selective inhibitors to prostaglandin endoperoxide synthases using fluorescence spectroscopy

Prostaglandin endoperoxide synthase (PGHS) is a heme protein that catalyzes the committed step in prostaglandin and thromboxane biosynthesis. Two isoforms of PGHS exist, a constitutive form termed PGHS-1 and an inducible form termed PGHS-2. We report here fluorescence resonance energy transfer analysis of isoform-selective inhibitors interacting with PGHS-1 and PGHS-2. By measuring fluorescence quenching due to the energy transfer of the inhibitor fluorescence to the heme prosthetic group of PGHS, we determined these inhibitors bind in the arachidonic acid substrate access channel with an R0 of 35 A for PGHS-1 with the PGHS-1 inhibitor and an R0 of 21 A for PGHS-2 with the PGHS-2 inhibitor. The observed fluorescence quenching is completely dynamic and dominated by quenching by the heme. Time-resolved results combined with molecular modeling determine the distance from the inhibitor to the heme moiety to be 20 A in PGHS-1 and 18 A in PGHS-2. Preliminary stopped-flow kinetic studies reveal that the rate of quenching is limited by a first-order protein transition, which is slow, and that bound inhibitor undergoes rapid exchange.

Time-resolved fluorescence of hemoglobin species

We used time-resolved fluorescence in the pico- to nanosecond time range to monitor the presence of tetramers, dimers and monomers in carbonmonoxyhemoglobin (COHb) solutions and to investigate how their distributions change under different experimental conditions. Comparison of fluorescence lifetime computed from the atomic coordinates of COHb (Vasquez et al., 1996) with those experimentally measured allowed identification of molecular species present in the hemoglobin solution. It was possible to observe modification of the distribution of tetramers, dimers, monomers and species with disordered hemes produced by different experimental conditions. Protein concentration affected the detectable lifetimes, indicating increasing amounts of dimers and monomers at low protein concentrations, while the amount of inverted hemes was not modified. Titration with up to 1 M NaCl modified only the extent of dissociation of hemoglobin into dimers, without affecting heme inversion and monomer formation. Hyperbaric pressure increased the amounts of dimers and monomers. This is the first time that monomeric subunits of hemoglobin have been detected at neutral pH in the normal system.

Conformational changes in oxyhemoglobin C (Glu beta 6-->Lys) detected by spectroscopic probing

Hemoglobin C (Glu beta 6-->Lys) shares with hemoglobin S (Glu beta 6-->Val) the site of mutation, but with different consequences: deoxyHbS forms polymers, whereas oxyHbC readily forms crystals. The molecular mechanism for this property of oxyHbC is unknown. Since no detailed oxyHbC crystal structural information exists, spectroscopic probing is used in this study to investigate possible solution-phase conformational changes in HbC compared with HbA. Intrinsic fluorescence combined with UV resonance Raman data demonstrate a weakening of the Trp beta 15-Ser beta 72 hydrogen bond that most likely leads to a displacement of the A helix away from the E helix.

A new front-face optical cell for measuring weak fluorescent emissions with time resolution in the picosecond time scale

Recent developments of ultrafast fluorimeters allow measuring time-resolved fluorescence on the picosecond time scale. This implies one is able to monitor lifetimes and anisotropy decays of highly quenched systems and of systems that contain fluorophores having lifetimes in the subnanosecond range; both systems that emit weak signals. The combination of weak signals and very short lifetimes makes the measurements prone to distortions which are negligible in standard fluorescence experiments. To cope with these difficulties, we have designed a new optical cell for front-face optics which offers to the excitation beam a horizontal free liquid surface in the absence of interactions with optical windows. The new cell has been tested with probes of known lifetimes and anisotropies. It proved very useful in detecting tryptophan fluorescence in hemoglobin. If only diluted samples are available, which cannot be used in front-face optics, regular square geometry can still be utilized by inserting light absorbers into a cuvette of 1 cm path length.

Effect of disordered hemes on energy transfer rates between tryptophans and heme in myoglobin

Our recent linear dichroism study of heme transitions (Gryczynski, Z., E. Bucci, and J. Kusba. 1993. Photochem. Photobiology. in press) indicate that heme cannot be considered a planar oscillator when it acts as an acceptor of radiationless excitation energy transfer from tryptophan. The linear nature of the heme absorption transition moment in the near-UV region implies a strong dependence of the transfer rate factors on the relative angular position of the heme and tryptophan, i.e., on the kappa 2 orientation parameter of the Förster equation. Using the atomic coordinates of SW myoglobin we have estimated the variation of kappa 2 parameter as a function of the heme absorption transition moment direction. The simulations proved that transfer is very efficient and anticipates lifetimes in the picosecond range. Also, they showed that transfer is very sensitive to rotations of the heme around its alpha-gamma-meso-axis, which may reduce the efficiency of transfer to almost zero values, producing lifetimes very similar to those of free tryptophan, in the nanosecond range. Comparisons between the lifetime values reported in the literature and those here estimated suggest that natural heme disorder, in which heme is rotated 180 degrees around its meso axis, is at the origin of the nanosecond lifetimes found in myoglobin systems.

Linear dichroism study of metalloporphyrin transition moments in view of radiationless interactions with tryptophan in hemoproteins

We measured the linear dichroism of several metalloporphyrins embedded in stretched polyvinyl alcohol (PVA) films to estimate the orientation of the absorption transition moments, which in hemoproteins are relevant to the radiationless energy transfer between tryptophan and heme. The metalloporphyrins were derivatives of protoporphyrin IX (PPIX), namely Fe(3+)-PPIX (ferric-heme) and Fe2+CO-PPIX (CO-heme), Mg-PPIX (Mg-heme) and Zn-PPIX (Zn-heme). Measurements were conducted between 300 and 700 nm. In all cases the linear dichroism was wavelength dependent, indicating the presence of several transition moments with different orientations. We focused our attention on the near-UV (300-380 nm) and Soret (380-450 nm) absorption bands. Deconvolution in terms of Gaussian components gave three components between 380 and 450 nm and only one in the 300-380 nm region. Deconvolution of the near-UV and Soret spectra of oxy-, deoxy- and carbonmonoxyhemoglobin gave very similar results, suggesting a very similar orientation of the various transition moments in the free and protein-embedded hemes. It should be stressed that the single 300-380 nm band is the only one responsible for the overlap integral that regulates the energy transfer from tryptophan to heme in hemoproteins (Gryczynski et al., Biophys. J. 63, 648-653, 1992). The dichroism of this single band indicated that its transition moment is oriented at about 60 degrees from the alpha-gamma meso-axis of the heme moiety. We conclude that the heme should be considered a linear oscillator when it acts as acceptor of energy transfer from tryptophans.