Resolution of the lifetimes and correlation times of the intrinsic tryptophan fluorescence of human hemoglobin solutions using 2 GHz frequency-domain fluorometry

Tryptophan-to-heme electron transfer in ferrous myoglobins

It was recently demonstrated that in ferric myoglobins (Mb) the fluorescence quenching of the photoexcited tryptophan 14 (*Trp(14)) residue is in part due to an electron transfer to the heme porphyrin (porph), turning it to the ferrous state. However, the invariance of *Trp decay times in ferric and ferrous Mbs raises the question as to whether electron transfer may also be operative in the latter. Using UV pump/visible probe transient absorption, we show that this is indeed the case for deoxy-Mb. We observe that the reduction generates (with a yield of about 30%) a low-valence Fe-porphyrin π [Fe(II)(porph(●-))] -anion radical, which we observe for the first time to our knowledge under physiological conditions. We suggest that the pathway for the electron transfer proceeds via the leucine 69 (Leu(69)) and valine 68 (Val(68)) residues. The results on ferric Mbs and the present ones highlight the generality of Trp-porphyrin electron transfer in heme proteins.

The extracellular chaperone haptoglobin prevents serum fatty acid-promoted amyloid fibril formation of β2-microglobulin, resistance to lysosomal degradation, and cytotoxicity

Fibril formation of β2-microglobulin and associated inflammation occur in patients on long term dialysis. We show that the plasma protein haptoglobin prevents the fatty acid-promoted de novo fibril formation of β2-microglobulin even at substoichiometric concentration. The fibrils are cytotoxic, and haptoglobin abolishes the cytotoxicity by preventing fibril formation. Haptoglobin does not alleviate the cytotoxicity of preformed fibrils. Fibrillar β2-microglobulin is resistant to lysosomal degradation. However, the species of β2-microglobulin populated in the presence of haptoglobin is susceptible to degradation. We observed that haptoglobin interacts with oligomeric prefibrillar species of β2-microglobulin but not with monomeric or fibrillar β2-microglobulin that may underlie the molecular mechanism. 1,1'-Bis(4-anilino)naphthalene-5,5'-disulfonic acid cross-linking to haptoglobin significantly compromises its chaperone activity, suggesting the involvement of hydrophobic surfaces. Haptoglobin is an acute phase protein whose level increases severalfold during inflammation, where local acidosis can occur. Our data show that haptoglobin prevents fibril formation of β2-microglobulin under conditions of physiological acidosis (between pH 5.5 and 6.5) but with relatively decreased efficiency. However, compromise in its chaperone activity under these conditions is more than compensated by its increased level of expression under inflammation. Erythrolysis is known to release hemoglobin into the plasma. Haptoglobin forms a 1:1 (mol/mol) complex with hemoglobin. This complex, like haptoglobin, interacts with the prefibrillar species of β2-microglobulin, preventing its fibril formation and the associated cytotoxicity and resistance to intracellular degradation. Thus, our study demonstrates that haptoglobin is a potential extracellular chaperone for β2-microglobulin even in moderately acidic conditions relevant during inflammation, with promising therapeutic implications in β2-microglobulin amyloid-related diseases.

Protein matrix elasticity determined by fluorescence anisotropy of its tryptophan residues

Rotational motions of Trp residues embedded within human hemoglobin matrix have been measured by using their steady-state fluorescence anisotropy. The mean square angular displacement theta2 of Trp residues, depending on the temperature, can be expressed by W = 1/2Ctheta2 where W is the thermal energy acting on the Trp residues and C the resilient torque constant of the protein matrix. To study the external medium influencing the protein dynamics, comparative experiments were made with protein in aqueous buffer and in the presence of 32% glycerol. The data show that between 5 degrees C and 25 degrees C, external medium acts on the protein matrix elasticity.

Heterogeneous motions within human apohemoglobin

Conservation of the secondary and tertiary protein organization of human apohemoglobin was observed at temperatures ranging from 7 to 25 degrees C using CD spectra in the far-UV (200-250 nm) and near-UV (250-300 nm) regions. The dynamics of apohemoglobin were probed using fluorescence quenching experiments on the Trp residues and an extrinsic dye (ANS or bis-ANS) located in the heme cavities. The long decay time of the dye emission (> 10 ns) reveals the dynamics of the protein matrix averaged over the whole molecule. The short decay time of the Trp residue emission (congruent with 3 ns) probes the dynamics of their close vicinities. When the temperature rises from 10 to 20 degrees C, the average intraproteic motions throughout the whole apohemoglobin matrix are greatly accelerated, whereas the hydrophobic protein regions around the alpha14, beta15 and beta37 Trp residues appear much less animated. These dynamic differences between the behavior of the softer matrix and the packed rigid regions containing the tryptophans could be one of the requisites for apohemoglobin stability. We suspect that the highly rigid tryptophan domains in human apohemoglobin are likely to be knots.

Entropy-driven intermediate steps of oxygenation may regulate the allosteric behavior of hemoglobin

When the oxygen binding isotherms of human, bovine and fallow deer (Dama-Dama) hemoglobins are measured at different temperatures either by optical or calorimetric techniques, analyses according to the Adair's formalism show that at least one of the intermediate steps of ligation has a positive enthalpy change, i.e., absorbs rather than emitting heat, indicating that it is entropy rather than enthalpy driven (Bucci, E., et al. 1991. Biochemistry. 30:3195-3199; Bucci, E., et al. 1993. Biochemistry. 32:3519-3526; Johnson, C., et al. 1992. Biochemistry. 31:10074-10082; Johnson, C., et al. 1995. Biophys. Chem. 59:107-117). This phenomenon is confirmed in systems in which the beta82 lysines of human hemoglobin are covalently cross-linked by acylation with dicarboxylic acids of increasing length, namely the fumaryl (four-carbon-long), adipoyl (six-carbon-long), and sebacoyl (10-carbon-long) residues. Consistently in all of the systems here reported, the enthalpy excursions are masked by compensatory entropy changes, which keep the free energy of ligand binding constant for the first three steps of oxygenation. Furthermore, the cooperativity index and the overall oxygen affinity seem to be correlated to the positive enthalpy excursions of the intermediate steps of ligation. Fumaryl-Hb (hemoglobin cross-linked with a fumaryl residue, four carbons) with the lowest absorption of heat has the highest affinity and lowest cooperativity index. Adipoyl-Hb (hemoglobin cross-linked with an adipoyl residue, six carbons) has the highest absorption of heat and the highest cooperativity index. It appears that nonuniform heat release by the intermediates of oxygenation is part of the allosteric phenomena in hemoglobin systems. There is not enough information that would allow assigning these phenomena to the interplay of the various conformations described for hemoglobin besides the classic T (Fermi et al. 1984. J. Mol. Biol. 175:159-174) and R (Shanaan. 1983. J. Mol. Biol. 171:31-59), as listed at the end of the Discussion. The possibility cannot be excluded that entropy-driven steps characterize new conformational transitions still to be described.

Introduction of negative charges to a crosslinked hemoglobin: lack of effect on plasma half time

Intramolecularly crosslinked hemoglobins do not dissociate into alpha 1 beta 1 dimers. As a result, they escape glomerular filtration and have plasma half times of 4 hours. This value is shorter than for albumin (5.2 hours) with similar molecular weight but higher negative charge. The present study was done to determine if increased negative charge on a hemoglobin covalently crosslinked with bis (3,5-dibromosalicyl) sebacate would lengthen its plasma half time. Negative charge was introduced by acylation with succinic anhydride. The product had a higher negative charge; however, plasma half time was not increased. A larger fraction of the succinylated material was excreted in the urine suggesting molecular instability.

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.

Positive and negative cooperativities at subsequent steps of oxygenation regulate the allosteric behavior of multistate sebacylhemoglobin

Cross-linked human hemoglobin (HbA) is obtained by reaction with bis(3,5-dibromosalicyl) sebacate. Peptide maps and crystallographic analyses confirm the presence of the 10 carbon atom long sebacyl residue cross-linking the two beta82 lysines of the beta-cleft (DecHb). The Adair's constants, obtained from the oxygen binding isotherms, show that at the first step of oxygenation normal hemoglobin and DecHb have a very similar oxygen affinity. In DecHb negative binding cooperativity is present at the second step of oxygenation, which has an affinity 27 times lower than at the first step. Positive cooperativity is present at the third binding step, whose affinity is 380 times that of the second step. The fourth binding step shows a weak negative cooperativity with an affinity one-half that of the third step. Crystals of deoxy-DecHb diffracted to 1.9 angstroms resolution. The resulting atomic coordinates are very similar to those of Fermi et al. [(1984) J. Mol.Biol. 175, 159-174] and Fronticelli et al. [(1994) J. Biol Chem. 269, 23965-23969] for deoxy-HbA. The electron density map of deoxy-DecHb indicates the presence of the 10 carbon bridge between the beta82 lysines. Molecular modeling confirms that insertion of the linker into the T structure requires only slight displacement of the two beta82 lysines. Instead, insertion of the linker into the R and R2 structures [Shaanan (1983) J. Mol. Biol. 171, 31-59; Silva et al. (1992) J. Biol. Chem. 267, 17248-17256] is hindered by serious sterical restrictions. The linker primarily affects the partially and fully liganded states of hemoglobin. The data suggest in DecHb concerted conformational changes at each step of oxygenation.

Studies on the dissociation of cryptomonad biliproteins

The spectroscopic properties of two biliproteins, phycocyanin 645 and phycoerythrin 566, have been studied by treating the proteins with two different agents, NaSCN at pH 6.0, or pH 4.0 without NaSCN. For phycoerythrin 566, treatment with NaSCN revealed that the visible CD spectrum of its chromophores was separated into a pair of different spectra, and each of these spectra was observed as a negative and one or more positive bands. For phycocyanin 645, two negative CD bands have been observed previously, together with two or more positive bands, in the dimer (alpha 2 beta 2) state, and NaSCN treatment caused elimination of one of these negative bands. The dimer was stable at pH 6.0, but at pH 4.0 the spectra of phycocyanin 645 had one less negative band than those at pH 6.0. Chromatography demonstrated that phycocyanin 645 was a monomer (alpha beta) at pH 4.0. Monomers of cryptomonad biliproteins have never been observed before. Excitation at 514 nm, in picosecond time-resolved fluorescence studies, produced lifetimes of 11.0 and 45.2 ps for dimers and monomers, respectively. Excitation at 566 nm yielded times of 1.38 and 1.24 ps, for dimers and monomers, respectively. CD in the far UV showed that monomers and dimers had very similar secondary structures. These results have been used to test an hypothesis that proposed two types of exciton splitting among the chromophores of phycocyanin 645, and perhaps phycoerythrin 566 could also have this chromophore organization.

UV resonance Raman and excited-state relaxation rate studies of hemoglobin

We have measured the UV resonance Raman (UVRR) spectra of human methemoglobin fluoride (metHbF) and examined the Raman saturation behavior of the metHbF trytophyl (Trp) and tyrosyl (Tyr) residues. Our high-quality UVRR spectra devoid of Raman saturation with 229- and 238.3-nm CW laser excitation allow us to determine small changes in Trp and Tyr residue Raman band frequencies and intensities caused by the hemoglobin R-T quaternary structural change induced by the allosteric effector inositol hexaphosphate. At 238.3-nm excitation, we observe a ca. 15 and 8% intensity increase for the Trp and Tyr bands, respectively, upon the R-T transition. In contrast, a small intensity decrease is observed with 225-nm excitation. These intensity alterations result from Trp and Tyr absorption and Raman excitation profile red-shifts which correlate with a strong 231.5-nm R-T absorption spectral change. These absorption and Raman excitation profile red-shifts and our model compound absorption studies together suggest a T-state increase in the hydrogen bond donation of the Trp-beta(2)37 and Tyr-alpha(1)42 residues at the alpha 1 beta 2 subunit interface. The Tyr-alpha 42 residue appears to be a hydrogen bond donor, rather than an acceptor. We determined the electronic excited-state relaxation rates of the Trp and Tyr residues in hemoglobin by using Raman saturation spectroscopy with 225-nm pulsed laser excitation. The observed average excited-state relaxation rate of the Trp residues is ca. 1/120 ps and is independent of the quaternary structure. This rate is slower that that observed for Trp residues of horse myoglobin. The average excited-state relaxation rate of the Tyr residues is ca. 1/60 ps for both the R and T quaternary forms. These are the first Tyr relaxation rates measured for any heme protein.

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.

Molecular dynamics of sickle and normal hemoglobins

Molecular dynamics (MD) simulations have been carried out for 62.5 ps on crystal structures of deoxy sickle cell hemoglobin (HbS) and normal deoxy hemoglobin (HbA) using the CHARMM MD algorithm, with a time step of 0.001 ps. In the trajectory analysis of the 12.5-62.5 (50 ps) simulation, oscillations of the radius of gyration and solvent-accessible surface area were calculated. HbS exhibited a general contraction during the simulation, while HbA exhibited a nearly constant size. The average deviations of simulated structures from the starting structures were found to be 1.8 A for HbA and 2.3 A for HbS. The average rms amplitudes of atomic motions (atomic flexibility) were about 0.7 A HbA and about 1.0 A for HbS. The amplitudes of backbone motion correlate well with temperature factors derived from x-ray crystallography. A comparison of flexibility between the alpha- and beta-chains in both HbA and HbS indicates that the beta-chains generally exhibited greater flexibility than the alpha-chains, and that the HbS beta-chains exhibit greater flexibility in the N-terminal and D- and F-helix regions than do those of HbA. The average amplitude of backbone torsional oscillations was about 9 degrees, a value comparable with that of other simulations, with enhanced torsional oscillation occurring primarily at the ends of helices or in loop regions between helices. Comparison of atomic flexibility and torsional oscillation results suggests that the increased beta-chain flexibility results from relatively concerted motions of secondary structure elements. The increased flexibility may play an important role in HbS polymerization.(ABSTRACT TRUNCATED AT 250 WORDS)

Discontinuous release of heat at successive steps of oxygenation in human and bovine hemoglobin at pH 9.0

We have measured the temperature dependence of the oxygen-binding isotherms of human and bovine hemoglobin at pH 9.0 in 0.1 M borate buffer. In both hemoglobins the ionization of the Bohr protons is finished at this pH; therefore, their heat does not interfere with the measurements. Two sets of curves have been obtained, which have been analyzed by either singular or global procedures for estimating the enthalpy changes of subsequent steps of oxygenation. The data indicate that in human hemoglobin the reaction with oxygen is enthalpy driven for steps 1, 2, and 4 while it is entropy driven for step 3. In bovine hemoglobin this phenomenon is even more evident: steps 2 and 4 are enthalpy driven while steps 1 and 3 are entropy driven. The discontinuous distribution of heat at subsequent steps of oxygenation suggests that the T to R transition in hemoglobin is not a monotonic process and involves conformations with novel characteristics.

Subunit interactions in hemoglobin probed by fluorescence and high-pressure techniques

The dissociation of the subunits of human adult oxyhemoglobin has been investigated by using steady-state fluorescence anisotropy, multifrequency phase fluorometry, and high hydrostatic pressure. Human hemoglobin obtained by using two purification procedures (bulk preparation by centrifugation or further fractionation using anion-exchange chromatography) was labeled with an extrinsic fluorescent probe, 5-(dimethylamino)naphthalene-1-sulfonyl chloride (DNS-Cl). The long fluorescence lifetime of this probe allows for the observation of the macromolecular tumbling, and thus provides a method for observing changes in the size of the complex upon subunit dissociation under differing solution conditions of proton and organic phosphate concentration. At pH 7, the dansylated preparations of bulk and fractionated hemoglobin showed a concentration-dependent decrease in the anisotropy which though not identical can only arise from the tetramer to dimer dissociation. We observed primarily the dimer at pH 9 and a small destabilization of the tetramer in the presence of saturating inositol hexaphosphate (IHP). High-pressure experiments allowed for the observation of the dissociation of the hemoglobin dimer into monomers. From these measurements, we estimate the dimer dissociation constant to be between 0.1 and 1 nM. We compare the present results on the subunit affinities in hemoglobin obtained from steady-state and time-resolved fluorescence data with those obtained previously by using gel filtration, sedimentation, and kinetic techniques. These comparisons are indicative of a certain degree of conformational heterogeneity in the hemoglobin preparations.

Effect of temperature on oxygen affinity and anion binding of bovine hemoglobin

Measurements of oxygen binding to bovine hemoglobin have been carried out over the temperature range 15-37 degrees C at pH 7.33. The standard enthalpy of oxygenation after correction for the heat of oxygen solution and of the Bohr protons is found to be -7.1 or -7.2 kcal/mol in the presence of 0.1 M chloride or bromide, respectively. This value is well below the -14.4 kcal/mol determined for human hemoglobin under identical experimental conditions. As reported by Fronticelli et al. (C. Fronticelli, E. Bucci and A. Razynska, J. Mol. Biol. 202 (1988) 343), the preferential binding of anions by bovine hemoglobin recognizes the various halides. Measurements at various temperatures reveal that this is true only above 25 degrees C. The halide recognition and the less exothermic enthalpy of oxygenation of bovine hemoglobin are probable due to oxygen-linked hydrophobic effects that are larger in bovine than in human hemoglobin.

Time-resolved emission spectra of hemoglobin on the picosecond time scale

We used front-face illumination to examine the steady-state and time-resolved emission from the intrinsic tryptophan emission of human hemoglobin (Hb). Experimental conditions were identified which eliminated all contributions of scattered light. The sensitivity obtained using front-face optics was adequate to allow measurement of the wavelength-dependent frequency response of the emission to 2 GHz. The intensity decays displayed pico- and nanosecond components in the emission at all wavelengths from 315 to 380 nm. The contribution of the picosecond component decreased from 72 to 37% over this range of wavelengths. Frequency-domain measurements were used to calculate the time-resolved emission spectra and decay-associated emission spectra. These spectra indicate that the picosecond components of the emission display maxima near 320 nm, whereas the nanosecond components are centered at longer wavelengths near 335 nm. The nanosecond components appear to be due to residual impurities which remain even in highly purified samples of Hb. However, we cannot eliminate the possibility that some of these components are due to Hb itself.