Tryptophan fluorescence of human hemoglobin. I. Significant change of fluorescence intensity and lifetimes in the T - R transition

Redox chemistry of the molecular interactions between tea catechins and human serum proteins under simulated hyperglycemic conditions

Carbonylation is an irreversible modification in oxidized proteins that has been directly related to a number of health disorders including Type 2 diabetes. Dietary antioxidants have been proposed to counteract the oxidative stress occurring under hyperglycemic conditions. An understanding of the nature and consequences of the molecular interactions between phytochemicals and human plasma proteins is of utmost scientific interest. Three tea catechins namely epicatechin (EC), epigallocatechin (EGC) and epigallocatechin-3-gallate (EGCG) were tested for (i) their affinity to bind to human serum albumin (HSA) and human hemoglobin (HH) and (ii) their ability to inhibit tryptophan (Trp) depletion and for the formation of specific protein carbonyls and pentosidine in the aforementioned proteins. Both proteins (20 mg mL(-1)) were allowed to react with postprandial plasmatic concentrations of the catechins (EC: 0.7 μM, EGC: 1.8 μM, and EGCG: 0.7 μM) under simulated hyperglycemic conditions (12 mM glucose/0.2 mM Fe(3+)/37 °C/10 days). The three catechins were able to inhibit Trp oxidation and protein carbonylation in both plasma proteins. Some anti-glycation properties were linked to their binding affinities. The molecular interactions reported in the present study may explain the alleged beneficial effects of tea catechins against the redox impairment linked to hyperglycemic conditions.

Interaction between the natural lipopeptide [Glu1, Asp5)] surfactin-C15 and hemoglobin in aqueous solution

The interaction between natural lipopeptide [Glu(1), Asp(5)] surfactin-C15 (surfactin) and hemoglobin (Hb) has been studied. Surface tension measurements show that the critical micelle concentration (cmc) of surfactin increases from 1.54 x 10(-5) to 3.86 x 10(-5) mol/L with Hb. The UV spectra display that the effect of surfactin on Hb exhibits strong concentration-dependent fashion and the aquometHb convert to hemichrome at high surfactin concentration. Small-angle neutron scattering (SANS) and freeze-fracture transmission electron microscopy (FF-TEM) measurements show that surfactin result in the formation of a fractal structure representing a "necklace model" of micelle-like clusters randomly distributed along the protein polypeptide chain at high surfactin concentration. Far-UV circular dichroism (CD) results confirmed that surfactin can disrupt the helical structure of protein at high concentrations, although the enhanced native-like behavior of protein by low concentration of surfactin was observed. The microenvironment change around Phe amino residues and disulfide bonds of Hb was obtained from near-UV CD spectra.

A fluorimetric and circular dichroism study of hemoglobin—Effect of pH and anionic amphiphiles

In this work, bovine hemoglobin (Hb) has been studied mainly by the fluorescence method. pH has been found to exert a profound effect on Hb structure. This has been confirmed by fluorescence and circular dichroism (CD) studies. The pH-induced change in quaternary structure of Hb indirectly affects its secondary structure. This in turn affects ligand binding to Hb at various pH. The binding of two amphiphiles, a bile salt and a surfactant, have been investigated. The pH-induced structural modification of Hb has been confirmed by studies with the well-known denaturant urea and the polarity probe ANS, which has been used as an extrinsic fluorophore.

Beta7E-beta132K salt bridge and sickle haemoglobin stability and conformation

The liganded (R-state) form of sickle cell haemoglobin (HbS) is of particular relevance at non-polymerizing concentrations as oxy HbS exhibits unusual properties compared with oxy HbA: mechanical precipitability (resulting from surface denaturation), greater unfolding at an air-water interface and a tendency to oxidize more readily. In human haemoglobins, the beta7 (A4) Glu residue forms an intrachain salt bridge with beta132 (H10) Lys in both liganded and deoxy structures. In the present study, recombinant haemoglobins with substitutions in the beta7 and beta132 sites were studied in order to determine the role of the beta7-beta132 salt bridge on Hb conformational integrity and stability. The elimination of this interhelix bridge correlates with enhanced surface denaturation and conformational alterations in the central cavity 2,3-diphosphoglycerate (DPG) cleft and alpha1beta2 interface. The A-helix beta7 Ala substitution generates a class of conformational change at the DPG pocket and alpha1beta2 interface that is distinct from that dictated by the H-helix beta132 Ala substitution. These results are significant with regard to the communication pathway between the alpha1beta1 and alpha1beta2 interfaces, and the new understanding of Hb allostery dependent upon tertiary structural constraints caused by effector binding to the R-state.

Distinct domain responses of R-state human hemoglobins A, C, and S to anions

Anionic regulation of hemoglobin (Hb) is of increasing interest for the design of Hb-based oxygen carriers. Even "external" amino-acid substitutions can alter the nature and extent of anionic control. This was shown by evaluation of the anion sensitivities of liganded, R-state, forms of HbA, HbC (beta6 Glu --> Lys) and HbS (beta6 Glu --> Val). The beta6 mutants differ in the anion-sensitivity of their central cavities, alpha1beta2 interfaces, and heme and beta93 Cys environments. The mutant Hbs also exhibit increased anion-dependent oxidation and surface denaturation. Moreover, differential chloride effects on oxygen binding by Hbs C, S compared to HbA occur after R-state stabilization by fluoresceination of beta93 Cys. It is concluded that the "external" substitutions in the mutant Hbs have structural consequences that are propagated to varying extents to other domains as a result of anion binding, and that these anion-dependent changes may underlie mechanisms leading to the observed increase in oxidation propensity and surface denaturation.

Steady-state fluorescence and circular dichroism of trout hemoglobins I and IV interacting with tributyltin

The effect of tributyltin chloride (TBTC) on rainbow trout (Salmo irideus) hemoglobin I (HbI) and hemoglobin IV (HbIV) was characterized by the steady-state fluorescence of intrinsic and extrinsic fluorescent probes. The fluorescence emission spectrum (lambdaex 280 nm) is greatly increased in intensity by the presence of the organotin in both proteins. Circular dichroism spectra in the same samples show a small decrease in theta222, a measure correlated with the percentage of the alpha-helical content. Morever, important changes in near-UV, Soret, and visible regions of CD were induced by TBTC. The correlation of data obtained with trout hemoglobins (HbI and HbIV) with similar measurements on globins suggests that the presence of heme is necessary for the interaction of the organotin compound with the proteins.

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.

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.

Stopped-flow front-face fluorometer: a prototype design to measure hemoglobin R----T transition kinetics

Stopped-flow techniques are successfully used to study the kinetics of the R----T transition of hemoglobin (Hb). We have previously used front-face fluorometry to demonstrate that (i) the intrinsic fluorescence of Hb primarily originates from beta 37 Trp; (ii) the intrinsic fluorescence is sensitive to the R----T transition; and (iii) the emission of the fluorescent probes bound to specific sites on the Hb molecule (beta 93 Cys) is sensitive to the R----T transition. These findings suggested that a stopped-flow front-face fluorometer could probe R----T transitions at specific sites, such as the aromatic amino acids and sites selectively binding extrinsic fluorophores. We have developed a prototype instrument using as the core a Gibson-Durrum stopped-flow apparatus on line with a digital data analysis system using a modified Marquardt algorithm. Excitation (470 nm) and emission light (520 nm) were selected by narrow band pass filters. To study the R----T transition, a solution of purified oxy Hb A covalently bound to the fluorescent probe 5-iodoacetamidofluorescein (Hb A-AF) (1.0 g%) was mixed rapidly with deoxygenated buffer (pH 7.35, 0.05 M potassium phosphate) containing 2 mg/ml of sodium dithionite. The hemoglobin, at a final concentration of 0.5 g% after mixing, is essentially completely tetrameric. A first-order reaction was observed with a rate constant near 8 s-1, similar to the oxygen dissociation rate reported for oxy Hb A.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic fluorescence of carp hemoglobin: a study of the R----T transition

The intrinsic fluorescence of hemoglobins is known to respond to ligand-induced changes in the quaternary structure of the protein. Carp hemoglobin is an interesting model to study the quaternary transition since its R----T equilibrium is pH-dependent and at low pH, in the presence of organic phosphate, it remains in the T or 'deoxy' quaternary structure, even when saturated with ligand. In this study, using front-face fluorometry, we show that the intrinsic fluorescence intensity exhibited by carp carboxyhemoglobin increases as the pH is lowered below 6.5 in the presence of inositol hexaphosphate. At low pH, carp methemoglobin is less affected by the addition of inositol hexaphosphate than is the CO derivative, while little or no change is observed in the met-azide derivative. We conclude: (1) the exact nature of the R to T state transition induced by inositol hexaphosphate differs for carp carboxy-, met- and met-azide hemoglobin derivatives; (2) the chromophores responsible for the changes observed with absorption spectroscopy may not be the same as those chromophores responsible for the fluorescence differences; and (3) alpha 46-Trp is tentatively assigned as one source of fluorescence emission. Furthermore, fluorescence properties of carp hemoglobin are compared to those of human hemoglobin.

Steady-state fluorescence emission from the fluorescent probe, 5-iodoacetamidofluorescein, bound to hemoglobin

In the past, fluorescence emission from an extrinsic fluorophore bound to heme-proteins would only be studied with the removal of the heme since fluorescence from the fluorophore could not be detected using right-angle optics. Using front-face fluorometry, a significant steady state emission signal originating from the probe bound to hemoglobin is detected. This is the first report of the detection of extrinsic fluorescence of a probe bound to a heme-protein. We also demonstrate that the extrinsic probe, 5-iodoacetamidofluorescein, is covalently bound to hemoglobin, specifically at beta 93 Cysteine. Ligand binding results in a change in the fluorophore fluorescence intensity as predicted by hemoglobin crystallographic studies. Efficiency of energy transfer measurements are made.

Intrinsic fluorometric determination of the stable state of aggregation in hemoglobins

Front-face fluorometry can detect steady-state intrinsic fluorescence of hemoglobins (R. E. Hirsch, R. S. Zukin, and R. L. Nagel, 1980, Biochem. Biophys. Res. Commun. 93, 432-439), a property that can be used to study the dimerization of human hemoglobins (R. E. Hirsch, N. A. Squires, C. Discepola, and R. L. Nagel, 1983, Biochem. Biophys. Res. Commun. 116, 712-718). We report that the stable dimeric hemoglobin components of the arcid clams Noetia ponderosa and Anadara ovalis exhibit fluorescence emission maxima shifted to longer wavelengths compared to tetrameric human hemoglobin. Conversely, the tetrameric major hemoglobin (Hb) component of A. ovalis exhibits an emission maximum similar to that of tetrameric Hb A. Hence, stable dimeric hemoglobins can be detected by emission maxima at longer wavelengths relative to Hb A. Fluorescence studies of ligand binding to these clam hemoglobins indicate structural and functional differences among these components and compared to Hb A. We conclude that different stable aggregation states of hemoglobins may be determined by intrinsic fluorescence when studied with front-face optics.

A fluorescence decay time study of tryptophan in isolated hemoglobin subunits

The time-resolved fluorescence behavior of tryptophan residues in isolated human hemoglobin subunits was determined using a sync-pumped dye laser system and time-correlated single photon counting detection. Two decay components having values near 80 ps and 2 ns were found in the fluorescence decay of the alpha-subunit. The data for the beta-chains were best fitted with 3 decay components of 90 ps, 2.5 ns and 6.4 ns. We propose that the decay times correspond to conformations of the proteins in which the disposition of the tryptophan to the heme residue differs.

Picosecond fluorescence decay of tryptophans in myoglobin

The fluorescence decay characteristics of Mb, MbCO, metMb (sperm whale), metMb (yellowfin tuna), and their apo derivatives were determined by using a picosecond streak camera and time-correlated single photon counting. The emission is dominated by tryptophans that transfer their energy to the heme on a subnanosecond time scale. Sperm whale Mb and derivatives have two tryptophans and their decays can be interpreted mainly as two exponentials, one of ca. 20 ps and the other of 130 ps, whereas tuna Mb has one tryptophan and its emission is nonexponential but dominated by one component of 31 ps. These results along with Förster energy transfer calculations allow us to assign the ca. 30-ps emission to Trp-14 and the 130-ps emission to Trp-7 in Mb. The streak camera was modified to determine the decay of the fluorescence anisotropy. In metMb (tuna) the fluorescence anisotropy decays in 100 ps, which is postulated to result from rapid motion of the Trp-14. Because energy transfer was used to gate the anisotropy, the fast motion of Trp-14 is proposed to correspond to only 10% of the equilibrium distribution of molecules.

The detection of hemoglobin dimers by intrinsic fluorescence

We have found that the intrinsic fluorescence emission maxima of oxy, met, and cyanmet hemoglobins have a concentration dependent shift to longer wavelengths. For oxy-hemoglobin, this effect is increased in the presence of 3M NaCl. At the protein concentrations studied, these liganded hemoglobins undergo dimerization. In contrast, horse-heart met myoglobin (which is a monomer), and deoxy Hb A and Hb Beth Israel (that have greatly decreased dissociation constants), exhibited a significantly smaller shift in fluorescence maxima. We conclude that hemoglobin dimers exhibit a bathochromic shift with respect to the tetramer. This shift is probably due to the increase in surface exposure of beta 37 Trp that occurs during hemoglobin dimerization.

Tryptophan fluorescence of human hemoglobin. II. Effect of inositol hexaphosphate on the T-R transition

Fluorescence spectra of tryptophan residues of human hemoglobin in the absence and presence of inositol hexaphosphate were measured at room temperature. The tryptophan fluorescence intensity of deoxy HbA was observed to decrease in accordance with the binding with inositol hexaphosphate. The fluorescence intensity of HbA, Hb Kempsey (beta 99 Asp-Asn), Hb Chesapeake (alpha 92 Arg-Leu) and NES-des-Arg Hb (des-141 alpha Arg and beta 93 Cys-N-ethylsuccinimide derivative) in the presence of inositol hexaphosphate exhibits a considerable decrease in the deoxy to oxy transition, while no or slight fluorescence intensity change was observed in the deoxy to oxy transition of Hb Kempsey and NES-des-Arg Hb in the absence of inositol hexaphosphate. The tryptophan fluorescence behavior suggest that the inositol hexaphosphate-induced structural change in these hemoglobins is attributable to the formation of a different T type of structure from that of the normal T-R transition.