Conformational relevance of the beta6Glu replaced by Val mutation in the beta subunits and in the beta(1-55) and beta(1-30) peptides of hemoglobin S

Determination of the transition-state entropy for aggregation suggests how the growth of sickle cell hemoglobin polymers can be slowed

Sickle cell anemia is associated with the mutant hemoglobin HbS, which forms polymers in red blood cells of patients. The growth rate of the polymers is several micrometers per second, ensuring that a polymer fiber reaches the walls of an erythrocyte (which has a 7-microm diameter) within a few seconds after its nucleation. To understand the factors that determine this unusually fast rate, we analyze data on the growth rate of the polymer fibers. We show that the fiber growth follows a first-order Kramers-type kinetics model. The entropy of the transition state for incorporation into a fiber is 95 J mol(-1) K(-1), very close to the known entropy of polymerization. This agrees with a recent theoretical estimate for the hydrophobic interaction and suggests that the gain of entropy in the transition state is due to the release of the last layer of water molecules structured around contact sites on the surface of the HbS molecules. As a result of this entropy gain, the free-energy barrier for incorporation of HbS molecules into a fiber is negligible and fiber growth is unprecedentedly fast. This finding suggests that fiber growth can be slowed by components of the red cell cytosol, native or intentionally introduced, which restructure the hydration layer around the HbS molecules and thus lower the transition state entropy for incorporation of an incoming molecule into the growing fiber.

Sickle-cell haemoglobin polymerization: is it the primary pathogenic event of sickle-cell anaemia?

Sickle cell anaemia is associated with a mutant haemoglobin, HbS, which forms polymers in the red blood cells of patients. The primary role of the HbS polymerization for the pathophysiology has been questioned: observations in patients and model organisms contradict deterministic scenarios of sickling crises triggered by polymerization. However, results with knock-out sickle-cell mice, which were cured by delaying HbS polymerization, reconfirm polymerization's primary role. To reconcile the contradictory observations, this article reviews recent findings on two steps in polymerization: homogeneous nucleation of fibres, and their growth. The fibre growth is faster by far than for any other protein ordered structure. This is due to a negligible free-energy barrier for incorporation into a fibre, determined by an entropy gain, stemming from the release of water molecules structured around HbS. The kinetics of fibre nucleation have shown that the formation of the polymer nucleus is preceded by a metastable droplet of a dense liquid. The properties of the dense liquid are sensitive functions of solution composition, including components in micro- and nanomolar amounts. This mechanism allows low-concentration solution components to strongly affect the nucleation kinetics, accounting for the high variability of the disease. These insights can potentially be utilized for control of HbS polymerization and treatment of the disease.

Two-step mechanism of homogeneous nucleation of sickle cell hemoglobin polymers

Sickle cell anemia is a debilitating genetic disease that affects hundreds of thousands of babies born each year worldwide. Its primary pathogenic event is the polymerization of a mutant, sickle cell, hemoglobin (HbS); and this is one of a line of diseases (Alzheimer's, Huntington's, prion, etc.) in which nucleation initiates pathophysiology. We show that the homogeneous nucleation of HbS polymers follows a two-step mechanism with metastable dense liquid clusters serving as precursor to the ordered nuclei of the HbS polymer. The evidence comes from data on the rates of fiber nucleation and growth and nucleation delay times, the interaction of fibers with polarized light, and mesoscopic metastable HbS clusters in solution. The presence of a precursor in the HbS nucleation mechanism potentially allows low-concentration solution components to strongly affect the nucleation kinetics. The variations of these concentrations in patients might account for the high variability of the disease in genetically identical patients. In addition, these components can potentially be utilized for control of HbS polymerization and treatment of the disease.

Metastable mesoscopic clusters in solutions of sickle-cell hemoglobin

Sickle cell hemoglobin (HbS) is a mutant, whose polymerization while in deoxy state in the venous circulation underlies the debilitating sickle cell anemia. It has been suggested that the nucleation of the HbS polymers occurs within clusters of dense liquid, existing in HbS solutions. We use dynamic light scattering with solutions of deoxy-HbS, and, for comparison, of oxy-HbS and oxy-normal adult hemoglobin, HbA. We show that solutions of all three Hb variants contain clusters of dense liquid, several hundred nanometers in size, which are metastable with respect to the Hb solutions. The clusters form within a few seconds after solution preparation and their sizes and numbers remain relatively steady for up to 3 h. The lower bound of the cluster lifetime is 15 ms. The clusters exist in broad temperature and Hb concentration ranges, and occupy 10(-5)-10(-2) of the solution volume. The results on the cluster properties can serve as test data for a potential future microscopic theory of cluster stability and kinetics. More importantly, if the clusters are a part of the nucleation mechanism of HbS polymers, the rate of HbS polymerization can be controlled by varying the cluster properties.

Inhibition of hemoglobin S polymerization in vitro by a novel 15-mer EF-helix beta73 histidine-containing peptide

Our mutational studies on Hb S showed that the Hb S beta73His variant (beta6Val and beta73His) promoted polymerization, while Hb S beta73Leu (beta6Val and beta73Leu) inhibited polymerization. On the basis of these results, we speculated that EF-helix peptides containing beta73His interact with beta4Thr in Hb S and compete with Hb S, resulting in inhibition of Hb S polymerization. We, therefore, studied inhibitory effects of 15-, 11-, 7-, and 3-mer EF-helix peptides containing beta73His on Hb S polymerization. The delay time prior to Hb S polymerization increased only in the presence of the 15-mer His peptide; the higher the amount, the longer the delay time. DIC image analysis also showed that the fiber elongation rate for Hb S polymers decreased with increasing concentration of the 15-mer His peptide. In contrast, the same 15-mer peptide containing beta73Leu instead of His and peptides shorter than 11 amino acids containing beta73His including His alone showed little effect on the kinetics of polymerization and elongation of polymers. Analysis by protein-chip arrays showed that only the 15-mer beta73His peptide interacted with Hb S. CD spectra of the 15-mer beta73His peptide did not show a specific helical structure; however, computer docking analysis suggested a lower energy for interaction of Hb S with the 15-mer beta73His peptide compared to peptides containing other amino acids at this position. These results suggest that the 15-mer beta73His peptide interacts with Hb S via the beta4Thr in the betaS-globin chain in Hb S. This interaction may influence hydrogen bond interaction between beta73Asp and beta4Thr in Hb S polymers and interfere in hydrophobic interactions of beta6Val, leading to inhibition of Hb S polymerization.

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.

Intermolecular interactions, nucleation, and thermodynamics of crystallization of hemoglobin C

The mutated hemoglobin HbC (beta 6 Glu-->Lys), in the oxygenated (R) liganded state, forms crystals inside red blood cells of patients with CC and SC diseases. Static and dynamic light scattering characterization of the interactions between the R-state (CO) HbC, HbA, and HbS molecules in low-ionic-strength solutions showed that electrostatics is unimportant and that the interactions are dominated by the specific binding of solutions' ions to the proteins. Microscopic observations and determinations of the nucleation statistics showed that the crystals of HbC nucleate and grow by the attachment of native molecules from the solution and that concurrent amorphous phases, spherulites, and microfibers are not building blocks for the crystal. Using a novel miniaturized light-scintillation technique, we quantified a strong retrograde solubility dependence on temperature. Thermodynamic analyses of HbC crystallization yielded a high positive enthalpy of 155 kJ mol(-1), i.e., the specific interactions favor HbC molecules in the solute state. Then, HbC crystallization is only possible because of the huge entropy gain of 610 J mol(-1) K(-1), likely stemming from the release of up to 10 water molecules per protein intermolecular contact-hydrophobic interaction. Thus, the higher crystallization propensity of R-state HbC is attributable to increased hydrophobicity resulting from the conformational changes that accompany the HbC beta 6 mutation.

Solution-active structural alterations in liganded hemoglobins C (beta6 Glu --> Lys) and S (beta6 Glu --> Val)

Based upon existing crystallographic evidence, HbS, HbC, and HbA have essentially the same molecular structure. However, important areas of the molecule are not well defined crystallographically (e.g. the N-terminal nonhelical portion of the alpha and beta chains), and conformational constraints differ in solution and in the crystalline state. Over the years, our laboratory and others have provided evidence of conformational changes in HbS and, more recently, in HbC. We now present data based upon allosteric perturbation monitored by front-face fluorescence, ultraviolet resonance Raman spectroscopy, circular dichroism, and oxygen equilibrium studies that confirm and significantly expand previous findings suggesting solution-active structural differences in liganded forms of HbS and HbC distal to the site of mutation and involving the 2,3-diphosphoglycerate binding pocket. The liganded forms of these hemoglobins are of significant interest because HbC crystallizes in the erythrocyte in the oxy form, and oxy HbS exhibits increased mechanical precipitability and a high propensity to oxidize. Specific findings are as follows: 1) differences in the intrinsic fluorescence indicate that the Trp microenvironments are more hydrophobic for HbS > HbC > HbA, 2) ultraviolet resonance Raman spectroscopy detects alterations in Tyr hydrogen bonding, in Trp hydrophobicity at the alpha1beta2 interface (beta37), and in the A-helix (alpha14/beta15) of both chains, 3) displacement by inositol hexaphosphate of the Hb-bound 8-hydroxy-1,3,6-pyrenetrisulfonate (the fluorescent 2,3-diphosphoglycerate analog) follows the order HbA > HbS > HbC, and 4) oxygen equilibria measurements indicate a differential allosteric effect by inositol hexaphosphate for HbC approximately HbS > HbA.

Fluorescence studies of normal and sickle beta apohemoglobin self-association

The acrylamide quenching of the intrinsic tryptophanyl fluorescence of normal and sickle beta apohemoglobins has been studied in 0.05 M potassium phosphate buffer, pH 7.5, at 5 degrees C over a protein concentration range from 1 to 50 microM. Analysis of quenching dynamics revealed a strong dependence on acrylamide concentration for the intrinsic fluorescence of both normal and sickle beta apohemoglobins, suggesting that one tryptophanyl residue [presumably that at position 37(C3)], was more accessible to collisional quencher than the other beta tryptophanyl residue [15(A12)]. Additional studies, which altered viscosity and subunit assembly experimental parameters, supported the assignment of residue 37 as the more dynamically accessible residue. Finally, the quenching data were also found to be dependent on protein concentration, implying that this difference in the mobility between the two residues is a sensitive probe of self-aggregation. Extrapolated dynamic quenching constants at low concentration of acrylamide were used to estimate the dimer-monomer equilibrium dissociation constants of normal and sickle beta apohemoglobins, and were found to be 5.6 and 2.4 microM, respectively, thus demonstrating distinct self-association properties of beta A and beta S apohemoglobins.

A heme c-peptide model system for the resonance Raman study of c-type cytochromes: characterization of the solvent-dependence of peptide-histidine-heme interactions

The visible absorption and Soret-excited resonance Raman spectra of ferrous microperoxidase-8 [MP8(II)], an octapeptide containing a heme c, are reported. These spectroscopies indicate that MP8(II), dissolved in aqueous buffered solutions, forms low-spin six-coordinated complexes in the 7-14 pH range. Intermolecular bonding interactions of MP8(II) in water account for this behavior. On the contrary, when the hemopeptide is dispersed in aqueous solutions containing detergent or an alcohol, the spectroscopic data show that the iron atom of MP8(II) is essentially high-spin five-coordinated in accordance with a monomeric structure of MP8(II). In addition to a high-spin signature to the heme skeletal modes, the high-frequency regions of resonance Raman spectra characterize an electronic influence of the thioether bridges on the frequency of stretching modes of C beta-C beta bonds (nu 2, nu 11, and nu 29). On the other hand, the low-frequency Raman spectra of monomeric MP8(II) at pH 7.5 present significant differences in the 150-250-cm-1 regions depending upon the solvent composition (pH, presence or absence of detergent, alcohol). These effects are attributed to frequency variations of the Fe-N(His)-involving mode which indicate changes in the H-bonding interactions of the axial His and therefore solvent-dependent changes of the octapeptide conformation. Our resonance Raman data further show that the axial His of monomeric MP8(II) could be totally deprotonated in aqueous cetyltrimethylammonium bromide solution at very alkaline pH (pKa = 13.3). The vibrational data (100-1700 cm-1) obtained for the various monomeric forms of MP8(II) are expected to be useful for determining the heme structure and environment in reduced c'-type cytochromes. Comparisons of resonance Raman data with X-ray crystallographic data available for different hemoproteins allow us to evaluate the ionization and H-bonding states of the His bound to the high-spin five-coordinated hemes. These data are discussed in terms of proximal influence of protein-His-heme interactions on the determination and the regulation of a particular biological function.

Restriction in the conformational flexibility of apoproteins in the presence of organic cosolvents: a consequence of the formation of "native-like conformation"

The influence of n-propanol on the overall alpha-helical conformation of beta-globin, apocytochrome C, and the functional domain of streptococcal M49 protein (pepM49) and its consequence on the proteolysis of the respective proteins has been investigated. A significant amount of alpha-helical conformation is induced into these proteins at pH 6.0 and 4 degrees C in the presence of relatively low concentrations of n-propanol. The induction of alpha-helical conformation into the proteins increased as a function of the propanol concentration, the maximum induction occurring around 30% n-propanol. In the case of alpha-globin, the fluorescence of its tryptophyl residues also increased as a function of n-propanol concentration, the midpoint of this transition being around 20% n-propanol. Furthermore, concomitant with the induction of helical conformation into these proteins, the proteolysis of their polypeptide chain by V8 protease also gets restricted. The alpha-helical conformation induced into alpha- and beta-globin by n-propanol decreased as the temperature is raised from 4 to 24 degrees C. In contrast, the alpha-helical conformation of both alpha- and beta-chain (i.e., globin with noncovalently bound heme) did not exhibit such a sensitivity to this change in temperature. However, distinct differences exist between the n-propanol induced "alpha-helical conformation" of globins and the "alpha-helical conformation" of alpha- and beta-chains. A cross-correlation of the n-propanol induced increase in the fluorescence of beta-globin with the corresponding increase in the alpha-helical conformation of the polypeptide chain suggested that the fluorescence increase represents a structural change of the protein that is secondary to the induction of the alpha-helical conformation into the protein (i.e., an integration of the helical conformation induced to the segments of the polypeptide chain to influence the microenvironment of the tryptophyl residues). Presumably, the fluorescence increase is a consequence of the packing of the helical segments of globin to generate a "native-like structure." The induction of alpha-helical conformation into these proteins in the presence of n-propanol and the consequent generation of "native-like conformation" is not unique to n-propanol. Trifluoroethanol, another helix-inducing organic solvent, also behaves in the same fashion as n-propanol. However, in contrast to the proteins described above, n-propanol could neither induce an alpha-helical conformation into performic acid oxidized RNAse-A nor restrict its proteolysis by proteases.(ABSTRACT TRUNCATED AT 400 WORDS)