Molecular topology in crystals and fibers of hemoglobin S

Kinetics of sickle hemoglobin polymerization. II. A double nucleation mechanism

A double nucleation mechanism for the polymerization of sickle hemoglobin is described. The mechanism accounts for all of the major kinetic observations: the appearance of a delay, the high concentration dependence of the delay time, and the stochastic behavior of slowly polymerizing samples in small volumes. The mechanism postulates that there are two pathways for polymer formation: polymerization is initiated by homogeneous nucleation in the solution phase, followed by nucleation of additional polymers on the surface of existing ones. This second pathway is called heterogeneous nucleation. Since the surface of polymers is continuously increasing with time, heterogeneous nucleation provides a mechanism for the extreme autocatalysis that is manifested as an apparent delay in the kinetic progress curves. In this mechanism, each spherulitic domain of polymers is considered to be initiated by a single homogeneous nucleation event. The mechanism explains the irreproducibility of the delay time for single domain formation as arising from stochastic fluctuations in the time at which the homogeneous nucleus for the first polymer is formed. Integration of the linearized rate equations that describe this model results in a simple kinetic form: A[cosh(Bt)-1] (Bishop & Ferrone, 1984). In the accompanying paper (Ferrone et al., 1985) it was shown that the initial 10 to 15% of progress curves, with delay times varying from a few milliseconds to over 10(5) seconds, is well fit by this equation. In this paper, we present an approximate statistical thermodynamic treatment of the equilibrium nucleation processes that shows how the nucleus sizes and nucleation equilibrium constants depend on monomer concentration. The equilibrium model results in expressions for B and B2A as a function of monomer concentration in terms of five adjustable parameters: the bimolecular addition rate of a monomer to the growing aggregate, the fraction of polymerized monomers that serve as heterogeneous nucleation sites, the free energy of intermolecular bonding within the polymer, and two parameters that describe the free energy change as a function of size for the bonding of the heterogeneous nucleus to a polymer surface. This model provides an excellent fit to the data for B and B2A as a function of concentration using physically reasonable parameters. The model also correctly predicts the time regime in which stochastic behavior is observed for polymerization in small volumes.

X-ray diffraction studies of 14-filament models of deoxygenated sickle cell hemoglobin fibers. Models based on electron micrograph reconstructions

The transforms of a large number of models of deoxygenated sickle hemoglobin fibers, related to that derived from image reconstruction of electron micrographs, have been calculated and compared with X-ray diffraction data of 15 A resolution. The model of the fiber, determined from the reconstructed image, is a helix consisting of 14 filaments that associate in a specific mode to form seven pairs, or protofilaments. Pairs were identified through the pattern of filament loss in partially disassembled fibers and by the separation between molecules, in adjacent filaments, of half a molecular diameter, along the fiber axis. An alternative mode of filament association can be derived also from the surface lattice of the reconstruction, which meets these criteria for the pairing of molecular filaments. Both pairing modes have been used in the search for structures whose transforms show the best agreement with the diffraction data. Models were generated by the systematic translation of six protofilaments, taken in symmetry related pairs, in steps of 3.5 A along the fiber axis relative to a fixed central protofilament. Each translation of a protofilament corresponds to a different fiber model, whose transform was compared with observed data. In all, over 11,000 transforms were calculated. Of all the models considered, three have been found whose residuals are minimal. At 30 A resolution, similar to that of electron micrographs, the model derived from image reconstruction and the three found through our search procedure are indistinguishable. At 15 A, however, the transforms of these models show better agreement with the observed data than the transform of the reconstructed image. Comparison of residuals shows that the model derived from the reconstructed image can be rejected with 99.5% probability relative to the model, with the same pairing scheme, found by our search procedures. The two other models, derived from the alternative pairing scheme, are also more credible than the reconstructed image, but at a lower confidence level. Each of our three models is equally acceptable. Their existence may reflect structural polymorphism of the fiber.

Structural basis and dynamics of the fiber-to-crystal transition of sickle cell hemoglobin

The kinetics of the assembly of structurally distinct, polymeric aggregates constituting the fiber-to-crystal transition of sickle cell hemoglobin in slowly stirred, deoxygenated solutions has been studied with the use of electron microscopy as a function of pH, as a function of the crystal structures of mutant forms of human deoxyhemoglobins employed as nucleating seeds, and as a function of hemoglobin S chemically modified at the Cys F9 (beta 93) position. The temporal order of appearance of fibers of approximately 210 A diameter, bundles of aligned fibers, macrofibers of greater than or equal to 650 A diameter, and microcrystals is observed. Microscopic fragments of end-stage crystals formed under slowly stirred conditions and introduced as nucleating seeds enhance the rate of crystallization only when added prior to the formation of large bundles of aligned fibers, while microscopic seed crystals added after the formation of bundles of aligned fibers do not alter the rate of crystallization. Over the pH range 6.3 to 7.1, the presence of macrofibers does not influence modulation of the kinetics of the transition with seed crystal fragments. Microscopic seed crystals of deoxyhemoglobin S and deoxyhemoglobin C formed under acidic conditions (pH less than 6.5) have a comparable influence on the kinetics of the fiber-to-crystal transition to that of end-stage crystals. Microscopic seed crystals of deoxyhemoglobin C formed under alkaline conditions (pH greater than 6.5) enhance the formation of macrofibers but do not alter the rate of crystallization. Under conditions associated with enhanced formation of macrofibers, metastable microscopic crystals having axial periodicities of approximately 64 A and approximately 210 A are observed in the intermediate phase of the transition, while end-stage crystals have axial unit cell dimensions identical to those of deoxyhemoglobin S crystallized from polyethylene glycol solutions of pH less than 6.5. Although the metastable crystals may arise from fragments of macrofibers, it is shown that they cannot be transformed directly into end-stage crystals under slowly stirred conditions without undergoing dissolution. These results stipulate that the pathway of the fiber-to-crystal transition proceeds according to the reaction: (Formula: see text) wherein the rate-limiting step is the alignment of fibers into large bundles, and macrofibers are not an intermediate of the fiber-to-crystal transition.(ABSTRACT TRUNCATED AT 400 WORDS)

Macrofiber structure and the dynamics of sickle cell hemoglobin crystallization

Fibers of deoxyhemoglobin S undergo spontaneous crystallization by a mechanism involving a variety of intermediate structures. These intermediate structures, in common with the fiber and crystal, consist of Wishner-Love double strands of hemoglobin S molecules arranged in different configurations. The structure of one of the key intermediates linking the fiber and crystal, called a macrofiber, has been studied by a variety of analytical procedures. The results of the analysis indicate that the intermediates involved in the fiber to crystal transition have many common structural features. Fourier analysis of electron micrographs of macrofibers confirms that they are composed of Wishner-Love double strands of hemoglobin molecules. Electron micrographs of macrofiber cross-sections reveal that the arrangement of the double strands in macrofibers resembles that seen in micrographs of the a axis projection of the crystal. This orientation provides an end-on view of the double strands which appear as paired dumb-bell-like masses. The structural detail becomes progressively less distinct towards the edge of the particle due to twisting of the double strands about the particle axis. Serial sections of macrofibers confirm that these particles do indeed rotate about their axes. The twist of the particle is right handed and its average pitch is 10,000 A. The effect of rotation on the appearance of macrofiber cross-sections 300 to 400 A thick can be simulated by a 15 degrees rotation of an a axis crystal projection. The relative polarity of the double strands in macrofibers and crystals can be determined easily by direct inspection of the micrographs. In both macrofibers and crystals they are in an anti-parallel array. On the basis of these observations we conclude that crystallization of macrofibers involves untwisting and alignment of the double strands.

Effects of the alpha 20 mutation on the polymerization of Hb S

The contribution of the alpha 20 residues in intermolecular contacts present in hemoglobin S fibers was investigated with mixtures of Hb Le Lamentin alpha 2(20)His----Gln beta 2A and of hemoglobin S alpha 2A beta 2(6)Glu----Val and with artificial hybrids alpha 2(20)His----Gln beta 2(6)Glu----Val. This study showed an increased solubility and delay time of polymerization of Hb S in solution only when the mutation at the alpha 20 residue is cis to the beta 6 Val contact. No modification of the polymerization process occurs when the mutation is trans to this beta 6 Val contact. This result is in agreement with the crystal model of Wishner and Love, who showed that one of the two alpha 20 residues of the Hb S tetramer was involved in an axial contact between hemoglobin S molecules in the crystals of Hb S ( Wishner , B.C., Ward, K.B., Lattman , E.E. and Love, W.E. (1975) J. Mol. Biol. 98, 179-194). The present observation is a new illustration of the validity of the crystal model for the structure of the fibers based on pairs of double filaments.

Inhibition of deoxyhemoglobin S polymerization by glyceraldehyde

Glyceraldehyde reacts with hemoglobin S in the intact erythrocyte to reduce the degree of polymerization, thereby inhibiting sickling of the erythrocyte. Only five of the 24 amino groups per alpha beta dimer react with glyceraldehyde; the adducts are present as ketoamine structures, formed by Amadori rearrangement of the initial Schiff base adducts on the protein. The reactive amino groups are the epsilon-amino group of Lys-16 of the alpha-chain, and the alpha-amino group of Val-1 as well as the epsilon-amino groups Lys-82, Lys-59, and Lys-120 of the beta-chain. Hybrid tetramers were prepared with the modification only on Lys-16 of the alpha-chain or on the reactive lysine residues of the beta-chain. The former derivative gels at a much higher hemoglobin concentration (23 g/dl) than either the latter derivative (16 g/dl) or unmodified deoxyhemoglobin S (15 g/dl). Thus, the modification at Lys-16 of the alpha-chain is a major factor in the inhibition of sickling by glyceraldehyde.

Polarity of the 14-strand fibers of sickle cell hemoglobin determined by cross-correlation methods

Images of negatively stained fibers of sickle cell hemoglobin have been analyzed by cross-correlation methods. These methods are used to compensate for the curvature and variable repeat distance characteristic of negatively stained fibers. Averaged images obtained by the correlation procedure display considerably more detail than the filtered images obtained earlier by Fourier methods. The averaged images are sufficiently detailed that the back-projection method for obtaining cross-sections can now be applied directly to the correlation-averaged images. This method was used previously to deduce the 14-strand structure on Fourier-filtered images that incorporated only the near-equatorial maxima. In this way the 14-strand structure has been reconfirmed without utilizing the partial Fourier reconstructions employed earlier that might conceivably have introduced spurious features. In addition, application of the correlation procedure with and without inversion of the reference reveals a consistent polarity in all of the fibers examined. The confirmation of the 14-strand structure by a new procedure and the determination of fiber polarity would appear to eliminate the alternative model of the fibers with a 16-strand structure with equal numbers of strands (eight) of each polarity.

Sickle cell hemoglobin fiber formation strongly inhibited by the Stanleyville II mutation (alpha 78 Asn leads to Lys)

A double mutant hemoglobin possessing both the Hb S (beta 6 Glu leads to Val) and the Hb Stanleyville II (alpha 78 Asn leads to Lys) mutations has been purified from the blood of a donor heterozygous for both of the mutations. The purification required two chromatography steps, with the second permitting resolution of the double mutant from Hb A2 remaining after the first step. Measurement of the competence for fiber formation by the double mutant hemoglobin was carried out by the centrifugation of gels to obtain Csat. The double mutant was found to have a greatly elevated Csat, 26.4 gm/dl, compared to 15.2 gm/dl for the Hb S control. The concentration of the pellet of the centrifuged gel for the double mutant was in the range 48-50 gm/dl, suggesting that no major rearrangement in the structure of the fibers had been induced by the Stanleyville II mutation. Electron microscopic observations on the fibers of the double mutant confirmed that a normal appearance was maintained.

Molecular and cellular pathogenesis of hemoglobin SC disease

Solution and cell studies were performed to ascertain why individuals with hemoglobin (Hb) SC have disease whereas those with Hb AS do not. The polymerization of deoxygenated mixtures containing sickle cell Hb (Hb S; alpha 2 beta 2(6)Glu leads to Val) and Hb C (alpha 2 beta 2(6)Glu leads to Lys) was investigated by measurements of delay times and solubilities. In mixtures containing more than 40% Hb S, polymerization takes place by the same mechanism as in solutions of Hb S alone, with no evidence for independent crystallization of Hb C. A detailed comparison of Hb S/Hb C and Hb S/Hb A mixtures with identical concentrations and proportions of Hb S show that there is no significant difference in the tendency of Hb C and Hb A to copolymerize with Hb S. In 50:50 Hb S/Hb C mixtures, polymerization is about 15 times more rapid than in 40:60 Hb S/Hb A mixtures at the same total Hb concentration. Measurements on density-fractionated erythrocytes show that SC cells contain a higher total Hb concentration and a more uniform distribution of reticulocytes compared to normal (AA) or sickle trait (AS) cells. The concentration distribution for C trait (AC) cells is much closer to that of SC cells than to AS or AA cells. It appears, therefore, that the presence of Hb C results in the SC cell beginning its life with an abnormally high Hb concentration. From these findings we conclude that both the larger proportion of Hb S and the higher intracellular Hb concentration contribute to the pathogenesis of Hb SC disease.