Probing the importance of the amino-terminal sequence of the beta- and gamma-chains to the properties of normal adult and fetal hemoglobins

Solution 1H NMR study of the active site molecular structure and magnetic properties of the cyanomet complex of the isolated, tetrameric beta-chain from human adult hemoglobin

The solution molecular structure and the electronic and magnetic properties of the heme pocket of the cyanomet complex of the isolated beta-chain of human adult hemoglobin, HbA, have been investigated by homonuclear 2D (1)H NMR in order to assess the extent of assignments allowed by (1)H NMR of a homo-tetrameric 65-kDa protein, to guide the future assignments of the heterotetrameric complex of HbA, and to compare the structure of the beta-chain to the crystallographically characterized complexes that contains the beta-chain. The target residues are those that exhibit significant (>|0.2| ppm) dipolar shifts, as predicted by a "preliminary" set of magnetic axes determined from a small set of easily assigned active site residues. All 104 target residues ( approximately 70% of total) were assigned by taking advantage of the temperature dependence predicted by the "preliminary" magnetic axes for the polypeptide backbone; they include all residues proposed to play a significant role in modulating the ligand affinity in the tetramer HbA. Left unassigned are the A-helix, the end of the G-helix and the beginning of the H-helix where dipolar shifts are less than |0.2| ppm. These comprehensive assignments allow the determination of a robust set of orientation and anisotropies of the paramagnetic susceptibility tensor that leads to quantitative interpretation of the dipolar shifts of the beta-chain in terms of the crystal coordinates of the beta-subunit in ligated HbA which, in turn, confirms a largely conserved molecular structure of the isolated beta-chain relative to that in the intact R-state HbA. The major magnetic axis, which is correlated with the tilt of the Fe-CN unit, is tilted approximately 10 degrees from the heme normal so that the Fe-CN unit is tilted toward the beta-meso-H in a fashion remarkably similar to the Fe-CO tilt in the beta-subunit of HbCO. It is concluded that a set of "preliminary" magnetic axes and the use of variable temperature 2D NMR spectra are crucial to effective assignments in the tetrameric cyanomet beta-chain and that this approach should be similarly effective in HbA.

Aggregation of normal and sickle hemoglobin in high concentration phosphate buffer

Sickle cell disease is caused by a mutant form of hemoglobin, hemoglobin S, that polymerizes under hypoxic conditions. The extent and mechanism of polymerization are thus the subject of many studies of the pathophysiology of the disease and potential treatment strategies. To facilitate such studies, a model system using high concentration phosphate buffer (1.5 M-1.8 M) has been developed. To properly interpret results from studies using this model it is important to understand the similarities and differences in hemoglobin S polymerization in the model compared to polymerization under physiological conditions. In this article, we show that hemoglobin S and normal adult hemoglobin, hemoglobin A, aggregate in high concentration phosphate buffer even when the concentration of hemoglobin is below the solubility defined for polymerization. This phenomenon was not observed using 0.05 M phosphate buffer or in another model system we studied that uses dextran to enhance polymerization. We have used static light scattering, dynamic light scattering, and differential interference contrast microscopy to confirm aggregation of deoxygenated and oxygenated hemoglobins below their solubility and have shown that this aggregation is not observable using turbidity measurements, a common technique for assessing polymerization. We have also shown that the aggregation increases with increasing temperature in the range of 15 degrees -37 degrees C and that it increases as the concentration of phosphate increases. These studies contribute to the working knowledge of how to properly apply studies of hemoglobin S polymerization that are conducted using the high phosphate model.

Hemoglobin Einstein: semisynthetic deletion in the B-helix of the alpha-chain

The influence of the deletion of the tetra peptide segment alpha(23-26) of the B-helix of the alpha-chain of hemoglobin-A on its assembly, structure, and functional properties has been investigated. The hemoglobin with the deletion, ss-Hemoglobin-Einstein, is readily assembled from semisynthetic alpha(1-141) des(23-26) globin and human betaA-chain. The deletion of alpha(23-26) modulates the O2 affinity of hemoglobin in a buffer/allosteric effector specific fashion, but has little influence on the Bohr effect. The deletion has no influence on the thermodynamic stability of the alpha1beta1 and the alpha1beta2 interface. The semisynthetic hemoglobin exhibits normal intersubunit interactions at the alpha1beta1 and alpha1beta2 interfaces as reflected by 1H-NMR spectroscopy. Molecular modeling studies of ss-Hemoglobin-Einstein suggest that the segment alpha(28-35) is in a helical conformation, while the segment alpha(19-22) is the nonhelical AB region. The shortened B-helix conserves the interactions of alpha1beta1 interface. The results demonstrate a high degree of plasticity in the hemoglobin structure that accommodates the deletion of alpha(23-26) without perturbing its overall global conformation.

Introduction of a new regulatory mechanism into human hemoglobin

Previous studies on bovine hemoglobin (HbBv) have suggested amino acid substitutions, which might introduce into human hemoglobin (HbA) functional characteristics of HbBv, namely a low intrinsic oxygen affinity regulated by Cl(-). Accordingly, we have constructed and characterized a multiple mutant, PB5, [beta(V1M + H2 Delta + T4I + P5A + A76K)] replacing four amino acid residues of HbA with those present at structurally analogous positions in HbBv, plus an additional substitution, beta T4I, which does not occur in either HbBv or HbA. This 'pseudobovine' hemoglobin has oxygen binding properties very similar to those of HbBv: the P(50) of HbA, PB5 and HbBv in the absence of Cl(-) are 1.6, 4.6 and 4.8 torr, respectively, and in 100 mM Cl(-) are 3.7, 10.5 and 12 torr, respectively. Moreover, PB5 has 3-fold slower autoxidation rate compared to HbA and HbBv. These are desirable characteristics for a human hemoglobin to be considered for use as a clinical artificial oxygen carrier. Although the functional properties of PB5 and HbBv are similar, van't Hoff plots indicate that the two hemoglobins interact differently with water, suggesting that factors regulating the R to T equilibrium are not the same in the two proteins. A further indication that PB5 is not a functional mimic of HbBv derives from PB5(control), a human hemoglobin with the same substitutions as PB5, except the beta T4I replacement. PB5(control) has a high oxygen affinity (P(50)=2.3 torr) in the absence of Cl(-), but retains the Cl(-) effect of PB5. The Cl(-) regulation of oxygen affinity in PB5 involves lysine residues at beta 8 and beta 76. PB4, which has the same substitutions as PB5 except beta A76K, and PB6, which has all the substitutions of PB5 plus beta K8Q, both have a low intrinsic oxygen affinity, like HbBv and PB5, but exhibit a decreased sensitivity to Cl(-). Since HbBv has lysine residues at both beta 8 and beta 76, these results imply that Cl(-) regulation in HbBv likewise involves these two residues. The mechanism responsible for the low intrinsic oxygen affinity of HbBv remains unclear. It is suggested that residues peculiar to HbBv at the alpha(1)beta(1) interface may play a role.

N-terminal contributions of the gamma-subunit of fetal hemoglobin to its tetramer strength: remote effects at subunit contacts

The greatly increased tetramer strength of liganded fetal hemoglobin compared with adult hemoglobin is shown by its 70-fold smaller tetramer-dimer dissociation constant. This property has been shown previously to be only partially caused by the 5-amino-acid differences at both types of interfaces in each hemoglobin. A major contributor to tetramer strengthening is the 18-amino-acid N-terminal A helix of the gamma-subunit of fetal hemoglobin, which differs from the beta-subunit of adult hemoglobin at eight amino acid residues. This long-distance communication between the A helix and the distant C helix and FG helical corner comprising the subunit contacts at the allosteric interface represents internal signaling. Physiologically, its greater tetramer strength endows fetal hemoglobin with the capacity to abstract oxygen from maternal adult hemoglobin. It also leads to resistance of fetal red cells to the malaria parasite because the HbF tetramer does not dissociate to dimers as readily as HbA; dimers are digested by malaria proteases but tetramers are not. In this communication, we report which sites on the A helix of the gamma-subunit are important for tetramer strengthening in HbF by substituting certain amino acids in the beta-subunit by the corresponding residues in the gamma-subunit. The recombinant hemoglobins containing up to five replacements together have been extensively characterized. Mass values were within 1 unit of theory. Gly 1 (gamma) of HbF with its high pK(a) of 8.1 compared with a 7.1 value for Val 1 (beta) of HbA creates a highly electropositive N terminus that may couple with the electronegative sequence just after it on the gamma-subunit. The Leu 3 to Phe replacement has no apparent role; however, position 5 is important because replacement of Pro 5 (beta) by Glu 5 (gamma) promotes tetramer strengthening. The Glu --> Asp replacement at position 7 enhances this effect because of the lower pK(a) of Asp but the Val --> Ile substitution at position 11 has no effect. Thus, the three positive/negative sites at positions 1, 5, and 7 account for practically all of the tetramer strength of HbF, as illustrated by an electrostatic surface potential analysis. The pathway by which information is transmitted to the distant allosteric subunit interfaces is currently under study. Oxygen-binding properties of the hemoglobins with charged substitutions more closely resemble those of HbA rather than those of HbF. Thus, whereas the A helix has a major role in controlling the strength of interactions at the tetramer-dimer allosteric interface, oxygen-binding properties of HbA and HbF are influenced by sequences in the C helix and at the FG helical corner constituting the allosteric interface.