Functional Properties of Hemoglobin Kempsey

Hb Waikato [α127(H10)Lys→Gln; HBA1: c.382A>C]: A Novel High Oxygen Affinity Variant

We report the identification of a novel, high oxygen affinity hemoglobin (Hb) variant [α127(H10)Lys→Gln; HBA1: c.382A>C]. The variant was detected in an adolescent male (proband) of Syrian descent by cation exchange high performance liquid chromatography (HPLC), during Hb A_1c analysis. A complete blood count (CBC) showed elevated red blood cells (RBCs) (6.08 × 10¹²/L), Hb (16.1 g/dL) and packed cell volume (PCV) (0.48 L/L). Capillary electrophoresis (CE) revealed the variant was more negatively charged and represented 18.2% of total Hb. Isopropanol stability was normal. Cyanosis in the subject prompted investigation of oxygen affinity, with a reduced p50 of 20.8 mm Hg and a left shifted oxygen dissociation curve demonstrating increased oxygen affinity. We propose the novel variant be named Hb Waikato, which reflects the Hospital Laboratory where the variant was discovered and region where the proband was born and herein describe characterization.

Conjugation of para-benzoquinone of Cigarette Smoke with Human Hemoglobin Leads to Unstable Tetramer and Reduced Cooperative Oxygen Binding

Besides multiple life-threatening diseases like lung cancer and cardiovascular disease, cigarette smoking is known to produce hypoxia, a state of inadequate oxygen supply to tissues. Hypoxia plays a pivotal role in the development of chronic obstructive pulmonary disease. Smoking during pregnancy imposes risk for the unborn child. In addition to carbon monoxide, conjugation of para-benzoquinone (pBQ), derived from cigarette smoke, with human hemoglobin (HbA) was also reported to contribute in hypoxia. In fact, conjugation of pBQ is more alarming than carbon monoxide as it is an irreversible covalent modification. In the present study, the functional assay of Hb-pBQ, performed through oxygen equilibrium curve, showed a significant decrease in both P₅₀ and cooperativity. However, the structural changes associated with the observed functional perturbation of the hemoglobin conjugate (Hb-pBQ) are unknown to date. Enhanced sensitivity and high resolution of nano-ESI mass spectrometry platform have enabled to investigate the native structure of oligomers of hemoglobin in a single scan. The structural integrity of Hb-pBQ measured through the dissociation equilibrium constants (K_d) indicated that compared to HbA, K_d of tetramer-dimer and dimer-monomer equilibria were increased by 4.98- and 64.3-folds, respectively. Using isotope exchange mass spectrometry, we observed perturbations in the inter-subunit interactions of deoxy and oxy states of Hb-pBQ. However, the three-dimensional architecture of Hb-pBQ, monitored through collision cross-sectional area, did not show any change. We propose that the significant destabilization of the functionally active structure of hemoglobin upon conjugation with pBQ results in tighter oxygen binding that leads to hypoxia. Graphical Abstract ᅟ.

Tertiary dynamics of human adult hemoglobin fixed in R and T quaternary structures

Protein dynamics of human adult hemoglobin and its mutants restricted in R and T quaternary states following ligand photolysis were studied by time-resolved resonance Raman spectroscopy. In the time-resolved spectra, we observed spectral changes of in-plane stretching modes of heme and the iron-histidine stretching mode of the Fe-His bond for all the hemoglobin samples. The βD99N mutant, which adopts the R state in both the ligand-bound and the deoxy forms, showed similar temporal behaviors in time-resolved resonance Raman spectra as wild-type recombinant hemoglobin until 10 μs, consistent with the fact that the mutant undergoes only the tertiary structural changes in the R state. The βN102T mutant, which adopts the T state in both the ligand-bound and the deoxy forms, showed much slower tertiary structural changes, suggesting that the EF helical motion is decelerated by the change of the intersubunit interactions. The present data indicate that the allosteric kinetic response between the interhelical hydrogen bonds of the EF helices and the intersubunit hydrogen bonds is bidirectional. The implications of these results for understanding the allosteric pathway of Hb are discussed in detail.

Collective dynamics underlying allosteric transitions in hemoglobin

Hemoglobin is the prototypic allosteric protein. Still, its molecular allosteric mechanism is not fully understood. To elucidate the mechanism of cooperativity on an atomistic level, we developed a novel computational technique to analyse the coupling of tertiary and quaternary motions. From Molecular Dynamics simulations showing spontaneous quaternary transitions, we separated the transition trajectories into two orthogonal sets of motions: one consisting of intra-chain motions only (referred to as tertiary-only) and one consisting of global inter-chain motions only (referred to as quaternary-only). The two underlying subspaces are orthogonal by construction and their direct sum is the space of full motions. Using Functional Mode Analysis, we were able to identify a collective coordinate within the tertiary-only subspace that is correlated to the most dominant motion within the quaternary-only motions, hence providing direct insight into the allosteric coupling mechanism between tertiary and quaternary conformation changes. This coupling-motion is substantially different from tertiary structure changes between the crystallographic structures of the T- and R-state. We found that hemoglobin's allosteric mechanism of communication between subunits is equally based on hydrogen bonds and steric interactions. In addition, we were able to affect the T-to-R transition rates by choosing different histidine protonation states, thereby providing a possible atomistic explanation for the Bohr effect.

Cross-linking with O-raffinose lowers oxygen affinity and stabilizes haemoglobin in a non-cooperative T-state conformation

O-R-polyHbA(0) is an intra- and intermolecularly O-raffinose cross-linked derivative of deoxygenated human haemoglobin developed as an oxygen therapeutic. When compared with its native protein (HbA(0)), O-R-polyHbA(0) was found to be locked in the T (tense) quaternary conformation with a lower oxygen affinity, a reduced Bohr effect (50% of HbA(0)) and no measurable cooperativity (h=1). The kinetics of oxygen and CO binding to the protein indicate lower 'on' rates and faster 'off' rates than HbA(0) and the absence of effects of inositol hexaphosphate (IHP) on the kinetics. Other properties consistent with a T-like conformation are inaccessibility of the betaCys-93 thiol group of O-R-polyHbA(0), the hyperfine splitting from nitrogen in the EPR spectrum of the Fe(II)NO complex of O-R-polyHbA(0) and decreased flexibility in the distal haem pocket, as indicated by low-spin bis-histidine complexes detected by EPR of oxidized chains. A comparison of the properties of O-R-polyHbA(0) with those of HbA(0) with and without IHP, as well as the reaction of nitrite with deoxygenated haemoglobin, provide additional insights into the variations in the conformation of T-state haemoglobin in solution (modifications of the T state produced by adding organic phosphates, like IHP and 2,3-diphosphoglycerate). Although the physiological ramifications of locking HbA(0) in the T conformation with the O-raffinose are still unknown, valuable insights into haemoglobin function are provided by these studies of O-R-polyHbA(0).

Adenoviral gene therapy of the Tay-Sachs disease in hexosaminidase A-deficient knock-out mice

The severe neurodegenerative disorder, Tays-Sachs disease, is caused by a beta-hexosaminidase alpha-subunit deficiency which prevents the formation of lysosomal heterodimeric alpha-beta enzyme, hexosaminidase A (HexA). No treatment is available for this fatal disease; however, gene therapy could represent a therapeutic approach. We previously have constructed and characterized, in vitro, adenoviral and retroviral vectors coding for alpha- and beta-subunits of the human beta-hexosaminidases. Here, we have determined the in vivo strategy which leads to the highest HexA activity in the maximum number of tissues in hexA -deficient knock-out mice. We demonstrated that intravenous co-administration of adenoviral vectors coding for both alpha- and beta-subunits, resulting in preferential liver transduction, was essential to obtain the most successful results. Only the supply of both subunits allowed for HexA overexpression leading to massive secretion of the enzyme in serum, and full or partial enzymatic activity restoration in all peripheral tissues tested. The enzymatic correction was likely to be due to direct cellular transduction by adenoviral vectors and/or uptake of secreted HexA by different organs. These results confirmed that the liver was the preferential target organ to deliver a large amount of secreted proteins. In addition, the need to overexpress both subunits of heterodimeric proteins in order to obtain a high level of secretion in animals defective in only one subunit is emphasized. The endogenous non-defective subunit is otherwise limiting.

Polymerization of recombinant Hb S-Kempsey (deoxy-R state) and Hb S-Kansas (oxy-T state)

In order to investigate the role of the R (relaxed) to T (tense) structural transition in facilitating polymerization of deoxy-Hb S, we have engineered and expressed two Hb S variants which destabilize either T state (Hb S-Kempsey, alpha 2 beta 2 Val-6,Asn-99) or R state structures (Hb S-Kansas, alpha 2 beta 2 Val-6, Thr-102). Polymerization of deoxy-Hb S-Kempsey, which shows high oxygen affinity and increased dimer dissociation, required about 2- and 6-fold higher hemoglobin concentrations than deoxy-Hb S for polymerization in low and high phosphate concentrations, and its kinetic pattern of polymerization was biphasic. In contrast, oxy- or CO Hb S-Kansas, which shows low oxygen affinity and increased dimer dissociation, polymerized at a slightly higher critical concentration than that required for polymerization of deoxy-Hb S in both low and high phosphate buffers. Polymerization of oxy- and CO Hb S-Kansas was linear and showed no delay time, which is similar to oversaturated oxy- or CO Hb S. These results suggest that nuclei formation, which occurs during the delay time prior to deoxy-Hb S polymerization, does not occur in T state oxy-Hb S-Kansas, even though the critical concentration for polymerization of T state oxy-Hb S-Kansas is similar to that of T state deoxy-Hb S.

FT-IR difference spectroscopy of hemoglobins A and Kempsey: evidence that a key quaternary interaction induces protonation of Asp beta 99

Fourier transform infrared difference spectra are reported for the CO adduct of human hemoglobin versus deoxyHb, in H2O and D2O. In addition to the well-known CO stretching and S-H(D) stretching bands, the difference spectra reveal numerous bands in the 1200-1700 cm-1 region, a number of which are assigned. Several amide modes are identified via their frequencies and D2O sensitivities. Bands arising from histidine protonation have also been found via comparison of the difference spectra at different pH(D) values, with the aid of aqueous histidine spectra. Of particular interest is the observation of a negative band at 1697 cm-1, which is assigned to the C = O stretch of carboxylic acid. This carboxylic acid is tentatively identified as the side chain of Asp beta 99, because it is missing in the difference spectrum of Hb Kempsey, a mutant in which Asp beta 99 is replaced by Asn. Asp beta 99 forms a critical contact with Tyr alpha 42 across the alpha 1 beta 2 interface in deoxyHb, which is broken upon ligation. Protonation of Asp beta 99 in deoxyHb is consistent with UV resonance Raman evidence that Tyr alpha 42 is the acceptor rather than the donor of the quaternary H-bond.

Properties of a recombinant human hemoglobin with aspartic acid 99(beta), an important intersubunit contact site, substituted by lysine

Site-directed mutagenesis of an important subunit contact site, Asp-99(beta), by a Lys residue (D99K(beta)) was proven by sequencing the entire beta-globin gene and the mutant tryptic peptide. Oxygen equilibrium curves of the mutant hemoglobin (Hb) (2-15 mM in heme) indicated that it had an increased oxygen affinity and a lowered but significant amount of cooperativity compared to native HbA. However, in contrast to normal HbA, oxygen binding of the recombinant mutant Hb was only marginally affected by the allosteric regulators 2,3-diphosphoglycerate or inositol hexaphosphate and was not at all responsive to chloride. The efficiency of oxygen binding by HbA in the presence of allosteric regulators was limited by the mutant Hb. At concentrations of 0.2 mM or lower in heme, the mutant D99K(beta) Hb was predominantly a dimer as demonstrated by gel filtration, haptoglobin binding, fluorescence quenching, and light scattering. The purified dimeric recombinant Hb mutant exists in 2 forms that are separable on isoelectric focusing by about 0.1 pH unit, in contrast to tetrameric hemoglobin, which shows 1 band. These mutant forms, which were present in a ratio of 60:40, had the same masses for their heme and globin moieties as determined by mass spectrometry. The elution positions of the alpha- and beta-globin subunits on HPLC were identical. Circular dichroism studies showed that one form of the mutant Hb had a negative ellipticity at 410 nm and the other had positive ellipticity at this wavelength. The findings suggest that the 2 D99K(beta) recombinant mutant forms have differences in their heme-protein environments.

Regulation of oxygen affinity by quaternary enhancement: does hemoglobin Ypsilanti represent an allosteric intermediate?

Recent crystallographic studies on the mutant human hemoglobin Ypsilanti (beta 99 Asp-->Tyr) have revealed a previously unknown quaternary structure called "quaternary Y" and suggested that the new structure may represent an important intermediate in the cooperative oxygenation pathway of normal hemoglobin. Here we measure the oxygenation and subunit assembly properties of hemoglobin Ypsilanti and five additional beta 99 mutants (Asp beta 99-->Val, Gly, Asn, Ala, His) to test for consistency between their energetics and those of the intermediate species of normal hemoglobin. Overall regulation of oxygen affinity in hemoglobin Ypsilanti is found to originate entirely from 2.6 kcal of quaternary enhancement, such that the tetramer oxygenation affinity is 85-fold higher than for binding to the dissociated dimers. Equal partitioning of this regulatory energy among the four tetrameric binding steps (0.65 kcal per oxygen) leads to a noncooperative isotherm with extremely high affinity (pmedian = .14 torr). Temperature and pH studies of dimer-tetramer assembly and sulfhydryl reaction kinetics suggest that oxygenation-dependent structural changes in hemoglobin Ypsilanti are small. These properties are quite different from the recently characterized allosteric intermediate, which has two ligands bound on the same side of the alpha 1 beta 2 interface (see ref. 1 for review). The combined results do, however, support the view that quaternary Y may represent the intermediate cooperativity state of normal hemoglobin that binds the last oxygen.

Effect of ligand-affinity differences of human hemoglobin variants on electrophoretic behavior and their isolation and functional characterization

A natural sulfated polysaccharide (agaropectin), contained in crude agar, can be used as a medium for electrophoretic separation of hemoglobin mutants, constituting a particular class of protein-ligand interactions. Mutations which either modify the electrostatic charge at the surface of the hemoglobin molecule or not, have been studied according to their putative interaction with the medium. Using conformational specificities of the hemoglobin molecule, we have also demonstrated that isoelectric focusing on a polyacrylamide gel in the absence of heme ligands represents a useful, convenient and rapid procedure for isolating silent Hb variants in their native form, provided that they exhibit an abnormal Bohr effect.

Relationship between tetramer-dimer assembly and the stability of Hb Malmö (alpha 2 beta 2 97Gln)

The effects of the mutation of the alpha 1 beta 2 contact in Hb Malmö (alpha 2 beta 2 97(FG4)His----Gln) on oxygen-binding properties, ease of dissociation into dimeric hemoglobin and stability were studied. The P50 value of Hb Malmö in the absence of organic phosphates was 1.9 mmHg, in contrast to 8.8 mmHg of Hb A. The n-value of Hb Malmö was 1.6. The overall free energy of interaction of oxygen with Hb Malmö was about 25% that of Hb A. The Adair constant, K1, of Hb Malmö was about 10-times larger than that of Hb A, but the K4 of Hb Malmö was similar to that of Hb A. The liganded form of Hb Malmö was found to dissociate into dimers more readily than Hb A by gel filtration on Sephadex G-100. Dissociation into dimeric hemoglobin was enhanced in dilute solutions. Increased instability during mechanical agitation of diluted samples was greater for Hb Malmö than for Hb A. The denaturation rate constants of tetramers of the oxyform of Hb A and Hb Malmö were about 20-times greater than those of dimers of these hemoglobins. The instability of Hb Malmö depends on a greater alpha 1 beta 2 dissociation constant compared with that of Hb A. These findings allow an examination of the role of the intersubunit contact in determining the functional properties and the stability of the hemoglobin molecule.

Mechanism of autoxidation of hemoglobin Kempsey (beta 99 Asp----Asn)

The mechanism of autoxidation of oxyhemoglobin Kempsey (beta 99 asp----asn) was studied at pH 7.0, 37 degrees C, using isoelectric focusing electrophoresis. During autoxidation, two intermediate hemoglobins, (alpha 2+ beta 3+)2 and (alpha 3+ beta 2+)2 were observed, and these were consecutively changed to methemoglobin. The autoxidation rates of oxyhemoglobin Kempsey were reduced about 43% by the addition of catalase and superoxide dismutase, but were not significantly changed by the addition of inositol hexaphosphate. This abnormal hemoglobin autoxidized more slowly than normal hemoglobin A. The differences in the autoxidation rates between these hemoglobins were explained by the changes in quaternary structure of these proteins. The reaction rate constant, k+1, k+2, k+3 and k+4 of each step including the reactions: (formula; see text) were determined by the analysis of fractional changes in these hemoglobin derivatives during the autoxidation. The difference in the reaction rate constants between k+1 and k+3 was explained by the nonequivalence of alpha and beta chains in tetrameric oxyhemoglobin Kempsey. The reaction mechanism of autoxidation of hemoglobin Kempsey was discussed on the basis of these kinetic results.

Quaternary-transformation-induced changes at the heme in deoxyhemoglobins

Quaternary-structure-induced differences in both the high- and low-frequency regions of the resonance Raman spectrum of the heme have been detected in a variety of hemoglobins. These differences may be the result of (1) changes in the amino acid sequence, induced by genetic and chemical modifications, and (2) alterations in the quaternary structure. For samples in solution in low ionic strength buffers, differences in the 1357-cm-1 line (an electron-density-sensitive vibrational mode) correlate with differences in the 216-cm-1 line (the iron-histidine stretching mode). Thus, changes in the iron-histidine bond and changes in the pi-electron density of the porphyrin depend upon a common heme-globin interaction. The quaternary-structure-induced changes in the vibrational modes associated with the heme demonstrate that there is extensive communication between the heme and the globin and impact on models for the energetics of cooperativity. The local interactions of the iron-histidine mode are energetically small and destabilize the deoxy heme in the T structure with respect to the R structure. Therefore, these interactions must be larger in the ligated protein than in the deoxy protein to obtain a negative free energy of cooperativity. Additionally, our data imply that the deprotonation of the proximal histidine does not play a major role in the energetics of cooperativity. On the other hand, models for cooperativity that require conformational changes in the iron-histidine bond or direct interaction between the porphyrin and the protein are qualitatively consistent with the observed variation of heme electronic structure in concert with protein quaternary structure.

Haemoglobin Radcliffe (alpha2beta299(Gi)Ala): a high oxygen-affinity variant causing familial polycythaemia

Three members of an Oxfordshire family have polycythaemia. In each case their whole-blood oxygen affinity is increased. This is due to a previously undescribed haemoglobin variant which has been named haemoglobin Radcliffe (alpha2beta299(Gl)Ala). In addition to having a high oxygen affinity haemoglobin Radcliffe shows virtually no haem-haem interaction and a diminished Bohr effect. It is synthesized at the same rate and is as stable as haemoglobin A. X-ray analysis indicates that crystals of deoxyhaemoglobin Radcliffe are isomorphous with those of deoxyhaemoglobin A. Solutions of haemoglobin Radcliffe were also studied by high-resolution proton nuclear magnetic resonance spectroscopy. The structure/function relationships of haemoglobin Radcliffe are discussed in the light of these studies.

The influence of organic phosphates on the Bohr effect of human hemoglobin valency hybrids

The Bohr effect of hemoglobin and that of the aquomet and cyanomet valency hybrids was measured in the presence and the absence of IHP (inositol hexaphosphate) and DPG (2,3-diphosphoglycerate). In the absence of these organic phosphates the four hybrids show similar, but suppressed Bohr effects as compared to hemoglobin. Addition of IHP and DPG results in all cases in an increase of the Bohr effect. The additional phosphate induced Bohr effect of the hybrids with the alpha chain in the oxidized form is almost identical to that of hemoglobin, while this effect of the hybrids with oxidized beta chains is slighly lower than that of hemoglobin. The results suggest (a) that the Bohr effect is correlated to the ligation state of the hemoglobin molecule rather than to its quaternary structure (b) that the additional phosphate induced Bohr effect is related to the change in quaternary structure of the tetramer, and (c) that with respect to the Bohr effect of the hybrids there is no difference between high and low spin species.

Hb-Alberta or alpha2beta2 (101(G3) Glu replaced by Gly), a new high-oxygen-affinity hemoglobin variant causing erythrocytosis

Hb-Alberta has been found in a 51 year old Caucasian male with erythrocytosis. The substitution in this variant involves the glutamyl residue in position 101(G3) of the beta chain which is replaced by a glycyl residue. Hb-Alberta accounts for about 45% in the heterozygote, and readily forms hybrid tetramers with other hemoglobins. The oxygen affinity of Hb-Alberta is greatly increased, its Bohr effect reduced, and its subunit interaction greatly diminished.