Cooperative effect of inositol hexakisphosphate, bezafibrate, and clofibric acid on the spectroscopic properties of the nitric oxide derivative of ferrous human hemoglobin

The cooperative effect of inositol hexakisphosphate (IHP), bezafibrate (BZF), and clofibric acid (CFA) on the spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous human hemoglobin (HbNO) has been investigated quantitatively. In the presence of IHP, BZF, and CFA, the X-band EPR spectra and the absorption spectra in the Soret region of HbNO display the same basic characteristics described in the presence of 2,3-diphosphoglycerate (2,3-DPG), which have been attributed to a low affinity conformation of the tetramer. Addition to HbNO of two allosteric effectors together (such as IHP and BZF, or IHP and CFA) further stabilizes the low affinity conformation of the ligated hemoprotein (i.e., HbNO). Moreover, in the presence of saturating amounts of IHP, the affinity of BZF and CFA for HbNO increases by about fifteenfold. Likewise, in the presence of both IHP and BZF, as well as in IHP and CFA, the oxygen affinity for ferrous human hemoglobin (Hb) is reduced with respect to that observed in the presence of IHP, BZF, or CFA alone, which in turn is lower than that reported in the absence of any allosteric effector. All the data were obtained at pH 7.0 (in 1.0 × 10⁻¹ M N-[2-hydroxyethyl]piperazine-N′-[2-ethanesulfonic acid]/NaOH buffer system plus 1.0 × 10⁻¹ M NaCl), as well as at 100 K and/or 20°C. The results here reported represent clearcut evidence for the cooperative and specific (i.e., functionally relevant) binding of IHP, BZF, and CFA to Hb.     

Different structural effects of allosteric modulators on subunits of tetrameric ferrous nitrosylated human hemoglobin: an EPR spectroscopic study

 The X-band EPR spectroscopic features of the ferrous nitrosylated derivative of α(Fe)₂β(Co)₂ and of α(Co)₂β(Fe)₂ metal hybrids of human hemoglobin (Hb) have been investigated at pH 7.0 and analyzed in parallel with those of the native nitrosylated tetramer (HbNO). The effect of 2,3-biphosphoglycerate (BPG), inositol hexakisphosphate (IHP) and bezafibrate (BZF) has been investigated in order to understand the perturbations induced on α and β subunits in the tetramer by the binding of allosteric effectors. A large perturbation is observed in both subunits upon BZF binding, while in the case of IHP only α-chains are affected; on the other hand, BPG leaves both chains essentially unperturbed. Thus, different binding modes of allosteric effectors to HbNO may occur, and the simultaneous addition of two effector molecules, namely BPG and BZF or IHP and BZF to HbNO, brings about different alterations of the X-band EPR spectroscopic properties. This behavior indicates that the intramolecular communication pathway(s) between the heme and the binding pockets of the heterotropic ligands (i.e., IHP and BZF, or BPG and BZF) are different, leading to distinct structural perturbations. Received: 19 September 1997 / Accepted: 16 December 1997     

Stabilization of the T-state of human hemoglobin by proflavine, an antiseptic drug.

The effect of proflavine (3,6-diaminoacridine), an antiseptic drug, on the spectroscopic and oxygen binding properties of ferrous human adult hemoglobin (Hb) has been investigated. Upon binding of proflavine to the nitric oxide derivative of ferrous human adult hemoglobin (HbNO), the X-band EPR spectrum displays the characteristics which have been attributed to the T-state of the ligated tetramer. In parallel, oxygen affinity for the deoxygenated derivative of ferrous human adult Hb decreases in the presence of proflavine. The effect of proflavine on the spectroscopic and ligand binding properties of ferrous human adult Hb is reminiscent that of 2,3-D-glycerate bisphosphate, the physiological modulator of Hb action.     

Effect of inositol hexakisphosphate on the spectroscopic properties of the nitric oxide derivative of ferrous horse and bovine hemoglobin

The effect of inositol hexakisphosphate (IHP) on the spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous horse and bovine hemoglobin (Hb) has been investigated. In the absence of IHP, the nitric oxide derivative of ferrous horse Hb shows spectroscopic properties similar to those of the corresponding derivative of ferrous human Hb that are generally taken as typical of the high affinity state of tetrametric hemoproteins. Similar to human Hb, the addition of IHP to the nitric oxide derivative of ferrous horse Hb induces a transition toward a species characterized by spectral properties typical of the low affinity state of hemoglobins. Nevertheless, the equilibrium constant for IHP binding to the nitric oxide derivative of ferrous horse Hb (= 1.5 x 10(2) M-1) is much lower than that reported for the association of the polyphosphate to the same derivative of ferrous human Hb (greater than 3 x 10(5) M-1). Conversely, the spectroscopic properties of the nitric oxide derivative of ferrous bovine Hb are characteristic of the low affinity state of tetrameric hemoproteins, both in the absence and in the presence of IHP. These results, taken together with the behavior of the nitric oxide derivative of ferrous human Hb, provide further evidence for the peculiar oxygen binding properties of horse and bovine Hb.     

Coupling of the oxygen-linked interaction energy for inositol hexakisphosphate and bezafibrate binding to human HbA0

The energetics of signal propagation between different functional domains (i.e. the binding sites for O2, inositol hexakisphospate (IHP), and bezafibrate (BZF)) of human HbA0 was analyzed at different heme ligation states and through the use of a stable, partially heme ligated intermediate. Present data allow three main conclusions to be drawn, and namely: (i) IHP and BZF enhance each others binding as the oxygenation proceeds, the coupling free energy going from close to zero in the deoxy state to -3.4 kJ/mol in the oxygenated form; (ii) the simultaneous presence of IHP and BZF stabilizes the hemoglobin T quaternary structure at very low O2 pressures, but as oxygenation proceeds it does not impair the transition toward the R structure, which indeed occurs also under these conditions; (iii) under room air pressure (i.e. pO2 = 150 torr), IHP and BZF together induce the formation of an asymmetric dioxygenated hemoglobin tetramer, whose features appear reminiscent of those suggested for transition state species (i.e. T- and R-like tertiary conformation(s) within a quaternary R-like structure).     

Spectroscopic properties of the nitric oxide derivative of ferrous man, horse, and ruminant hemoglobins: a comparative study.

The spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous man, horse, buffalo, deer, mouflon, musk ox, ox, and reindeer hemoglobin (HbNO) have been investigated in the absence of any allosteric effector at pH 6.5 (in 0.1 M 2-[N-morpholino]ethanesulphonic acid/NaOH chloride-free buffer system), as well as at 100 K and/or 20 degrees C. Man and horse HbNO show spectroscopic properties that are generally taken as typical of the high affinity state of ferrous tetrameric Hb's; on the other hand, the spectroscopic properties of ruminant (i.e., buffalo, deer, mouflon, musk ox, ox, and reindeer) HbNO are characteristic of the low affinity conformation. These results are in keeping with the functional properties of the mammalian Hb's considered and have been related to the peculiar low oxygen affinity of ruminant Hb's.     

Influence of allosteric effectors on the heme conformation of dromedary ferrous nitrosylhemoglobin detected by XANES spectroscopy

The changes of the Fe heme-active site conformation of dromedary (Camelus dromedarius) nitrosylhemoglobin (HbNO) induced by inositol hexakisphosphate (IHP) and chlofibric acid (CFA) have been studied by using X-ray absorption near-edge structure (XANES) spectroscopy. Structural information has been determined by multiple scattering analysis of the Fe K-edge XANES spectra. The proximal histidine is found to move away from iron centers by about 0.4 Angstrom on the average over the four hemes upon binding of CFA or stoichiometric amount of IHP. In molar excess of polyanion or in the simultaneous presence of IHP, CFA and chloride, the proximal histidine moves back to a position very close to that observed in pure buffer; yet, the structure modulation induced by the allosteric effectors is not completely reversible. Such findings parallel with the functional properties and the spectroscopic (e.g., EPR and absorbance) characteristics of HbNO.     

Effect of inositol hexakisphosphate on the spectroscopic properties of the nitric oxide derivative of ferrous naturally glycated human hemoglobin HbA1c

The effect of inositol hexakisphosphate (IHP) on the spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous naturally glycated human hemoglobin HbA1c (HbA1cNO) has been investigated quantitatively. The results obtained show that 1) both in the absence and presence of IHP, the EPR and absorbance spectra of HbA1cNO show the same basic characteristics described for the nitrosyl derivative of ferrous HbA0, the nonglycated major component of human hemoglobin (HbA0NO); and 2) HbA1cNO binds IHP with an apparent dissociation equilibrium constant (upsilon = 1.8 x 10(-2) M), which is at least four orders of magnitude higher than that estimated for the polyphosphate interaction with HbA0NO (less than or equal to 3 x 10(-6) M). These data provide further independent evidence that interaction(s) of polyphosphates at the specific cleft between beta-chains along the dyad-axis is sterically hindered in HbA1c by the presence of the two glucose residues covalently bound to the N-termini of beta-chains, this finding being in agreement with the reduced effect of polyanions on HbA1c spectral and ligand-binding properties.     

Proton-linked subunit heterogeneity in ferrous nitrosylated human adult hemoglobin: an EPR study

The effect of pH on the X-band electron paramagnetic resonance (EPR) spectrum of ferrous nitrosylated human adult tetrameric hemoglobin (HbNO) as well as of ferrous nitrosylated monomeric alpha- and beta-chains has been investigated, at -163 degrees C. At pH 7.3, the X-band EPR spectrum of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains displays a rhombic shape. Lowering the pH from 7.3 to 3.0, tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains undergo a transition towards a species characterized by a X-band EPR spectrum with a three-line splitting centered at 334mT. These pH-dependent spectroscopic changes may be taken as indicative of the cleavage, or the severe weakening, of the proximal HisF8-Fe bond. In tetrameric HbNO, the pH-dependent spectroscopic changes depend on the acid-base equilibrium of two apparent ionizing groups with pK(a) values of 5.8 and 3.8. By contrast, the pH-dependent spectroscopic changes occurring in ferrous nitrosylated monomeric alpha- and beta-chains depend on the acid-base equilibrium of one apparent ionizing group with pK(a) values of 4.8 and 4.7, respectively. The different pK(a) values for the proton-linked spectroscopic transition(s) of tetrameric HbNO and ferrous nitrosylated monomeric alpha- and beta-chains suggest that the quaternary assembly drastically affects the strength of the proximal HisF8-Fe bond in both subunits. This probably reflects a 'quaternary effect', i.e., structural changes in both subunits upon tetrameric assembly, which is associated to a relevant variation of functional properties (i.e., proton affinity).     

Quaternary structure of carbonmonoxyhemoglobins in solution: structural changes induced by the allosteric effector inositol hexaphosphate

We have applied the residual dipolar coupling (RDC) method to investigate the solution quaternary structures of (2)H- and (15)N-labeled human normal adult recombinant hemoglobin (rHb A) and a low-oxygen-affinity mutant recombinant hemoglobin, rHb(alpha96Val-->Trp), both in the carbonmonoxy form, in the absence and presence of an allosteric effector, inositol hexaphosphate (IHP), using a stretched polyacrylamide gel as the alignment medium. Our recent RDC results [Lukin, J. A., Kontaxis, G., Simplaceanu, V., Yuan, Y., Bax, A., and Ho, C. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 517-520] indicate that the quaternary structure of HbCO A in solution is a dynamic ensemble between two previously determined crystal structures, R (crystals grown under high-salt conditions) and R2 (crystals grown under low-salt conditions). On the basis of a comparison of the geometric coordinates of the T, R, and R2 structures, it has been suggested that the oxygenation of Hb A follows the transition pathway from T to R and then to R2, with R being the intermediate structure [Srinivasan, R., and Rose, G. D. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 11113-11117]. The results presented here suggest that IHP can shift the solution quaternary structure of HbCO A slightly toward the R structure. The solution quaternary structure of rHbCO(alpha96Val-->Trp) in the absence of IHP is similar to that of HbCO A in the presence of IHP, consistent with rHbCO(alpha96Val-->Trp) having an affinity for oxygen lower than that of Hb A. Moreover, IHP has a much stronger effect in shifting the solution quaternary structure of rHbCO(alpha96Val-->Trp) toward the R structure and toward the T structure, consistent with IHP causing a more pronounced decrease in its oxygen affinity. The results presented in this work, as well as other results recently reported in the literature, clearly indicate that there are multiple quaternary structures for the ligated form of hemoglobin. These results also provide new insights regarding the roles of allosteric effectors in regulating the structure and function of hemoglobin. The classical two-state/two-structure allosteric mechanism for the cooperative oxygenation of hemoglobin cannot account for the structural and functional properties of this protein and needs to be revised.     

The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin

The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin was investigated by accurately measuring and analyzing the oxygen equilibrium curves of human adult hemoglobin in the presence and absence of various concentrations of one or two of the following materials: chloride (Cl-), 2,3-diphosphoglycerate (DPG), and inositol hexaphosphate (IHP). Each equilibrium curve was analyzed according to the Adair equation to evaluate the four-step oxygen equilibrium constants (Adair constants) and the median oxygen pressure. The binding constants of the anions for the molecular species of hemoglobin carrying j oxygen molecules, Hb(O2)j(j=0,1,...,4), were evaluated from the dependences of the Adair constants and the median oxygen pressure on the anion concentration by introducing a model which takes the competitive binding of Cl- and DPG or IHP into account. Assumptions made in the model are: (a) the hemoglobin molecule has two oxygen-linked binding sites for Cl- which are equivalent and independent and (b) no Cl- can be bound to hemoglobin to which DPG or IHP is already bound and vice versa. Thus, we could obtain values for the intrinsic binding constants of Cl- and DPG, i.e., the constants in the absence of other competitive anions. For IHP, only the binding constants and apparent binding constants for Hb and Hb(O2)2 were obtained. Values of the Cl- binding constants and apparent binding constants for DPG and IHP, i.e., the binding constants in the presence of Cl- for Hb and Hb(O2)4, were in reasonable agreement with literature values. From the binding constants we calculated anion binding curves for Hb(O2)j(J=0,1,...,4), the number of anions bound to Hb(O2)J, And the relationship between fractional anion saturation of hemoglobin and fractional oxygen saturation. The numbers of released anions are not uniform with respect to oxygenation step. This non-uniformity is the reason for the changes in the shape of the oxygen equilibrium curve with anion concentration changes and for the non-uniform dependences of the Adair constants on anion concentration, and also results in non-linear relations between anion saturation and oxygen saturation. The anion binding constants and various binding properties of the anions derived from those constants are consistent with those observed by other investigators using different techniques, indicating that the present model describes the oxygen-linked competitive anion binding well.     

Electron paramagnetic resonance analysis of nitrosylhemoglobin in humans during NO inhalation

The reactions of nitric oxide with hemoglobin play an important role in explaining the vascular biology of this free radical. It is perhaps surprising that the level of nitrosylhemoglobin (HbNO) in which NO is bound to the ferrous hemoglobin heme in whole human blood under basal and stimulated conditions is a matter of some controversy, with measurements ranging from <1 nm to close to 10 mum. In order to examine HbNO levels in human blood by using EPR spectroscopy, we have developed a regression-based spectral analysis technique that has a detection level of about 200 nm HbNO. We have utilized this methodology to detect the level of HbNO under basal conditions and during NO inhalation. The major findings of this study are as follows. (i) HbNO can be accurately detected and quantified in whole blood with a detection limit of approximately 200 nm. (ii) By using regression analysis, levels of HbNO as low as 0.5-1 mum can be deconvoluted into component species. (iii) HbNO is present at less than 200 nm at basal conditions in both arterial and venous blood and is formed at a level of 0.5-2.5 mum upon inhalation of 80 ppm NO. (iv) The levels of HbNO detected by EPR are remarkably close (within a factor of 2) to those detected by tri-iodide-based chemiluminescence and much smaller than those detected by photolysis chemiluminescence. (v) The half-time of HbNO in vivo is approximately 40 min.     

The reaction of deoxy-sickle cell hemoglobin with hydroxyurea.

In addition to its capacity to increase fetal hemoglobin levels, other mechanisms are implicated in hydroxyurea's ability to provide beneficial effects to patients with sickle cell disease. We hypothesize that the reaction of hemoglobin with hydroxyurea may play a role. It is shown that hydroxyurea reacts with deoxy-sickle cell hemoglobin (Hb) to form methemoglobin (metHb) and nitrosyl hemoglobin (HbNO). The products of the reaction as well as the kinetics are followed by absorption spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. Analysis of the kinetics shows that the reaction can be approximated by a pseudo-first order rate constant of 3.7x10(-4) (1/(s.M)) for the disappearance of deoxy-sickle cell hemoglobin. Further analysis shows that HbNO is formed at an observed average rate of 5.25x10(-5) (1/s), three to four times slower than the rate of formation of metHb. EPR spectroscopy is used to show that the formation of HbNO involves the specific transfer of NO from the NHOH group of hydroxyurea. The potential importance of this reaction is discussed in the context of metHb and HbNO being able to increase the delay time for sickle cell hemoglobin polymerization and HbNO's vasodilating capabilities through conversion to S-nitrosohemoglobin.     

Stabilization of the T-state of hemoglobin

The effect of inositol hexaphosphate and bezafibrate on binding of O2 and CO to HbAO at high concentrations (1 mM) has been evaluated using thin layer optical techniques. Data analysis shows 1) the occurrence of greatly reduced ligand dependent cooperativity (Hill slope of 2.23 for CO and 1.51 for O2), and 2) the presence of significant triply ligated species. The data fits a nested allosteric two-state MWC model in which the T state consists of two allosteric substrates, Tt and Tr, where Tt binds only to the alpha chains and Tr binds to both alpha and beta chains. The model indicates that the triply ligated species consists of a predominant amount of T form, agreeing with kinetic observations of CO ligated hemoglobin. The maximum amount of triply ligated R molecules (CO or O2) implicated is less than 1%, a result similar to that found previously for binding studies made in the absence of BZF and IHP.     

Effect of organic phosphates on methemoglobin reduction by ascorbic acid.

The rate of methemoglobin reduction by ascorbic acid was accelerated in the presence of ATP,2,3-diphosphoglycerate (2,3-DPG), and inositol hexaphosphate (IHP). The acceleration was as much as three times, four times, and ten times in the presence of ATP, 2.3-DPG, and IHP at pH 7.0, respectively. The changes of the concentrations of methemoglobin and ascorbic acid during the methemoglobin reduction were determined, and the reaction was found to proceed stoichiometrically in the presence of IHP. The reduction rate of methemoglobin by ascorbic acid was compared at different concentrations of organic phosphates (ATP,2,3-DPG, and IHP) at various pH values (6.3, 7.0, 7.7). From the changes in the reduction rate under different concentrations of organic phosphates, the dissociation constants of ATP, 2,3-DPG, and IHP to methemoglobin could be determined and were estimated to be 3.3 X 10(-4) M, 2 X 10(-3) M, and 8 X 10(-6) M at pH 7.0, respectively. On the basis of these results, the acceleration mechanism of methemoglobin reduction by ascorbic acid due to the presence of organic phosphates was described. The physiological role of 2,3-DPG in human red cells was discussed in relation to the reduction of methemoglobin by ascorbic acid.     

Acetylation of human hemoglobin by methyl acetylphosphate. Evidence of broad regio-selectivity revealed by NMR studies.

The development of chemical modification agents that reduce the tendency of sickle hemoglobin (HbS) to aggregate represents an important chemotherapeutic goal. Methyl acetylphosphate (MAP) has been reported to bind to the 2,3-diphosphoglycerate (2,3-DPG) binding site of hemoglobin, where it selectively acetylates residues, resulting in increased solubility of HbS. We have prepared [1-(13)C]MAP and evaluated the adduct formation with hemoglobin using (1)H-(13)C HMQC and HSQC NMR studies. These spectra of the acetylated hemoglobin adducts showed 10-11 well resolved adduct peaks, indicating that the acetylation was not highly residue specific. The chemical shift pattern observed is in general similar to that obtained recently using [1'-(13)C]aspirin as the acetylating agent (Xu, A. S. L., Macdonald, J. M., Labotka, R. J., and London, R. E. (1999) Biochim. Biophys. Acta 1432, 333-349). Blocking the 2, 3-DPG binding site with inositol hexaphosphate (IHP) resulted in a selective reduction in intensity of adduct resonances, presumably corresponding to residues located in the 2,3-DPG binding cleft. The pattern of residue protection appeared to be identical to that observed in our previous study using IHP and labeled aspirin. Pre-acetylation of hemoglobin using unlabeled MAP, followed by acetylation with [1'-(13)C]aspirin indicated a general protective effect, with the greatest reduction of intensity for resonances corresponding to acetylated residues in the 2,3-DPG binding site. These studies indicated that both MAP and aspirin exhibit similar, although not identical, acetylation profiles and target primarily the betaLys-82 residue in the 2,3-DPG binding site, as well as sites such as betaLys-59 and alphaLys-90, which are not located in the beta-cleft of hemoglobin.     

Hb琼海的氧平衡特征与78K下的穆斯堡尔谱

经用Beneseh法检测发现,Hb琼海(β78(EF_2)Leu→Arg)的氧亲合力明显高于HbA,但其Bohr效应和亚基间协同作用却正常。Hb琼海的穆斯堡尔谱表明它颇不稳定,较易形成衍生物,本文讨论了Hb琼海的结构与功能及其稳定性之间的关系。     

The dissociation of NO from nitrosylhemoglobin

The reaction between nitrosylhemoglobin and an excess of deoxymyoglobin has been used to study the kinetics of ligand dissociation from Hb4(NO)4 and Hb4(no)1 species. The kinetics of the dissociation of the first NO molecule from Hb4(no)4 was studied by the ligand replacement method. The results indicate that: (a) the ligand dissociation reaction in Hb4(NO)4 is a cooperative process. This is consistent with the results of Moore and Gibson (Moore, E.G., and Gibson, Q.H. (1977) J. Biol. Chem. 251, 2788-2794). (b) alpha and beta chains in the T state formed by adding IHP to Hb4(NO)4 show kinetic heterogeneity. (c) A similar kinetic heterogeneity is shown by alpha and beta chains in the species Hb4NO in the absence of IHP.(d) The value for the NO dissociation rate constant calculated from the slow phases observed in (b) and (c) is similar to that estimated for the R state. These results suggest that the R to T transition brought about with or without inositol hexaphosphate changes the ligand affinity of one type of the chains much more than of the other. On the basis of IR and EPR studies, it is suggested that alpha chains undergo larger functional changes in R to T transition (or vice versa) in nitrosylhemoglobin. The kinetic parameters for HbNO are compared with those of HbO2 and HbCO and the implications of the results for the reaction mechanism are discussed.     

Effect of inositol hexakisphosphate on the EPR properties of the nitric oxide derivative of ferrous dromedary (Camelus dromedarius) hemoglobin. Evidence for two polyanion binding sites

The effect of inositol hexakisphosphate on the EPR properties of the nitric oxide derivative of ferrous dromedary (Camelus dromedarius) hemoglobin has been investigated at 110 K. In the absence of inositol hexakisphosphate, the nitrosyl derivative of dromedary hemoglobin shows an EPR spectrum with a rhombic shape and a weak hyperfine splitting in the gz region, a feature that is generally taken as characteristic of the high-affinity state of tetrameric hemoproteins. On addition of 1 mole of inositol hexakisphosphate/tetramer, three new hyperfine lines (Az = 1.7 mT), centered at gz = 2.01, appear; this type of spectrum is indicative of the low-affinity state of hemoglobins. A further addition of inositol hexakisphosphate, corresponding to a 20-fold molar excess, completely reverses the polyphosphate-dependent transition, giving an EPR spectrum that is exactly superimposable to that observed in the absence of the allosteric effector, i.e., is typical of the high-affinity state of the macromolecule. Both in the absence and presence of inositol hexakisphosphate, the EPR spectra are virtually independent of pH in the range explored (from 5.5 to 7.5). These results, taken together with the behavior of the nitric oxide derivative of human hemoglobin, provide further evidence for the existance in dromedary hemoglobin of two polyanion binding sites that affect in an opposite way the conformational equilibrium of the macromolecule.     

Oxygen equilibrium and EPR studies on alpha1beta1 hemoglobin dimer

We undertook this project to clarify whether hemoglobin (Hb) dimers have a high affinity for oxygen and cooperativity. For this, we prepared stable Hb dimers by introducing the mutation Trp-->Glu at beta37 using our Escherichia coli expression system at the alpha1beta2 interface of Hb, and analyzed their molecular properties. The mutant hybrid Hbs with a single oxygen binding site were prepared by substituting Mg(II) protoporphyrin for ferrous heme in either the alpha or beta subunit, and the oxygen binding properties of the free dimers were investigated. Molecular weight determination of both the deoxy and CO forms showed all these molecules to be dimers in the absence of IHP at different protein concentrations. Oxygen equilibrium measurements showed high affinity and non-cooperative oxygen binding for all mutant Hb and hybrid Hb dimers. However, EPR results on the [alpha(N)(Fe-NO)beta(M)(Mg)] hybrid showed some alpha1beta1 interactions. These results provide some clues as to the properties of Hb dimers, which have not been studied extensively owing to practical difficulties in their preparation.