Structure and function of haemoglobin Philly (Tyr C1 (35) beta replaced by Phe)

Interrelationship among Fe-His Bond Strengths, Oxygen Affinities, and Intersubunit Hydrogen Bonding Changes upon Ligand Binding in the β Subunit of Human Hemoglobin: The Alkaline Bohr Effect

Regulation of the oxygen affinity of human adult hemoglobin (Hb A) at high pH, known as the alkaline Bohr effect, is essential for its physiological function. In this study, structural mechanisms of the alkaline Bohr effect and pH-dependent O₂ affinity changes were investigated via ¹H nuclear magnetic resonance and visible and UV resonance Raman spectra of mutant Hbs, Hb M Iwate (αH87Y) and Hb M Boston (αH58Y). It was found that even though the binding of O₂ to the α subunits is forbidden in the mutant Hbs, the O₂ affinity was higher at alkaline pH than at neutral pH, and concomitantly, the Fe-His stretching frequency of the β subunits was shifted to higher values. Thus, it was confirmed for the β subunits that the stronger the Fe-His bond, the higher the O₂ affinity. It was found in this study that the quaternary structure of α(Fe³⁺)β(Fe²⁺-CO) of the mutant Hb is closer to T than to the ordinary R at neutral pH. The retained Aspβ94-Hisβ146 hydrogen bond makes the extent of proton release smaller upon ligand binding from Hisβ146, known as one of residues contributing to the alkaline Bohr effect. For these T structures, the Aspα94-Trpβ37 hydrogen bond in the hinge region and the Tyrα42-Aspβ99 hydrogen bond in the switch region of the α₁-β₂ interface are maintained but elongated at alkaline pH. Thus, a decrease in tension in the Fe-His bond of the β subunits at alkaline pH causes a substantial increase in the change in global structure upon binding of CO to the β subunit.

An Origin of Cooperative Oxygen Binding of Human Adult Hemoglobin: Different Roles of the α and β Subunits in the α2β2 Tetramer

Human hemoglobin (Hb), which is an α2β2 tetramer and binds four O2 molecules, changes its O2-affinity from low to high as an increase of bound O2, that is characterized by 'cooperativity'. This property is indispensable for its function of O2 transfer from a lung to tissues and is accounted for in terms of T/R quaternary structure change, assuming the presence of a strain on the Fe-histidine (His) bond in the T state caused by the formation of hydrogen bonds at the subunit interfaces. However, the difference between the α and β subunits has been neglected. To investigate the different roles of the Fe-His(F8) bonds in the α and β subunits, we investigated cavity mutant Hbs in which the Fe-His(F8) in either α or β subunits was replaced by Fe-imidazole and F8-glycine. Thus, in cavity mutant Hbs, the movement of Fe upon O2-binding is detached from the movement of the F-helix, which is supposed to play a role of communication. Recombinant Hb (rHb)(αH87G), in which only the Fe-His in the α subunits is replaced by Fe-imidazole, showed a biphasic O2-binding with no cooperativity, indicating the coexistence of two independent hemes with different O2-affinities. In contrast, rHb(βH92G), in which only the Fe-His in the β subunits is replaced by Fe-imidazole, gave a simple high-affinity O2-binding curve with no cooperativity. Resonance Raman, 1H NMR, and near-UV circular dichroism measurements revealed that the quaternary structure change did not occur upon O2-binding to rHb(αH87G), but it did partially occur with O2-binding to rHb(βH92G). The quaternary structure of rHb(αH87G) appears to be frozen in T while its tertiary structure is changeable. Thus, the absence of the Fe-His bond in the α subunit inhibits the T to R quaternary structure change upon O2-binding, but its absence in the β subunit simply enhances the O2-affinity of α subunit.

Functional characterization of the single hemoglobin of the migratory bird Ciconia ciconia

Hemolysate from white stork displayed a single hemoglobin component, thus resulting into two bands and two globin peaks in dissociating PAGE and reversed phase-HPLC, respectively. Stripped hemoglobin showed an oxygen affinity higher than that of human HbA, a small Bohr effect, and a cooperative oxygen binding. A small decrease of oxygen affinity, of the same extent in all the pH range examined, was observed by addition of chloride, thus indicating an unusual chloride-independent Bohr effect (DeltalogP50/Deltalog pH=-0.24). Saturating amounts of inositol hexakisphosphate, largely decreased hemoglobin-oxygen affinity (DeltalogP(50)=1.17 at pH 7.0), and increased the extent of its Bohr effect (DeltalogP50/DeltalogpH=-0.45). The phosphate binding curve allowed to measure a very high overall binding constant (K=1.18 x 10(5) M(-1)). The effect of temperature on the oxygen affinity was measured, and the enthalpy change of oxygenation resulted almost independent on pH. Structural-functional relationships are discussed by considering some amino acid residues situated at alpha1/beta1 and alpha1/beta2 interfaces, such as alpha38 and alpha89 positions. The presence of only one hemoglobin component, a rare event among birds, and its functional properties have been related to the physiological oxygen requirements of this soaring migrant bird and to its technique of flight during migration.

Hydrogen bonding to Trp beta37 is the first step in a compound pathway for hemoglobin allostery

Human hemoglobin is widely thought to change from the R to the T quaternary structure in a single rate process requiring tens of microseconds. Here we present kinetic evidence that the R --> T allosteric pathway in hemoglobin requires more than one step. We use magnetic circular dichroism (MCD) spectroscopy of the aromatic amino acid bands to show that formation of a tryptophan-aspartate hydrogen bond in the hinge region of the dimer-dimer interface is part of an obligatory R --> T step proceeding more than a factor of 10 faster than the kinetic step previously identified in heme-band absorption studies.

Site-directed mutations of human hemoglobin at residue 35beta: a residue at the intersection of the alpha1beta1, alpha1beta2, and alpha1alpha2 interfaces

Because Tyr35beta is located at the convergence of the alpha1beta1, alpha1beta2, and alpha1alpha2 interfaces in deoxyhemoglobin, it can be argued that mutations at this position may result in large changes in the functional properties of hemoglobin. However, only small mutation-induced changes in functional and structural properties are found for the recombinant hemoglobins betaY35F and betaY35A. Oxygen equilibrium-binding studies in solution, which measure the overall oxygen affinity (the p50) and the overall cooperativity (the Hill coefficient) of a hemoglobin solution, show that removing the phenolic hydroxyl group of Tyr35beta results in small decreases in oxygen affinity and cooperativity. In contrast, removing the entire phenolic ring results in a fourfold increase in oxygen affinity and no significant change in cooperativity. The kinetics of carbon monoxide (CO) combination in solution and the oxygen-binding properties of these variants in deoxy crystals, which measure the oxygen affinity and cooperativity of just the T quaternary structure, show that the ligand affinity of the T quaternary structure decreases in betaY35F and increases in betaY35A. The kinetics of CO rebinding following flash photolysis, which provides a measure of the dissociation of the liganded hemoglobin tetramer, indicates that the stability of the liganded hemoglobin tetramer is not altered in betaY35F or betaY35A. X-ray crystal structures of deoxy betaY35F and betaY35A are highly isomorphous with the structure of wild-type deoxyhemoglobin. The betaY35F mutation repositions the carboxyl group of Asp126alpha1 so that it may form a more favorable interaction with the guanidinium group of Arg141alpha2. The betaY35A mutation results in increased mobility of the Arg141alpha side chain, implying that the interactions between Asp126alpha1 and Arg141alpha2 are weakened. Therefore, the changes in the functional properties of these 35beta mutants appear to correlate with subtle structural differences at the C terminus of the alpha-subunit.

Chain-selective isotopic labeling for NMR studies of large multimeric proteins: application to hemoglobin

Multidimensional, multinuclear NMR has the potential to elucidate the mechanisms of allostery and cooperativity in multimeric proteins under near-physiological conditions. However, NMR studies of proteins made up of non-equivalent subunits face the problem of severe resonance overlap, which can prevent the unambiguous assignment of resonances, a necessary step in interpreting the spectra. We report the application of a chain-selective labeling technique, in which one type of subunit is labeled at a time, to carbonmonoxy-hemoglobin A (HbCO A). This labeling method can be used to extend previous resonance assignments of key amino acid residues, which are important to the physiological function of hemoglobin. Among these amino acid residues are the surface histidyls, which account for the majority of the Bohr effect. In the present work, we report the results of two-dimensional heteronuclear multiple quantum coherence (HMQC) experiments performed on recombinant (15)N-labeled HbCO A. In addition to the C2-proton (H epsilon(1)) chemical shifts, these spectra also reveal the corresponding C4-proton (H delta(2)) resonances, correlated with the N epsilon(2) and N delta(1) chemical shifts of all 13 surface histidines per alpha beta dimer. The HMQC spectrum also allows the assignment of the H delta(1), H epsilon(1), and N epsilon(1) resonances of all three tryptophan residues per alpha beta dimer in HbCO A. These results indicate that heteronuclear NMR, used with chain-selective isotopic labeling, can provide resonance assignments of key regions in large, multimeric proteins, suggesting an approach to elucidating the solution structure of hemoglobin, a protein with molecular weight 64.5 kDa.

Near-ultraviolet magnetic circular dichroism spectroscopy of protein conformational states: correlation of tryptophan band position and intensity with hemoglobin allostery

The near-UV magnetic circular dichroism spectroscopy of the aromatic amino acid bands of hemoglobin was investigated as a potential probe of structural changes at the alpha(1)beta(2) interface during the allosteric transition. Allosteric effectors were used to direct carp and chemically modified human hemoglobins into the R (relaxed) or T (tense) state in order to determine the heme-ligation-independent spectral characteristics of the quaternary states. The tryptophan magnetic circular dichroism (MCD) peak observed at 293 nm in the R state of N-ethylsuccinimide- (NES-) des-Arg-modified human hemoglobin (Hb) was shifted to a slightly longer wavelength in the T state, consistent with the shift expected for tryptophan acting as a proton donor in a T-state hydrogen bond. Moreover, the increase observed in the T-state MCD intensity of this band relative to the R-state intensity was consistent with the effect expected for proton donation by tryptophan on the basis of the Michl perimeter model of aromatic MCD. The peak-to-trough magnitude of the R - T MCD difference spectrum is equal to 30% of the total R-state peak intensity contributed by all six tryptophans present in the human tetramer; the relative magnitude specific to the two beta37 tryptophans undergoing conformational change is estimated accordingly to be 3 times larger. The Trp-beta37 spectral shift, about 200 cm(-)(1), is in good agreement with the shifts observed in other H-bonded proton donors and provides corroborating spectral evidence for the formation in solution of a T-state Trp beta37-Asp alpha94 hydrogen bond observed in X-ray diffraction studies of deoxyHb crystals.

Substitution of the heme binding module in hemoglobin alpha- and beta-subunits. Implication for different regulation mechanisms of the heme proximal structure between hemoglobin and myoglobin

In our previous work, we demonstrated that the replacement of the "heme binding module," a segment from F1 to G5 site, in myoglobin with that of hemoglobin alpha-subunit converted the heme proximal structure of myoglobin into the alpha-subunit type (Inaba, K., Ishimori, K. and Morishima, I. (1998) J. Mol. Biol. 283, 311-327). To further examine the structural regulation by the heme binding module in hemoglobin, we synthesized the betaalpha(HBM)-subunit, in which the heme binding module (HBM) of hemoglobin beta-subunit was replaced by that of hemoglobin alpha-subunit. Based on the gel chromatography, the betaalpha(HBM)-subunit was preferentially associated with the alpha-subunit to form a heterotetramer, alpha(2)[betaalpha(HBM)(2)], just as is native beta-subunit. Deoxy-alpha(2)[betaalpha(HBM)(2)] tetramer exhibited the hyperfine-shifted NMR resonance from the proximal histidyl N(delta)H proton and the resonance Raman band from the Fe-His vibrational mode at the same positions as native hemoglobin. Also, NMR spectra of carbonmonoxy and cyanomet alpha(2)[betaalpha(HBM)(2)] tetramer were quite similar to those of native hemoglobin. Consequently, the heme environmental structure of the betaalpha(HBM)-subunit in tetrameric alpha(2)[betaalpha(HBM)(2)] was similar to that of the beta-subunit in native tetrameric Hb A, and the structural conversion by the module substitution was not clear in the hemoglobin subunits. The contrastive structural effects of the module substitution on myoglobin and hemoglobin subunits strongly suggest different regulation mechanisms of the heme proximal structure between these two globins. Whereas the heme proximal structure of monomeric myoglobin is simply determined by the amino acid sequence of the heme binding module, that of tetrameric hemoglobin appears to be closely coupled to the subunit interactions.

Quaternary structure sensitive tyrosine residues in human hemoglobin: UV resonance raman studies of mutants at alpha140, beta35, and beta145 tyrosine

Recent studies noted the contribution of alpha42Tyr to the T-R-dependent UV resonance Raman (UVRR) spectral changes of HbA [Nagai, M., et al. (1996) J. Mol. Struct. 379, 65-75; Huang, S., et al. (1997) Biochemistry 36, 6197-6206], but the observed UVRR changes of the Tyr residue cannot be fully interpreted with alpha42Tyr alone. To identify the remaining contributions, the 235 nm-excited UVRR spectra of Tyr mutant Hbs at alpha140, beta35, and beta145 were investigated here. The Fe-His stretching mode demonstrated that all of these mutant Hbs take the T structure in the deoxy form under these experimental conditions. The UVRR change of the Trp residue of these mutants upon the T-R transition was the same as that in HbA, indicating that the T-R-dependent UVRR change of beta37Trp is not due to stacking with Tyr residues but is due to the formation or destruction of a hydrogen bond. The recombinant Hbs beta35Tyr --> Phe and beta35Tyr --> Thr both exhibited UVRR spectra identical with that of HbA, meaning that beta35Tyr is not responsible. In the spectra of des(beta146His,beta145Tyr)Hb with inositol hexaphosphate, the frequency shift of the Tyr RR bands was the same as that in HbA but the intensity enhancement in the CO form was small, suggesting that beta145Tyr contributes to a part of the intensity change, but scarcely relates to the frequency shift. In the spectra of Hb Rouen (alpha140Tyr --> His), the frequency shifts of bands at 1617 (Y8a) and 1177 (Y9a) cm-1 following ligation were half of those in HbA, while the intensity enhancement was not detected. This result means that alpha140Tyr is responsible for both the frequency shift and the intensity changes. It is suggested that the frequency shift of the Tyr RR bands upon the T --> R transition is due to changes in the hydrogen bonding state of alpha42- and alpha140Tyr and that the intensity enhancement is due to changes in the environment of the penultimate Tyr in both alpha and beta subunits (alpha140 and beta145). These alterations in the vibrational spectra clearly demonstrate which tyrosine residues are involved in the T-R transition as a result of modification of their local environments.

The artificial alpha1beta1-contact mutant hemoglobin, Hb Phe-35beta, shows only small functional abnormalities

It was previously reported that Hb Philly with a mutation of Phe for Tyr at 35(C1)beta showed non-cooperative oxygen binding with a very high affinity and instability leading to hemolysis. Further, it lacked the 1H-NMR signal at 13.1 ppm from 2,2-dimethyl-2-silapentane-5-sulfonate in normal hemoglobin (Hb A), so that this signal was assigned to a hydrogen bond formed by Tyr-35(C1)beta. Surprisingly, our artificial mutant hemoglobin with the same mutation as Hb Philly showed slightly lowered oxygen affinity, almost normal cooperativity, the 1H-NMR signal at 13.1 ppm and no sign of instability. Our results indicate that the mutation reported for Hb Philly and the assignment of the 13.1 ppm signal need reexamination.

Structural and functional effects of pseudo-module substitution in hemoglobin subunits. New structural and functional units in globin structure

Functional and structural significance of the "module" in proteins has been investigated for globin proteins. Our previous studies have revealed that some modules in globins are responsible for regulating the subunit association and heme environmental structures, whereas the module substitution often induces fatal structural destabilization, resulting in failure of functional regulation. In this paper, to gain further insight into functional and structural significance of the modular structure in globins, we focused upon the "pseudo-module" in globin structure where boundaries are located at the center of modules. Although the pseudo-module has been supposed not to retain a compactness, the betaalpha(PM3)-subunit, in which one of the pseudo-modules, the F1-H6 region, of the alpha-subunit is implanted into the beta-subunit, conserved stable globin structure, and its association property was converted into that of the alpha-subunit, as the case for the module substituted globin, the betaalpha(M4)-subunit. These results suggest that modules are not unique structural and functional units for globins. Interestingly, however, the recent reconsideration of the module boundary indicates that the modules in globins can be further divided into two small modules, and one of the boundaries for the new small modules coincides with that of the pseudo-module we substituted in this study. Although it would be premature to conclude the significance of the modular structure in globins, it can be safely said that we have found new structural units in globin structure, probably new modules.

Ultraviolet resonance Raman studies of quaternary structure of hemoglobin using a tryptophan beta 37 mutant

Environmental changes of tyrosine and tryptophan residues of hemoglobin (Hb) upon its T to R transition of quaternary structure were investigated with ultraviolet resonance Raman (UVRR) spectroscopy excited at 235 nm. DeoxyHb A (T-form) showed a UVRR spectrum distinctly different from those of the ligated Hbs (R-form) including oxyHb, COHb, and metHb A, whereas the ligated Hbs exhibited similar UVRR spectra irrespective of the ligand species and the oxidation state of the heme. To characterize the spectral change of Trp-beta 37 at the alpha 1 beta 2 interface due to the quaternary structure transition, the UVRR spectra of Hb A were compared with the corresponding spectra of Hb Hirose (Trp-beta 37-->Ser). A difference spectrum between deoxyHb A and deoxyHb Hirose showed only Trp resonance Raman (RR) bands, which were reasonably ascribed to Trp-beta 37 in deoxyHb A. RR bands at 873 cm-1 (W17) and at 1360 and 1343 cm-1 (W7, Fermi doublet) indicated that the indole ring of Trp-beta 37 in deoxyHb A formed a strong hydrogen bond at the N1H site in hydrophobic environments. Tyr residues in deoxyHb Hirose seemed to be in the same environments as those of deoxyHb A. In contrast, the difference spectrum between Hb A and Hb Hirose in the ligated state displayed peaks for RR bands of both Trp and Tyr. The difference spectra were unaltered by the addition of 5 mM inositol hexaphosphate. This means that the differences were not caused by the tetramer to dimer dissociation but by a conformation change within a tetramer. Comparison of the Hb A-Hb Hirose difference spectra in the oxy and deoxy states revealed that the oxygenation-induced changes of Trp RR bands arose mostly from Trp-beta 37 with the small portion of remaining changes coming from Trp-beta 15, demonstrating that Trp-beta 37 plays a pivotal role in the quaternary structural change in Hb A.

Effects of beta 6 amino acid hydrophobicity on stability and solubility of hemoglobin tetramers

The relationship between different amino acids at the beta 6 position of hemoglobin and tetramer stability was addressed by a site-directed mutagenesis approach. Precipitation rates during mechanical agitation of oxyhemoglobins with Gln, Ala, Val, Leu and Trp at the beta 6 position increased 2, 5, 13, 21 and 53 times, respectively, compared with that for Hb A. There was a linear relationship between the log of the precipitation rate constant and amino acid hydrophobicity at the beta 6 position, suggesting that enhanced precipitation of oxy Hb S during mechanical agitation results in part from increased hydrophobicity of beta 6 Val. Deoxyhemoglobin solubility increased in the order of beta 6 Ile, Leu, Val, Trp, Gln, Ala and Glu suggesting that hydrophobic interactions between beta 6 Val and the acceptor site of another hemoglobin molecule during deoxy-Hb S polymerization not only depend on hydrophobicity but also on stereospecificity of the amino acid side chain at the beta 6 position. Furthermore, our results indicate that hydrophobic amino acids at the beta 6 position which promote tetramer instability in the oxy form do not necessarily promote polymerization in the deoxy form.

Adaptation of bird hemoglobins to high altitudes: demonstration of molecular mechanism by protein engineering

Of two closely related species of geese, one, the greylag goose, lives in the Indian plains all year round, while the other, the bar-headed goose, lives at the Tibetan lakes and migrates across the Himalayas to winter in India. Another species, the Andean goose, lives in the High Andes all year round. Possession of a Hb with high oxygen affinity helps to adapt bar-headed and Andean geese to high altitudes. The Hb amino acid sequences of the bar-headed and the greylag geese differ by four substitutions, of which only one is unique among bird sequences: Pro-119 alpha (H2)----Ala. Perutz proposed that the two-carbon gap left by this substitution at the alpha 1 beta 1 contact raises the oxygen affinity, because it relaxes the tension in the deoxy or T structure [Perutz, M. F. (1983) Mol. Biol. Evol. 1, 1-28]. It was later found that the Hb of the Andean goose has a gap in the same position, due to the complementary substitution Leu-55 beta (D6)----Ser. We have tested Perutz's hypothesis by introducing each of these substitutions into human globin synthesized in Escherichia coli. The reconstituted Hbs combine cooperatively with oxygen. Their oxygen affinities exceed that of normal human Hb by an even larger factor than that found between the high-flying geese and the greylag goose. The mutant Hb Met-55 beta (D6)----Ser was crystallized. Its structure is the same as that of HbA, except in the immediate environment of the gap left by the substitution of the serine for the methionine side chain, which evidently causes the increased oxygen affinity of this Hb.

Functional and NMR studies of Hb Sassari (Asp-126 alpha----His); role of the inter-subunit contacts in the affinity control of human hemoglobin

The oxygen affinity of hemoglobin Sassari (Asp-126 alpha----His), a variant substituted in the alpha 1 beta 1 interface, was found to be 8-times greater relative to normal adult human hemoglobin. Study of the exchangeable hydrogen-bonded protons by NMR spectroscopy shows only minor changes at the alpha 1 beta 1 interface. In particular, the resonance previously assigned to the proton of the hydrogen bond Asp-126 alpha 1. . . Tyr-35 beta 1 in normal hemoglobin is still present in the variant spectrum, suggesting that His-126 alpha can also form a hydrogen bond with the Tyr-35 beta. The possible explanation of the increased affinity of hemoglobin Sassari and other variants substituted in the same structural region is discussed in terms of perturbations of the equilibrium between the two quaternary states.

Unstable hemoglobins

About 70 variants of Hb A with associated hemolytic disorders have been reported during the past 30 years. I have classified them according to four grades of severity of chronic hemolysis. Acute episodes of severe hemolysis may be seen in all classes. In addition, some 80 variants without overt hemolysis have given positive results with in vitro hemoglobin instability tests. The stereochemical bases for instability can be conjectured in most cases, although few unstable hemoglobins have actually been studied by X-ray crystallography. The mechanisms for denaturation of normal Hb A and its acceleration in unstable hemoglobins were proposed some 15 years ago. The alterations of membrane lipids and proteins leading to red cell senescence and the relevance of hemoglobin denaturation to this process are presently being investigated. Several "hyperunstable" variants are clinically silent, or equivalent to a thalassemia, probably because of very efficient degradation of the abnormal chains.

A proton nuclear Overhauser effect investigation of the subunit interfaces in human normal adult hemoglobin

High-resolution proton nuclear magnetic resonance spectroscopy and nuclear Overhauser effects for the low-field exchangeable proton resonances of human normal adult hemoglobin in aqueous solvents are being used to confirm and extend the assignments of these resonances to specific protons at the intersubunit interfaces of the molecule. Most of these exchangeable proton resonances of human normal adult hemoglobin have been found to be absent in the spectra of isolated alpha or beta subunits. This finding indicates that they are specific spectral markers for the quaternary structure of the hemoglobin tetramer. Based on the nuclear Overhauser effect results, we have assigned the exchangeable proton resonance at +7.4 ppm downfield from H2O to the hydrogen-bonded proton between alpha 103(G10)His and beta 108(G10)Asn at the alpha 1 beta 1 interface. The nuclear Overhauser effect results have also confirmed the assignments of the exchangeable proton resonances at +9.4 and +8.2 ppm downfield from H2O previously proposed by workers in this laboratory based on a comparison of human normal adult hemoglobin and appropriate mutant hemoglobins. This independent confirmation of previously proposed assignments is necessary in view of the possible long-range conformational effects of single amino-acid substitutions in mutant hemoglobin molecules.

NMR study of hybrid hemoglobins containing unnatural heme: effect of heme modification on their tertiary and quaternary structures

The effect of heme modification on the tertiary and quaternary structures of hemoglobins was examined by utilizing the NMR spectra of the reconstituted [mesohemoglobin (mesoHb), deuterohemoglobin (deuteroHb)] and hybrid heme (meso-proto, deutero-proto) hemoglobins (Hbs). The heme peripheral modification resulted in the preferential downfield shift of the proximal histidine N1H signal for the beta subunit, indicating nonequivalence of the structural change induced by the heme modification in the alpha and beta subunits of Hb. In the reconstituted and hybrid heme Hbs, the exchangeable proton resonances due to the intra- and intersubunit hydrogen bonds, which have been used as the oxy and deoxy quaternary structural probes, were shifted by 0.2-0.3 ppm from that of native Hb upon the beta-heme substitution. This suggests that, in the fully deoxygenated form, the quaternary structure of the reconstituted Hbs is in an "imperfect" T state in which the hydrogen bonds located at the subunit interface are slightly distorted by the conformational change of the beta subunit. Moreover, the two heme orientations are found in the alpha subunit of deuteroHb, but not in the beta subunit of deuteroHb, and in both the alpha and beta subunits of mesoHb. The tertiary and quaternary structural changes in the Hb molecule induced by the heme peripheral modification were also discussed in relation to their functional properties.

Hemoglobin San Diego/beta zero thalassemia in a Greek adult

Interaction of beta zero thalassemia with Hb San Diego [a high affinity hemoglobin variant, alpha 2 beta (2)109(G11)Val----Met] in a 29-year-old Greek male is described. A marked polycythemia with low MCH and MCV, but minor clinical problems were observed. Functional properties of the isolated variant are described.

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.

A high-resolution proton nuclear-magnetic-resonance investigation of carp hemoglobin. Conformational differences between carp and human normal adult hemoglobins in solution

The high-resolution proton nuclear magnetic resonance spectra of carp hemoglobin have been compared to those of human normal adult hemoglobin. Carp deoxy and carbonmonoxy hemoglobins in the deoxy-type quaternary state exhibit two downfield exchangeable proton resonances as compared to four seen in human normal adult deoxyhemoglobin. This suggests that two of the hydrogen bonds present in human normal adult deoxyhemoglobin are absent or occur in very different environments in carp hemoglobin. One of the exchangeable proton resonances of carp hemoglobin, while present in the deoxy-type quaternary state of the carbonmonoxy and deoxy derivatives, is absent in the oxy-type quaternary state of both, in agreement with the assignments of these quaternary structures by other methods. The ring-current-shifted proton resonances (sensitive tertiary structural markers) of carp carbonmonoxyhemoglobin are substantially different from those of human normal adult hemoglobin. The aromatic proton resonance region of carp hemoglobin has fewer resonances than that of human normal adult hemoglobin, consistent with its much reduced histidine content. The hyperfine-shifted proximal histidyl NH-exchangeable proton resonances of carp hemoglobin suggest that during the transition from the oxy to the deoxy quaternary structure, there is a greater alteration in the heme pocket of one type of subunits (presumably the beta chain) than that in the other subunit. The present results suggest that there are differences in both tertiary and quaternary structures between carp and human normal adult hemoglobins which could contribute to the great differences in the functional properties between these two proteins.

A proton nuclear magnetic resonance investigation of human hemoglobin A2. Implications on the intermolecular contacts in sickle hemoglobin fibers and on the Bohr effect of human normal adult hemoglobin

High-resolution proton nuclear magnetic resonance spectroscopy at 300 and 600 MHz has been used to investigate the conformation of a minor hemoglobin component of human blood, hemoglobin A2 (alpha 2 delta 2), in solution. We have found that (i) the replacement of the beta chains by the delta chains in hemoglobin A2 conserves the alpha 1 delta 2 interface but slightly perturbs the alpha 1 delta 1 interface, and (ii) one surface histidine residue in the deoxy form and one in the carbonmonoxy form of hemoglobin A2 have local conformations and/or electrostatic environments which are different from the corresponding ones in human normal adult hemoglobin. By comparing the proton nuclear magnetic resonance titration of individual histidine residues in hemoglobin A2 and in human normal adult hemoglobin, we can conclude that in human normal adult hemoglobin, both beta 116 and beta 117 histidine residues are titratable in both the deoxy and the carbonmonoxy forms. Thus, these two histidine residues can contribute to the Bohr effect of human normal adult hemoglobin. The present nuclear magnetic resonance data on hemoglobin A2 and those previously obtained in our laboratory on sickle hemoglobin suggest that the antisickling property of hemoglobin A2 does not originate from an alteration of the intermolecular contact site at the beta 6 position, but involves additional amino-acid residues which are different in the beta and delta chains. We have found that the replacement of the beta 116 and beta 117 histidine residues in the delta chains does not play a significant role in the antisickling effect of hemoglobin A2 and, thus, these amino-acid residues do not participate in the intermolecular interactions responsible for the polymerization of sickle hemoglobin.

Effects of anions on the ligand-linked subunit assembly of human hemoglobin: the mutual effects of Cl- and EDTA

We have studied the mutual effects of chloride ion and EDTA on the dimer-tetramer assembly of human deoxyhemoglobin and oxyhemoglobin. It is found that these two anions have similar but interdependent effects. In low C1- (.01 M) increasing concentrations of EDTA are found to decrease both forward and reverse rate constants for deoxyhemoglobin, whereas no effect is observed at 0.1 M C1-. These results suggest that binding of anions at the alpha 1 beta 2 intersubunit contact may stabilize both the dimeric and tetrameric forms of the deoxy molecule, thus inhibiting both the dissociation and reassociation reactions. The overall effects of EDTA and low C1- on the dimer-tetramer equilibrium constants are found to be distinctly different in deoxy and oxyhemoglobins with a major effect on the oxy form. These findings establish validity of the results from previous thermodynamic studies carried out in approximately physiological C1- concentrations along with the small amounts of EDTA which are used to minimize artifacts of oxidation. As observed for deoxyhemoglobin, it is found that in 0.1 M C1- ion there is no further effect of EDTA on the oxyhemoglobin dimer-tetramer equilibrium.

Functional studies of two new abnormal hemoglobins with their mutation located at intersubunit contacts: Hb Hotel Dieu beta99 (G1) Asp replaced by Gly and Hb Pitie Salpetriere beta34 (B16) Val replaced by Phe

The functional properties of two new abnormal hemoglobins with high oxygen affinity were studied. Hb Hôtel Dieu beta99 (G1) Asp replaced by Gly is situated in the alpha1beta2 contact. Hb Pitié Salpétrière beta34 (B16) Val replaced by Phe is situated in the alpha1beta1 contact. Both hemoglobins exhibited similar functional properties with a 10-fold increased oxygen affinity, a decreased cooperativity, a decreased Bohr effect and a normal or slightly decreased 2,3-diphosphoglycerate effect. The structure-function relationship is discussed in the light of these results.

Functional abnormalities of hemoglobin Toyoake (142 (H20)beta, Ala leads to Pro)

Oxygen equilibrium of Hb Toyoake (142 (H20)beta, Ala leads to Pro) is characterized by an oxygen affinity 6-times higher than that of Hb A, a slightly decreased alkaline Bohr effect, diminished cooperativity, with Hill's coefficient decreased by 1.2, and reduced response to 2,3-diphosphoglycerate and inositol hexaphosphate. These properties are in qualitative agreement with those shown previously from oxygen equilibrium data for hemolysate containing Hb Toyoake. The heat of oxygenation was -13.5 kcal/mol for Hb Toyoake and -12.9 kcal/mol for Hb A at pH 7.4 in 0.1 M Cl- and they became equal when corrected for the heat of oxygen-linked proton and Cl- release. OxyHb Toyoake autooxidized faster than oxyHb A. The visible absorption spectrum and electron paramagnetic resonance spectrum of oxidized Hb Toyake indicated that oxidation of this hemoglobin, either by autooxidation or by K3Fe(CN)6, is followed by gradual conversion into hemichrome derivatives. The soret peak of deoxyHb Toyoake was lowered compared to that of deoxyHb A and the magnitude of narrow-banded oxy-minus-deoxy difference spectrum around 290 nm was smaller for Hb Toyoake than for Hb A, indicating that the former remains predominantly in the R state upon deoxygenation. The functional abnormalities, including tendency to lose heme groups previously reported, were interpreted in terms of structural disturbance by proline at 142beta of 141 leucine, 143 histidine, 145 tyrosine, and 146 histidine residues of the same beta chain.

Molecular stability of Hb Philly (alpha 2 beta 2 35(Cl) Tyr leads to Phe). Rhe relationship of hemoglobin stability to ligand state as defined by heat and mechanical shaking tests

The molecular stability of Hb Philly (alpha 2 beta 2 35(Cl) Tyr leads to Phe) with different ligand states was compared with that of Hb A and Hb S using mechanical shaking and heat stability tests. The rates of mechanical denaturation of the oxy-forms of these hemoglobins decreased in the order of Hb S, Hb Philly, and Hb A, with relative ratios of 9.5: 5.6: 1.0. Upon oxidation to the met-forms, Hb Philly became mechanically most unstable, with ratios of 13.3: 23.0: 1.8, respectively. The deoxy-forms, of Hb A and Hb S were very stable, while that of Hb Philly was as unstable as the oxy-form. The addition of inositol hexaphosphate (IHP) to deoxy-Hb Philly stabilized the molecules. Since IHP restores the cooperative oxygen binding of Hp Philly, deoxy-Hb Philly appears to combine with IHP to change the quaternary structure required for cooperative oxygen binding and for stabilization of the molecule.

Oxygen affinity and stability of hemoglobin Dunn alpha 6(A4)Asp replaced by Asn): use of isoelectric focusing in recognition of a new abnormal hemoglobin

A new slow-moving hemoglobin was found in low proportion in an asymptomatic black woman. Isoelectric focusing helped to distinguish it from other hemoglobins with similar electrophoretic mobility, and amino acid analysis showed that aspartic acid alpha 6 (A4) had been replaced by asparagine. Oxygen affinity was increased, but the Bohr and DPG effects were normal. Stability of the purified hemoglobin was decreased, but that of hemolysates was normal. Abnormal oxygen affinity of this variant, and that of hemoglobin Sawara (alpha 6(A4)Asp replaced by Ala), may reflect loss of a salt bridge between Asp alpha 6 and Lys-alpha 127(H10) which would tend to favor the high-affinity R conformation of the molecule.

Comparative effects of CO2 on the affinity for O2 of fetal and adult erythrocytes

1. Oxygen-linked carbamino formation in fetal erythrocytes was compared to that measured in adult erythrocytes. 2. Whole oxygen binding curves were recorded on washed intact erythrocytes either fresh or D-glycerate-2,3-P depleted with a continuous recording technique. Erythrocytes were resuspended in buffer media of different pH and PCO2 varying from 0-10.7 kPa (80 torr) at physiological ionic strength. Oxygen linked carbamates were estimated as deltalog PO2/delta log PCO2 at constant pH and constant saturation levels from 10-90% oxygen saturation. 3. The overall CO2 effect (deltalog P50/deltalog PCO2) was consistently lower in fetal erythrocytes than in the adult. The deltalog PO2/deltalog PCO2 ratio was markedly dependent on oxygen saturation in both types of erythrocytes and highest at the early part of the oxygen binding curve. This was more so in fetal erythrocytes. 4. Carbamino formation was lower in fetal erythrocytes than in adult erythrocytes at any pH value, indicating a higher apparent pK of the alpha amino groups involved in CO2 binding in fetal erythrocytes. This may be related to the different primary structures of the non alpha chains of HbFII and HbAI. 5. The large effect of low PCO2 on both fetal and adult erythrocytes was related to the higher affinity for CO2 of deoxyhemoglobin compared to oxyhemoglobin and a model for CO2 binding analogous to that described by de Bruin et al. [6] for anion binding is proposed. 6. It is concluded that the lower CO2 binding to fetal erythrocytes is in keeping with the lower allosteric effect of other major effectors of hemoglobin within the cells. This leads to a higher affinity for O2 of fetal erythrocytes well suited for O2 transport in utero.

Proton nuclear magnetic resonance studies of hemoglobins Osler (beta145HC2 Tyr replaced by Asp) and McKee Rocks (beta145HC2 Tyr replaced by term): an assignment for an important tertiary structural probe in hemoglobin

High-resolution proton nuclear magnetic resonance studies of deoxyhemoglobins Osler (beta145HC2 Tyr replaced by Asp) and McKees Rocks (beta 145HC2 Tyr replaced by term) indicate that these hemoglobins are predominately in the oxy quaternary structure in 0.1 M [bis(2-hydroxyethyl)imino]-tris(hydroxymethyl) methane buffer at pH 7. Upon the addition of inositol hexaphosphate, the proton nuclear magnetic resonance spectra of these hemoglobins become similar to those characteristic of a hemoglobin molecule in the deoxy quaternary structure. The exchangeable proton resonance which is found at -6.4 ppm from H2O in the spectrum of normal human adult deoxyhemoglobin is absent in the spectra of these two mutant hemoglobins. Consequently we believe the hydrogen bond between the hydroxyl group of tyrosine-beta145HC2 and the carboxyl oxygen of valine-beta98FG5 gives rise to this resonance. This assignment allows us to use the -6.4ppm resonance as an important tertiary structural probe in the investigation of the cooperative oxygenation of hemoglobin.

Proton nuclear magnetic resonance studies of hemoglobin Providence (beta82EF6 Lys replaced by Asn or Asp): a residue involved in anion binding

High-resolution proton nuclear magnetic resonance studies of hemoglobins Providence-Asn (beta82EF6 Lys replaced by Asn) and Providence-Asp (beta82EF6 Lys replaced by Asp) show that different amino acid substitutions at the same position in the hemoglobin molecule have different effects on the structure of the protein molecule. Hemoglobin Providence-Asp appears to be in a low-affinity tertiary structure in both the deoxy and carbonmonoxy forms. Deoxyhemoglobin Providence-Asn has its beta heme resonance shifted downfield slightly from its position in normal adult hemoglobin; however, the tertiary structures of the heme pocket of hemoglobins A and Providence-Asn are very similar when both proteins are in the carbonmonoxy form. These results are consistent with the oxygen equilibrium measurements of Bonaventura, J., et al. [(1976) J. Biol. Chem. 251, 7563] which show that both Hb Providence-Asn and Hb Providence-Asp have oxygen affinities lower than normal adult hemoglobin, with Hb Providence-Asp having the lowest. Our studies of the effects of sodium chloride on the hyperfine shifted proton resonances of deoxyhemoglobins A, Providence-Asn, and Providence-Asp indicate that the beta82EF6 lysine is probably one, but not the only binding site for chloride ions.