X-ray diffraction study of di and tetra-ligated T-state hemoglobin from high salt crystals

Modulating hemoglobin allostery for treatment of sickle cell disease: current progress and intellectual property

INTRODUCTION

Sickle cell disease (SCD) is a debilitating inherited disorder that affects millions worldwide. Four novel SCD therapeutics have been approved, including the hemoglobin (Hb) modulator Voxelotor.

AREAS COVERED

This review provides an overview of discovery efforts toward modulating Hb allosteric behavior as a treatment for SCD, with a focus on aromatic aldehydes that increase Hb oxygen affinity to prevent the primary pathophysiology of hypoxia-induce erythrocyte sickling.

EXPERT OPINION

The quest to develop small molecules, especially aromatic aldehydes, to modulate Hb allosteric properties for SCD began in the 1970s; however, early promise was dogged by concerns that stalled support for research efforts. Persistent efforts eventually culminated in the discovery of the anti-sickling agent 5-HMF in the 2000s, and reinvigorated interest that led to the discovery of vanillin analogs, including Voxelotor, the first FDA approved Hb modulator for the treatment of SCD. With burgeoning interest in the field of Hb modulation, there is a growing landscape of intellectual property, including drug candidates at various stages of preclinical and clinical investigations. Hb modulators could provide not only the best chance for a highly effective oral therapy for SCD, especially in the under-developed world, but also a way to treat a variety of other human conditions.

Hemoglobin: Structure, Function and Allostery

This chapter reviews how allosteric (heterotrophic) effectors and natural mutations impact hemoglobin (Hb) primary physiological function of oxygen binding and transport. First, an introduction about the structure of Hb is provided, including the ensemble of tense and relaxed Hb states and the dynamic equilibrium of Hb multistate. This is followed by a brief review of Hb variants with altered Hb structure and oxygen binding properties. Finally, a review of different endogenous and exogenous allosteric effectors of Hb is presented with particular emphasis on the atomic interactions of synthetic ligands with altered allosteric function of Hb that could potentially be harnessed for the treatment of diseases.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Therapeutic strategies to alter the oxygen affinity of sickle hemoglobin

The pathophysiology of sickle cell disease involves the polymerization of sickle hemoglobin in its T state, which develops under low oxygen saturation. One therapeutic strategy is to develop pharmacologic agents to stabilize the R state of hemoglobin, which has higher oxygen affinity and is expected to have slower kinetics of polymerization, potentially delaying the sickling of red cells during circulation. This strategy has stimulated the investigation of aromatic aldehydes, aspirin derivatives, thiols, and isothiocyanates that can stabilize the R state of hemoglobin in vitro. One representative aromatic aldehyde agent, 5-hydoxymethyl-2-furfural, protects sickle cell mice from the effects of hypoxia.

Structure of fully liganded Hb ζ2β2s trapped in a tense conformation

A variant Hb ζ2β2(s) that is formed from sickle hemoglobin (Hb S; α2β2(s)) by exchanging adult α-globin with embryonic ζ-globin subunits shows promise as a therapeutic agent for sickle-cell disease (SCD). Hb ζ2β2(s) inhibits the polymerization of deoxygenated Hb S in vitro and reverses characteristic features of SCD in vivo in mouse models of the disorder. When compared with either Hb S or with normal human adult Hb A (α2β2), Hb ζ2β2(s) exhibits atypical properties that include a high oxygen affinity, reduced cooperativity, a weak Bohr effect and blunted 2,3-diphosphoglycerate allostery. Here, the 1.95 Å resolution crystal structure of human Hb ζ2β2(s) that was expressed in complex transgenic knockout mice and purified from their erythrocytes is presented. When fully liganded with carbon monoxide, Hb ζ2β2(s) displays a central water cavity, a ζ1-β(s)2 (or ζ2-β(s)1) interface, intersubunit salt-bridge/hydrogen-bond interactions, C-terminal βHis146 salt-bridge interactions, and a β-cleft, that are highly unusual for a relaxed hemoglobin structure and are more typical of a tense conformation. These quaternary tense-like features contrast with the tertiary relaxed-like conformations of the ζ1β(s)1 dimer and the CD and FG corners, as well as the overall structures of the heme cavities. This crystallographic study provides insights into the altered oxygen-transport properties of Hb ζ2β2(s) and, moreover, decouples tertiary- and quaternary-structural events that are critical to Hb ligand binding and allosteric function.

Hemoglobin-ligand binding: understanding Hb function and allostery on atomic level

The major physiological function of hemoglobin (Hb) is to bind oxygen in the lungs and deliver it to the tissues. This function is regulated and/or made efficient by endogenous heterotropic effectors. A number of synthetic molecules also bind to Hb to alter its allosteric activity. Our purpose is to review the current state of Hb structure and function that involves ensemble of tense and relaxed hemoglobin states and the dynamic equilibrium of the multistate due to the binding of endogenous heterotropic or synthetic allosteric effectors. The review also discusses the atomic interactions of synthetic ligands with the function or altered allosteric function of Hb that could be potentially harnessed for the treatment of diseases. This article is part of a Special Issue entitled: Protein Structure and Function in the Crystalline State.

The effects of Efaproxyn (efaproxiral) on subcutaneous RIF-1 tumor oxygenation and enhancement of radiotherapy-mediated inhibition of tumor growth in mice

Efaproxiral, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity, facilitating oxygen release from hemoglobin, which is likely to increase tissue pO(2). The purpose of this study was to determine the effect of efaproxiral on tumor oxygenation and growth inhibition of RIF-1 tumors that received X radiation (4 Gy) plus oxygen breathing compared to radiation plus oxygen plus efaproxiral daily for 5 days. Two lithium phthalocyanine (LiPc) deposits were implanted in RIF-1 tumors in C3H mice for tumor pO(2) measurements using EPR oximetry. Efaproxiral significantly increased tumor oxygenation by 8.4 to 43.4 mmHg within 5 days, with maximum increases at 22-31 min after treatment. Oxygen breathing alone did not affect tumor pO(2). Radiation plus oxygen plus efaproxiral produced tumor growth inhibition throughout the treatment duration, and inhibition was significantly different from radiation plus oxygen from day 3 to day 5. The results of this study provide unambiguous quantitative information on the effectiveness of efaproxiral to consistently and reproducibly increase tumor oxygenation over the course of 5 days of treatment, modeling the clinical use of efaproxiral. Also, based on the tumor growth inhibition, the study shows the efaproxiral-enhanced tumor oxygenation was radiobiologically significant. This is the first study to demonstrate the ability of efaproxiral to increase tumor oxygenation and to increase the tumor growth inhibition of radiotherapy over 5 days of treatment.

Increased oxygenation of intracranial tumors by efaproxyn (efaproxiral), an allosteric hemoglobin modifier: In vivo EPR oximetry study

PURPOSE

To determine quantitatively the changes in oxygenation of intracranial tumors induced by efaproxiral, an allosteric hemoglobin modifier. Efaproxiral reduces hemoglobin-oxygen binding affinity, which facilitates oxygen release from hemoglobin into surrounding tissues and potentially increases the pO(2) of the tumors.

METHODS AND MATERIALS

The study was performed on 10 male Fisher 344 rats with 9L intracranial tumors. Electron paramagnetic resonance (EPR) oximetry was used to measure quantitatively the changes in the pO(2) in the tumors. Lithium phthalocyanine (LiPc) crystals were implanted in the tumors and in the normal brain tissue in the opposite hemispheres. We monitored the cerebral pO(2) starting 7 to 10 days after the tumor cells were implanted. NMR imaging determined the position and size of tumor in the brain. After an initial baseline EPR measurement, efaproxiral (150 mg/kg) was injected intravenously over 15 minutes, and measurements of tumor and normal brain oxygen tension were made alternately at 10-minute intervals for the next 60 minutes; the procedure was repeated for 6 consecutive days.

RESULTS

Efaproxiral significantly increased the pO(2) of both the intracranial tumors and the normal brain tissue on all days. The maximum increase was reached at 52.9 to 59.7 minutes and 54.1 to 63.2 minutes after injection, respectively. The pO(2) returned to baseline values at 106 to 126.5 minutes after treatment. The maximum tumor and normal tissue pO(2) values achieved after efaproxiral treatment from Day 1 through Day 6 ranged from 139.7 to 197.7 mm Hg and 103.0 to 135.9 mm Hg, respectively. The maximum increase in tumor tissue pO(2) values from Day 2 to Day 5 was greater than the maximum increase in normal tissue pO(2).

CONCLUSION

We obtained quantitative data on the timing and extent of efaproxiral-induced changes in the pO(2) of intracerebral 9L tumors. These results illustrate a unique and useful capability of in vivo EPR oximetry to obtain repeated noninvasive measurements of tumor oxygenation over a number of days. The information on the dynamics of tumor pO(2) after efaproxiral administration illustrates the ability of efaproxiral to increase intracranial tumor oxygenation.

Effect of RSR13, an allosteric hemoglobin modifier, on oxygenation in murine tumors: an in vivo electron paramagnetic resonance oximetry and bold MRI study

PURPOSE

RSR13, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity facilitating oxygen release from hemoglobin, resulting in increases in tissue pO(2). The purpose of this study was noninvasively to monitor the time course and effect of RSR13 on tumor oxygenation, directly using in vivo electron paramagnetic resonance (EPR oximetry), and indirectly using blood oxygen level dependent magnetic resonance imaging (BOLD MRI).

METHODS AND MATERIALS

The study was performed in transplanted radiation-induced fibrosarcoma tumors (RIF-1) in 18 female C3H/HEJ mice, which had two lithium phthalocyanine (LiPc) deposits implanted in the tumor when the tumors reached about 200-600 mm(3). Baseline EPR measurements were made daily for 3 days. Then, for 6 consecutive days and after an initial baseline EPR measurement, RSR13 (150 mg/kg) or vehicle (same volume) was injected intraperitoneally, and measurements of intratumoral oxygen were made at 10-min intervals for the next 60 min. In each mouse, every third day, instead of EPR oximetry, BOLD MRI measurements were made for 60 min after administration of the RSR13.

RESULTS

Based on EPR measurements, RSR13 produced statistically significant temporal increases in tumor pO(2) over the 60-min time course, which reached a maximum at 35-43 min postdose. The average time required to return to the baseline pO(2) was 70-85 min. The maximum increase in tumor tissue pO(2) values after RSR13 treatment from Day 1 to Day 5 (8.3-12.4 mm Hg) was greater than the maximum tumor tissue pO(2) value for Day 6 (4.7 mm Hg, p < 0.01). The maximum increase in pO(2) occurred on Day 2 (12.4 mm Hg) after RSR13 treatment. There was little change in R(2)*, indicating that the RSR13 had minimal detectable effects on total deoxyhemoglobin and hemoglobin-oxygen saturation.

CONCLUSION

The extent of the increase in tumor pO(2) achieved by RSR13 would be expected to lead to a significant increase in the effectiveness of tumor radiotherapy. The lack of a change in the BOLD MRI signal suggests that the tumor physiology was largely unchanged by RSR13. These results illustrate a unique and useful capability of in vivo EPR oximetry and BOLD MRI to obtain repeated measurements of tumor oxygenation and physiology. The dynamics of tumor pO(2) after RSR13 administration may be useful for the design of clinical protocols using allosteric hemoglobin effectors.

RSR13 plus cranial radiation therapy in patients with brain metastases: comparison with the Radiation Therapy Oncology Group Recursive Partitioning Analysis Brain Metastases Database

PURPOSE

This phase II, open-label, multicenter study assessed the efficacy and safety of the potential radiation enhancer RSR13 plus cranial radiation therapy (RT) in patients with brain metastases. The primary end point was patient survival in comparison with the Radiation Therapy Oncology Group Recursive Partitioning Analysis Brain Metastases Database (RTOG RPA BMD).

PATIENTS AND METHODS

Eligibility criteria were age > or = 18 years, Karnofsky performance score > or = 70, and brain metastases with solid tumor histology. Patients received cranial RT, 30 Gy in 10 fractions of 3 Gy each, preceded by RSR13, 50 to 100 mg/kg intravenously over 30 minutes. Univariate and multivariate comparisons of survival and cause of death were made between class II study patients and RTOG BMD patients.

RESULTS

Fifty-seven RPA class II patients were enrolled. With a minimum follow-up of 24 months, the median survival time and 1- and 2-year survival rates were 6.4 months, 23%, and 11% for the RSR13-treated patients compared with 4.1 months, 15%, and 3% for the RTOG BMD patients (P =.0174). In an exact-matched case analysis (n = 38), median survival time for RSR13 patients was 7.3 months versus 3.4 months for the RTOG BMD patients (P =.006). There was a 54% reduction in the risk of death for RSR13 patients (P =.0267). RSR13-related adverse events of greater than or equal to grade 3 toxicity that occurred in more than one patient included hypoxia, headache, anemia, fatigue, hypertension, and intracranial hypertension.

CONCLUSION

RSR13 plus cranial RT resulted in a significant improvement in survival, as well as a reduction in death due to brain metastases, compared with class II patients in the RTOG BMD.

High and low oxygen affinity conformations of T state hemoglobin

To understand the interplay between tertiary and quaternary transitions associated with hemoglobin function and regulation, oxygen binding curves were obtained for hemoglobin A fixed in the T quaternary state by encapsulation in wet porous silica gels. At pH 7.0 and 15 degrees C, the oxygen pressure at half saturation (p50) was measured to be 12.4 +/- 0.2 and 139 +/- 4 torr for hemoglobin gels prepared in the absence and presence of the strong allosteric effectors inositol hexaphosphate and bezafibrate, respectively. Both values are in excellent agreement with those found for the binding of the first oxygen to hemoglobin in solution under similar experimental conditions. The corresponding Hill coefficients of hemoglobin gels were 0.94 +/- 0.02 and 0.93 +/- 0.03, indicating, in the frame of the Monod, Wyman, and Changeux model, that high and low oxygen-affinity tertiary T-state conformations have been isolated in a pure form. The values, slightly lower than unity, reflect the different oxygen affinity of alpha- and beta-hemes. Significantly, hemoglobin encapsulated in the presence of the weak effector phosphate led to gels that show intermediate oxygen affinity and Hill coefficients of 0.7 to 0.8. The heterogeneous oxygen binding results from the presence of a mixture of the high and low oxygen-affinity T states. The Bohr effect was measured for hemoglobin gels containing the pure conformations and found to be more pronounced for the high-affinity T state and almost absent for the low-affinity T state. These findings indicate that the functional properties of the T quaternary state result from the contribution of two distinct, interconverting conformations, characterized by a 10-fold difference in oxygen affinity and a different extent of tertiary Bohr effect. The very small degree of T-state cooperativity observed in solution and in the crystalline state might arise from a ligand-induced perturbation of the distribution between the high- and low-affinity T-state conformations.

The X-ray structure determination of bovine carbonmonoxy hemoglobin at 2.1 A resoultion and its relationship to the quaternary structures of other hemoglobin crystal froms

Crystallographic studies of the intermediate states between unliganded and fully liganded hemoglobin (Hb) have revealed a large range of subtle but functionally important structural differences. Only one T state has been reported, whereas three other quaternary states (the R state, B state, and R2 or Y state) for liganded Hb have been characterized; other studies have defined liganded Hbs that are intermediate between the T and R states. The high-salt crystal structure of bovine carbonmonoxy (CO bovine) Hb has been determined at a resolution of 2.1 A and is described here. A detailed comparison with other crystallographically solved Hb forms (T, R, R2 or Y) shows that the quaternary structure of CO bovine Hb closely resembles R state Hb. However, our analysis of these structures has identified several important differences between CO bovine Hb and R state Hb. Compared with the R state structures, the beta-subunit N-terminal region has shifted closer to the central water cavity in CO bovine Hb. In addition, both the alpha- and beta-subunits in CO bovine Hb have more constrained heme environments that appear to be intermediate between the T and R states. Moreover, the distal pocket of the beta-subunit heme in CO bovine Hb shows significantly closer interaction between the bound CO ligand and the Hb distal residues Val 63(E11) and His 63(E7). The constrained heme groups and the increased steric contact involving the CO ligand and the distal heme residues relative to human Hb may explain in part the low intrinsic oxygen affinity of bovine Hb.

High-resolution crystal structure of deoxy hemoglobin complexed with a potent allosteric effector

The crystal structure of human deoxy hemoglobin (Hb) complexed with a potent allosteric effector (2-[4-[[(3,5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid) = RSR-13) is reported at 1.85 A resolution. Analysis of the hemoglobin:effector complex indicates that two of these molecules bind to the central water cavity of deoxy Hb in a symmetrical fashion, and that each constrains the protein by engaging in hydrogen bonding and hydrophobic interactions with three of its four subunits. Interestingly, we also find that water-mediated interactions between the bound effectors and the protein make significant contributions to the overall binding. Physiologically, the interaction of RSR-13 with Hb results in increased oxygen delivery to peripheral tissues. Thus, this compound has potential therapeutic application in the treatment of hypoxia, ischemia, and trauma-related blood loss. Currently, RSR-13 is in phase III clinical trials as a radiosensitizing agent in the treatment of brain tumors. A detailed structural analysis of this compound complexed with deoxy Hb has important implications for the rational design of future analogs.

Oxygen binding by alpha(Fe2+)2beta(Ni2+)2 hemoglobin crystals

Oxygen binding by hemoglobin fixed in the T state either by crystallization or by encapsulation in silica gels is apparently noncooperative. However, cooperativity might be masked by different oxygen affinities of alpha and beta subunits. Metal hybrid hemoglobins, where the noniron metal does not bind oxygen, provide the opportunity to determine the oxygen affinities of alpha and beta hemes separately. Previous studies have characterized the oxygen binding by alpha(Ni2+)2beta(Fe2+)2 crystals. Here, we have determined the three-dimensional (3D) structure and oxygen binding of alpha(Fe2+)2beta(Ni2+)2 crystals grown from polyethylene glycol solutions. Polarized absorption spectra were recorded at different oxygen pressures with light polarized parallel either to the b or c crystal axis by single crystal microspectrophotometry. The oxygen pressures at 50% saturation (p50s) are 95 +/- 3 and 87 +/- 4 Torr along the b and c crystal axes, respectively, and the corresponding Hill coefficients are 0.96 +/- 0.06 and 0.90 +/- 0.03. Analysis of the binding curves, taking into account the different projections of the alpha hemes along the optical directions, indicates that the oxygen affinity of alpha1 hemes is 1.3-fold lower than alpha2 hemes. Inspection of the 3D structure suggests that this inequivalence may arise from packing interactions of the Hb tetramer within the monoclinic crystal lattice. A similar inequivalence was found for the beta subunits of alpha(Ni2+)2beta(Fe2+)2 crystals. The average oxygen affinity of the alpha subunits (p50 = 91 Torr) is about 1.2-fold higher than the beta subunits (p50 = 110 Torr). In the absence of cooperativity, this heterogeneity yields an oxygen binding curve of Hb A with a Hill coefficient of 0.999. Since the binding curves of Hb A crystals exhibit a Hill coefficient very close to unity, these findings indicate that oxygen binding by T-state hemoglobin is noncooperative, in keeping with the Monod, Wyman, and Changeux model.

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).

Heterotropic effectors exert more significant strain on monoligated than on unligated hemoglobin

The effect of allosteric effectors, such as inositol hexakisphosphate and/or bezafibrate, has been investigated on the unliganded human adult hemoglobin both spectroscopically (employing electronic absorption, circular dichroism, resonance Raman, and x-ray absorption near-edge spectroscopies) and functionally (following the kinetics of the first CO binding step up to a final 4% ligand saturation degree). All data indicate that the unliganded T-state is not perturbed by the interaction with either one or both effectors, suggesting that their functional influence is only exerted when a ligand molecule is bound to the heme. This is confirmed by the observation that CO dissociation from partially liganded hemoglobin ( </= 0.04) is strongly altered by the presence of either effector, and the effect is enhanced whenever the two effectors are simultaneously present. Altogether, these data are a direct demonstration of the occurrence of a strain induced by the presence of a ligand molecule bound to the heme, and for the first time there is a clear indication that the expression of the functional heterotropic effect by these non-heme ligands requires this strain, which is not present in the unliganded molecule.

RSR13, an allosteric effector of haemoglobin, and carbogen radiosensitize FSAII and SCCVII tumours in C3H mice

Pre-clinical evaluation has demonstrated that 2-[4-(((3,5-dimethylanilino)carbonyl)methyl)phenoxy]-2-methylpropi onic acid (RSR13) acts as an allosteric effector of haemoglobin (Hb). RSR13 binding to Hb results in decreased haemoglobin-oxygen (Hb-O2) affinity, improved tumour oxygenation, and enhanced radiation-induced cell killing in several experimental tumour systems. In the present work, ex vivo clonogenic survival analyses are applied in two murine tumour systems to characterize the relationship between the magnitude of decrease in Hb-O2 affinity and radiosensitization, the influence of inspired pO2 upon this effect, and the efficacy of combining RSR13 and radiation during a course of repeated radiation exposures. For FSaII tumours in C3H mice breathing air, 100 mg kg(-1) RSR13 administered intraperitoneally produced an enhancement ratio (ER) of 1.3, but there was marked desensitization at a RSR13 dose of 300 mg kg(-1) (ER 0.6). The most likely reason for the increased radioresistance was insufficient oxygen loading of Hb in the pulmonary circulation due to reduced haemoglobin-oxygen affinity because carbogen breathing combined with 300 mg kg(-1) RSR13 reversed the effect and produced an ER of 1.8. In SCCVII tumours in C3H mice irradiated with eight fractions of 2.5 Gy over 4 days, the surviving fraction was reduced to 58-67% of control values when RSR13 was combined with radiation on days 1 and 2, days 3 and 4, or days 1-4. These results confirm that combining RSR13 and irradiation within a fractionated course of clinically relevant low-dose exposures provides significant radiosensitization. Additional preclinical experimentation is needed to define better the optimum dose-scheduling conditions for clinical applications.

Novel water-mediated hydrogen bonds as the structural basis for the low oxygen affinity of the blood substitute candidate rHb(alpha 96Val-->Trp)

One of the most promising approaches for the development of a synthetic blood substitute has been the engineering of novel mutants of human hemoglobin (Hb) A which maintain cooperativity, but possess lowered oxygen affinity. We describe here two crystal structures of one such potential blood substitute, recombinant (r) Hb(alpha 96Val-->Trp), refined to 1.9 A resolution in an alpha-aquomet, beta-deoxy T-state, and to 2.5 A resolution in a carbonmonoxy R-state. On the basis of molecular dynamics simulations, a particular conformation had been predicted for the engineered Trp residue, and the lowered oxygen affinity had been attributed to a stabilization of the deoxy T-state interface by alpha 96Trp-beta 99Asp hydrogen bonds. Difference Fourier maps of the T-state structure clearly show that alpha 96Trp is in a conformation different from that predicted by the simulation, with its indole side chain directed away from the interface and into the central cavity. In this conformation, the indole nitrogen makes novel water-mediated hydrogen bonds across the T-state interface with beta 101Glu. We propose that these water-mediated hydrogen bonds are the structural basis for the lowered oxygen affinity of rHb(alpha 96Val-->Trp), and discuss the implications of these findings for future molecular dynamics studies and the design of Hb mutants.

Conformation of the sebacyl beta1Lys82-beta2Lys82 crosslink in T-state human hemoglobin

The crystal structure of human T state hemoglobin crosslinked with bis(3,5-dibromo-salicyl) sebacate has been determined at 1.9 A resolution. The final crystallographic R factor is 0.168 with root-mean-square deviations (RMSD) from ideal bond distance of 0.018 A. The 10-carbon sebacyl residue found in the beta cleft covalently links the two betaLys82 residues. The sebacyl residue assumes a zigzag conformation with cis amide bonds formed by the NZ atoms of betaLys82's and the sebacyl carbonyl oxygens. The atoms of the crosslink have an occupancy factor of 1.0 with an average temperature factor for all atoms of 34 A2. An RMSD of 0.27 for all CA's of the tetramer is observed when the crosslinked deoxyhemoglobin is compared with deoxyhemoglobin refined by using a similar protocol, 2HHD [Fronticelli et al. J. Biol. Chem. 269: 23965-23969, 1994]. Thus, no significant perturbations in the tertiary or quaternary structure are introduced by the presence of the sebacyl residue. However, the sebacyl residue does displace seven water molecules in the beta cleft and the conformations of the beta1Lys82 and beta2Lys82 are altered because of the crosslinking. The carbonyl oxygen that is part of the amide bond formed with the NZ of beta2Lys82 forms a hydrogen bond with side chain of beta2Asn139 that is in turn hydrogen-bonded to the side chain of beta2Arg104. A comparison of the observed conformation with that modeled [Bucci et al. Biochemistry 35:3418-3425, 1996] shows significant differences. The differences in the structures can be rationalized in terms of compensating changes in the estimated free-energy balance, based on differences in exposed surface areas and the observed shift in the side-chain hydrogen-bonding pattern involving beta2Arg104, beta2Asn139, and the associated sebacyl carbonyl group.

Alpha-subunit oxidation in T-state crystals of a sebacyl cross-linked human hemoglobin with unusual autoxidation properties

In the crystal structure of human T-state hemoglobin with a sebacyl residue cross-linking the two beta-subunit Lys82,s (DecHb), the Fe atoms of the alpha-subunit hemes are found to be oxidized with a water molecule bound. The three-dimensional structure and heme geometries were compared to those of deoxyhemoglobin and other partially and fully oxidized hemoglobins [R. Liddington, Z. Derewenda, E. Dodson, R. Hubbard, G. Dodson, J. Mol. Biol. 228 (1992) 551]. The heme geometries of the alpha-subunits are consistent with those observed in oxidized structures. The proximal histidines of the alpha-subunits move toward the heme plane shifting the F-helix and FG-corner in a manner observed for partially oxidized human hemoglobin. This supports the hypothesis that these perturbations may precede the T- to R-state transition. Circular dichroism studies comparing DecHb and natural human hemoglobin in the deoxy and CO ligated forms confirm that the conformations of the deoxy forms are identical, but the ligated forms have slight differences in the solution structures. DecHb is found to be more resistant to autoxidation than natural hemoglobin. The time course of autoxidation of DecHb shows that it is virtually absent for the first 1500 min followed by a rapid increase. Thus, the discovery of the oxidation of the alpha-subunits in the deoxy-crystals is quite unexpected. The data confirm that ligation of the alpha-subunits precedes that of the beta-subunits. This may suggest a low ligand affinity of the alpha-diligated form of hemoglobin.

Structure and oxygen affinity of crystalline des-his-146beta human hemoglobin in the T state

To correlate directly structure with function, the oxygen affinity and the three-dimensional structure of crystals of the T quaternary state of des-His-146beta human hemoglobin have been determined by polarized absorption microspectrophotometry and x-ray diffraction crystallography. In des-His-146beta, the COOH-terminal histidine residues of the beta chains of hemoglobin A have been removed. Oxygen binding to crystalline des-His hemoglobin is non-cooperative and independent of pH. The oxygen affinity is 1.7-fold greater than that of the crystalline state of hemoglobin A. Removal of His-146beta results in a small movement of the truncated COOH-terminal peptide and in a very small change in quaternary structure. Previously, similar studies on T state crystals of des-Arg-141alpha hemoglobin showed that removal of the COOH termini of the alpha chains results in much larger effects on oxygen affinity and on quaternary structure. Kinetic studies in solution reveal that at pH 7.0, the rates of CO combination with deoxygenated des-His-146beta in the absence and presence of inositol hexaphosphate are 2.5- and 1.3-fold, respectively, more rapid than for hemoglobin A. The values for des-Arg are 7.6- and 3.9-fold. The properties of the T state of hemoglobin both in the crystal and in solution are influenced to a greater degree by the interactions associated with Arg-141alpha than those associated with His-146beta.

T state hemoglobin binds oxygen noncooperatively with allosteric effects of protons, inositol hexaphosphate, and chloride

Hemoglobin is the paradigm of allosteric proteins. Over the years, cooperative oxygen binding has been explained by different models predicting that the T state of hemoglobin binds oxygen either noncooperatively or with some degree of cooperativity or with strong cooperativity. Therefore, a critical test that discriminates among models is to determine the oxygen binding by the T state of hemoglobin. Fixation of hemoglobin in the T state has been achieved either by crystallization from polyethylene glycol solutions or by encapsulation in wet porous silica gels. Hemoglobin crystals bind oxygen noncooperatively with reduced affinity compared with solution, with no Bohr effect and with no influence of other allosteric effectors. In this study, we have determined accurate oxygen-binding curves to the T state of hemoglobin in silica gels with the same microspectrophotometric apparatus and multiwavelengths analysis used in crystal experiments. The T state of hemoglobin in silica gels binds oxygen noncooperatively with an affinity and a Bohr effect similar to those observed in solution for the binding of the first oxygen molecule. Other allosteric effectors such as inositol hexaphosphate, bezafibrate, and chloride significantly affect oxygen affinity. Therefore, T state hemoglobins that are characterized by strikingly different functional properties share the absence of cooperativity in the binding of oxygen. These findings are fully consistent with the Monod, Wyman, and Changeux model and with most features of Perutz's stereochemical model, but they are not consistent with models of both Koshland and Ackers.

Quaternary structure sensitive tyrosine interactions in hemoglobin: a UV resonance Raman study of the double mutant rHb (beta99Asp-->Asn, alpha42Tyr-->Asp)

Two interactions involving tyrosines have been implicated in the communication pathway that links ligand binding to quaternary state changes in hemoglobin. Tyr alpha(1)42 stabilizes the alpha1beta2 T state interface through the formation of a hydrogen bond to Asp beta(2)99. The side chains of the penultimate Tyr residues (alpha140 and beta145) occupy the pockets made by helicies F and H in the deoxy form with the phenolic hydroxyl hydrogen bonded to the carbonyl group of Val FG5. Early crystallographic studies indicated that in the R form the penultimate Tyr is expelled out of the pocket, thus eliminating the hydrogen bond. This hydrogen bond has been considered to play an important role in maintaining the low-oxygen-affinity state (T state) in deoxy HbA, but a later higher resolution crystallographic study (Shannon, 1983) failed to reveal such movement of this Tyr during the R --> T transition. Nevertheless, conversion of this Tyr to Phe increases oxygen affinity considerably, suggesting that hydrogen bonding is involved in oxygen affinity modulation. Earlier ultraviolet resonance Raman results reported by Spiro and co-workers [Rodgers et al. (1992) J. Am. Chem. Soc. 114, 3697-3709] were used to conclude that the significant quaternary structure dependent changes observed in tyrosine Raman bands are due to the formation of the T state hydrogen bond with Tyr alpha42 acting as a proton acceptor rather than being the anticipated proton donor, as would be expected if Asp beta99 were ionized. This surprising result rests on the assumption that changes in the environment of Tyr alpha42 are the overwhelming contributor to the R - T UV Raman difference spectrum. In this study, a cooperative double mutant lacking Tyr alpha42, [rHb (Asp beta99 --> Asn, Tyr alpha42 --> Asp)], is used to determine the relative contributions of Tyr alpha42 and the penultimate tyrosines to the R - T UV resonance Raman difference spectrum. The results both directly support the claim that Tyr alpha42 is the proton acceptor in the T state and expose the potential role of the penultimate tyrosines in coupling the quaternary state to the ligand reactivity.

Positive and negative cooperativities at subsequent steps of oxygenation regulate the allosteric behavior of multistate sebacylhemoglobin

Cross-linked human hemoglobin (HbA) is obtained by reaction with bis(3,5-dibromosalicyl) sebacate. Peptide maps and crystallographic analyses confirm the presence of the 10 carbon atom long sebacyl residue cross-linking the two beta82 lysines of the beta-cleft (DecHb). The Adair's constants, obtained from the oxygen binding isotherms, show that at the first step of oxygenation normal hemoglobin and DecHb have a very similar oxygen affinity. In DecHb negative binding cooperativity is present at the second step of oxygenation, which has an affinity 27 times lower than at the first step. Positive cooperativity is present at the third binding step, whose affinity is 380 times that of the second step. The fourth binding step shows a weak negative cooperativity with an affinity one-half that of the third step. Crystals of deoxy-DecHb diffracted to 1.9 angstroms resolution. The resulting atomic coordinates are very similar to those of Fermi et al. [(1984) J. Mol.Biol. 175, 159-174] and Fronticelli et al. [(1994) J. Biol Chem. 269, 23965-23969] for deoxy-HbA. The electron density map of deoxy-DecHb indicates the presence of the 10 carbon bridge between the beta82 lysines. Molecular modeling confirms that insertion of the linker into the T structure requires only slight displacement of the two beta82 lysines. Instead, insertion of the linker into the R and R2 structures [Shaanan (1983) J. Mol. Biol. 171, 31-59; Silva et al. (1992) J. Biol. Chem. 267, 17248-17256] is hindered by serious sterical restrictions. The linker primarily affects the partially and fully liganded states of hemoglobin. The data suggest in DecHb concerted conformational changes at each step of oxygenation.

Effect of chloride on oxygen binding to crystals of hemoglobin Rothschild (beta 37 Trp-->Arg) in the T quaternary structure

Oxygen binding to crystals of hemoglobin Rothschild (beta 37 Trp-->Arg) in the T quaternary structure has been investigated by polarized absorption microspectrophotometry. These crystals were grown from poly(ethylene glycol) solutions containing low concentrations of salt. In the absence of chloride, they have a significantly higher oxygen affinity than crystals of human hemoglobin A grown in a similar manner, and exhibit Hill coefficients lower than 1. There is no Bohr effect from pH 6 to 9. We have found that chloride decreases the oxygen affinity of Hb Rothschild crystals, an effect which is absent in crystals of HbA. This dependence of affinity on chloride is almost certainly associated with the chloride binding sites which have been localized crystallographically at the mutant arginine residues (Kavanaugh et al., 1992). Since chloride binding appears to lower the oxygen affinities of both the alpha and beta chains, the linkage between the binding of oxygen and the dissociation of chloride results in significant cooperativity in oxygen binding to the crystals.

Oxygen binding by single crystals of hemoglobin

Reversible oxygen binding curves for single crystals of hemoglobin in the T quaternary structure have been measured using microspectrophotometry. Saturations were determined from complete visible spectra measured with light linearly polarized parallel to the a and c crystal axes. Striking differences were observed between the binding properties of hemoglobin in the crystal and those of hemoglobin in solution. Oxygen binding to the crystal is effectively noncooperative, the Bohr effect is absent, and there is no effect of chloride ion. Also, the oxygen affinity is lower than that of the T quaternary structure in solution. The absence of the Bohr effect supports Perutz's hypothesis on the key role of the salt bridges, which are known from X-ray crystallography to remain intact upon oxygenation. The low affinity and absence of the Bohr effect can be explained by a generalization of the MWC-PSK model (Monod, Wyman, & Changeux, 1965; Perutz, 1970; Szabo & Karplus, 1972) in which both high- and low-affinity tertiary conformations, with broken and unbroken salt bridges, respectively, are populated in the T quaternary structure. Because the alpha and beta hemes make different projections onto the two crystal axes, separate binding curves for the alpha and beta subunits could be calculated from the two measured binding curves. The approximately 5-fold difference between the oxygen affinities of the alpha and beta subunits is much smaller than that predicted from the crystallographic study of Dodson, Liddington, and co-workers, which suggested that oxygen binds only to the alpha hemes.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric modifiers of hemoglobin: 2-[4-[[(3,5-disubstituted anilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid derivatives that lower the oxygen affinity of hemoglobin in red cell suspensions, in whole blood, and in vivo in rats

Two new potent allosteric effectors of hemoglobin, RSR-4 [2-[4-[[(3,5-dichloroanilino)carbonyl]-methyl]phenoxy]-2- methylpropionic acid] and RSR-13 [2-[4-[[(3,5-dimethlanilino)carbonyl]methyl]-phenoxy]-2-methylp rop ionic, are compared to the previously reported compounds L3,5 and L3,4,5 [Lalezari, I., Lalezari, P., Poyart, C., Marden, M., Kister, J., Bohn, B., Fermi, G., & Perutz, M. F. (1990) Biochemistry 29, 1515]. Unlike L3,5 and L3,4,5, RSR-4 and RSR-13 are less impeded by physiological concentrations of serum albumin. RSR-4 has also been shown to be more effective than L3,5 in shifting the allosteric equilibrium of bovine Hb toward the low-affinity T-state. X-ray crystal studies show that both RSR-4 and RSR-13 bind to only one pair of symmetry-related sites in the Hb central water cavity whereas previous studies on L3,5 and L3,4,5 demonstrated a second pair of symmetry-related binding sites near Arg 104 beta. Three major interactions between these allosteric effectors and Hb include the acid group with the guanidinium group of C-terminal Arg 141 alpha, the effector's amide oxygen with the ammonium ion of Lys 99 alpha, and the phi electrons of the halogenated or methylated aromatic ring and Asn 108 beta. No explanation has been found for the difference in number of binding sites observed for RSR-4 and RSR-13 (two sites) compared to L3,5 and L3,4,5 (four sites); also no correlation has been made between the number of binding sites and degree of allosteric shift in the oxygen equilibrium curve.(ABSTRACT TRUNCATED AT 250 WORDS)