The effect of potassium chloride on the Bohr effect of human hemoglobin

Hemoglobin oxygen affinity in patients with cystic fibrosis

In patients with cystic fibrosis lung damages cause arterial hypoxia. As a typical compensatory reaction one might expect changes in oxygen affinity of hemoglobin. Therefore position (standard half saturation pressure P₅₀st) and slope (Hill’s n) of the O₂ dissociation curve as well as the Bohr coefficients (BC) for CO₂ and lactic acid were determined in blood of 14 adult patients (8 males, 6 females) and 14 healthy controls (6 males, 8 females). While Hill’s n amounted to approximately 2.6 in all subjects, P₅₀st was slightly increased by 1mmHg in both patient groups (controls male 26.7±0.2, controls female 27.0±0.1, patients male 27.7±0.5, patients female 28.0±0.3 mmHg; mean and standard error, overall p<0.01). Main cause was a rise of 1–2 µmol/g hemoglobin in erythrocytic 2,3-biphosphoglycerate concentration. One patient only, clearly identified as an outlier and with the mutation G551D, showed a reduction of both P₅₀st (24.5 mmHg) and [2,3-biphosphoglycerate] (9.8 µmol/g hemoglobin). There were no differences in BCCO₂, but small sex differences in the BC for lactic acid in the controls which were not detectable in the patients. Causes for the right shift of the O₂ dissociation curve might be hypoxic stimulation of erythrocytic glycolysis and an increased red cell turnover both causing increased [2,3-biphosphoglycerate]. However, for situations with additional hypercapnia as observed in exercising patients a left shift seems to be a more favourable adaptation in cystic fibrosis. Additionally when in vivo PO₂ values were corrected to the standard conditions they mostly lay left of the in vitro O₂ dissociation curve in both patients and controls. This hints to unknown fugitive factors influencing oxygen affinity.

Lack of conventional oxygen-linked proton and anion binding sites does not impair allosteric regulation of oxygen binding in dwarf caiman hemoglobin

In contrast to other vertebrate hemoglobins (Hbs) whose high intrinsic O2 affinities are reduced by red cell allosteric effectors (mainly protons, CO2, organic phosphates, and chloride ions), crocodilian Hbs exhibit low sensitivity to organic phosphates and high sensitivity to bicarbonate (HCO3(-)), which is believed to augment Hb-O2 unloading during diving and postprandial alkaline tides when blood HCO3(-) levels and metabolic rates increase. Examination of α- and β-globin amino acid sequences of dwarf caiman (Paleosuchus palpebrosus) revealed a unique combination of substitutions at key effector binding sites compared with other vertebrate and crocodilian Hbs: β82Lys→Gln, β143His→Val, and β146His→Tyr. These substitutions delete positive charges and, along with other distinctive changes in residue charge and polarity, may be expected to disrupt allosteric regulation of Hb-O2 affinity. Strikingly, however, P. palpebrosus Hb shows a strong Bohr effect, and marked deoxygenation-linked binding of organic phosphates (ATP and DPG) and CO2 as carbamate (contrasting with HCO3(-) binding in other crocodilians). Unlike other Hbs, it polymerizes to large complexes in the oxygenated state. The highly unusual properties of P. palpebrosus Hb align with a high content of His residues (potential sites for oxygenation-linked proton binding) and distinctive surface Cys residues that may form intermolecular disulfide bridges upon polymerization. On the basis of its singular properties, P. palpebrosus Hb provides a unique opportunity for studies on structure-function coupling and the evolution of compensatory mechanisms for maintaining tissue O2 delivery in Hbs that lack conventional effector-binding residues.

Investigation of the chloride effect on hemoglobin by adsorptive transfer voltammetry

A strategy of ex situ electrochemical method has been proposed for investigating the chloride effect on hemoglobin (Hb). Unlike the common electrochemical method that measures the chloride effect on Hb in bulk solution (in situ), the effects of chloride anion on Hb were investigated ex situ by adsorptive transfer voltammetry (AdTV) in this work. Gold electrode modified by self-assembled monolayer of 3-mercaptopropanoic acid (AuE/MPA) was prepared and then incubated in a series of Hb solutions containing different concentrations of chloride anion for adsorbing Hb-Cl (AuE/MPA/Hb-Cl). The resulting electrode was then measured in phosphate buffer solution by cyclic voltammetry. The corresponding voltammograms showed obvious promotion of the direct electron transfer of Hb with remarkable increase of peak currents, decrease of peak-to-peak separations, and negative shift of the formal potentials. As complementation, the adsorption behavior of Hb-Cl on AuE/MPA, the structural information of Hb-Cl, and the electrocatalytic ability of AuE/MPA/Hb-Cl toward hydrogen peroxide were investigated by surface plasmon resonance, circular dichroism spectrum, ultraviolet-visible spectrum and amperometry, respectively. The results indicate that the chloride effect resulted in more electroactive sites of Hb on the surface of electrode. Meanwhile, the specific and nonspecific interactions between Hb and chloride anion can be discriminated from the electrochemical parameters obtained by AdTV.

Modern cross-linking strategies for synthesizing acellular hemoglobin-based oxygen carriers

Unmodified cell-free hemoglobin (Hb) is structurally unstable when transfused into the blood stream (Valeri et al., Artif Cells Blood Substit Immobil Biotechnol. 2000;28:451-475; Chan et al., Toxicol Pathol. 2000;28:635-642; Eike, Dissertation, 2005; Eike and Palmer, Biotechnol Prog. 2004;20:946-952). This review examines some of the latest chemical strategies used over the last 5 years to intra- and intermolecularly cross-link Hb, thereby stabilizing its quaternary structure. Therefore, this work will address the following aspects: (1) site-specific chemical modifications of Hb and (2) non-site-specific chemical modifications of Hb, including, but not limited to, PolyHeme, Hemopure, Oxyglobin, and SOD-Hb. Current strategies for synthesizing PEGylated Hb is outside the scope of this review and will not be discussed herein. For a more thorough review of PEGylated Hb, the reader is directed to the following works: Cabrales and Friedman, Transfus Alternatives in Transfus Med. 2007;9:281-293 and Winslow, Biochim Biophys Acta, 2008;1784(10):1382-1386.

Transition of hemoglobin between two tertiary conformations: determination of equilibrium and thermodynamic parameters from the reaction of 5,5'-dithiobis(2-nitrobenzoate) with the CysF9[93]beta sulfhydryl group

The equilibrium constant of the reaction of 5,5'-dithiobis(2-nitrobenzoate) with the CysF9[93]beta sulfhydryl group of hemoglobin decreases by 2 to 3 orders of magnitude between pH 5.6 and 9. The reaction is coupled to the ionizations of two groups on the protein. At 25 degrees C one group has a pK(a) of 5.31+/-0.2 when hemoglobin is in its (tertiary) r conformation, typified by the thiolate anion form of CysF9[93]beta; this changes to 7.73+/-0.4 in the (tertiary) t conformation, typified by the mixed disulfide form of the sulfhydryl. The second group ionizes with a pK(a) of 7.11+/-0.4 in the r conformation; this changes to 8.38+/-0.2 in the t conformation. K(rt), the equilibrium constant for the r<-->t isomerization process, is 0.22+/-0.06. The standard enthalpy and entropy changes for the isomerization are DeltaH(o)(rt)=24.2 kJ mol(-1) and DeltaS(o)(rt)=68.8 JK(-1)mol(-1), respectively.

Evolution of vertebrate haemoglobins: Histidine side chains, specific buffer value and Bohr effect

This review highlights the use of analytical tools, recently developed in the comparative method of evolutionary biology, for the study of haemoglobin (Hb) adaptation. It focuses on the functional consequences of a previously largely ignored structural feature of Hb, namely the degree and positional specificity of histidine (His) substitution in Hb chains. The importance of His side chains for hydrogen ion buffering, blood CO(2) transport capacity and the molecular mechanism of the Bohr effect in vertebrate Hbs is discussed. Using phylogenetically independent contrasts, a significant correlation between the specific buffer value of Hb and the number of predicted physiological buffer groups from Hb sequence data is shown. In a new result, the evolution of the number of physiological buffer groups in 77 vertebrate species is reconstructed on a phylogenetic tree. The analysis predicts that teleost fishes, passeriform birds and some snakes have independently evolved a much-reduced specific buffer value of Hb, possibly for enhancing the efficiency of an acid load to change oxygen affinity via the Bohr effect. This analysis demonstrates how in comparative physiology analysis of genetic databases in an evolutionary framework can identify candidate species for further experimental in vitro and whole animal studies.

New insights into the proton-dependent oxygen affinity of Root effect haemoglobins

A long-standing puzzle with regard to protein structure/function relationships is the proton-dependent modification of haemoglobin (Hb) structure that causes oxygen to be unloaded from Root effect Hbs into the swim bladders and eyes of fish even against high oxygen pressure gradients. Although oxygen unloading in Root effect Hbs has generally been attributed to proton-dependent stabilization of the T-state, protonation of Root effect Hbs can alter their ligand affinities in both R- and T-state conformations and either stabilize the T-state or destabilize the R-state. The C-terminal residues that are so important in the Bohr effect of human Hb appear to be involved in the Root effects of some fish Hbs and not in others, indicating that several evolutionary pathways have resulted in expression of highly pH-dependent Hbs. New data are presented that show surprising similarities in the pH- and anion-dependence of sulfhydryl group reactivity and anaerobic oxidation of human and fish Hbs. The available evidence supports the concept that in both Bohr effect and Root effect Hbs a large steric component acts in addition to quaternary shifts between R and T conformations to regulate ligand affinity. Allosteric effectors moderate these steric effects within both R- and T-state conformations and allow for an elegant match between Hb function and the wide-ranging physiological needs of diverse organisms.

Calculation of physiological acid-base parameters in multicompartment systems with application to human blood

A general formalism for calculating parameters describing physiological acid-base balance in single compartments is extended to multicompartment systems and demonstrated for the multicompartment example of human whole blood. Expressions for total titratable base, strong ion difference, change in total titratable base, change in strong ion difference, and change in Van Slyke standard bicarbonate are derived, giving calculated values in agreement with experimental data. The equations for multicompartment systems are found to have the same mathematical interrelationships as those for single compartments, and the relationship of the present formalism to the traditional form of the Van Slyke equation is also demonstrated. The multicompartment model brings the strong ion difference theory to the same quantitative level as the base excess method.

Allosteric effects of chloride ions at the intradimeric alpha1beta1 and alpha2beta2 interfaces of human hemoglobin

The structural transition induced by ligand binding in human hemoglobin encompasses quaternary structure changes at the interfaces between the two alphabeta dimers. In contrast, the interfaces between alpha and beta subunits within the same dimer (i.e., alpha1beta1 and alpha2beta2 interfaces) are structurally invariant. Previous work from this laboratory using NMR spectroscopy has identified four sites at the intradimeric alpha1beta1 and alpha2beta2 interfaces that, although structurally invariant, experience significant changes in the rates of proton exchange upon ligand binding. These sites are Hisalpha103(G10) and Hisalpha122(H5) in each alpha subunit of the hemoglobin tetramer. In the present work, we show that the proton exchange at the Hisalpha103(G10) sites is affected by the interactions of hemoglobin with chloride ions. Increasing concentrations of chloride ions at pH 6.45 and at 37 degrees C enhance the exchange rate of the Hisalpha103(G10) N(epsilon 2) proton. The enhancement is greater in deoxygenated than in ligated hemoglobin. In the framework of the local unfolding model for proton exchange, these results suggest that the structural free energy and/or the proton transfer reactions at the Hisalpha103(G10) sites depend on the concentration of chloride ions. Therefore, the ligand-induced changes at the Hisalpha103(G10) sites are modulated by the allosteric effect of chloride ions on hemoglobin.

Distinct domain responses of R-state human hemoglobins A, C, and S to anions

Anionic regulation of hemoglobin (Hb) is of increasing interest for the design of Hb-based oxygen carriers. Even "external" amino-acid substitutions can alter the nature and extent of anionic control. This was shown by evaluation of the anion sensitivities of liganded, R-state, forms of HbA, HbC (beta6 Glu --> Lys) and HbS (beta6 Glu --> Val). The beta6 mutants differ in the anion-sensitivity of their central cavities, alpha1beta2 interfaces, and heme and beta93 Cys environments. The mutant Hbs also exhibit increased anion-dependent oxidation and surface denaturation. Moreover, differential chloride effects on oxygen binding by Hbs C, S compared to HbA occur after R-state stabilization by fluoresceination of beta93 Cys. It is concluded that the "external" substitutions in the mutant Hbs have structural consequences that are propagated to varying extents to other domains as a result of anion binding, and that these anion-dependent changes may underlie mechanisms leading to the observed increase in oxidation propensity and surface denaturation.

Concentration-dependent effects of anions on the anaerobic oxidation of hemoglobin and myoglobin

The redox potentials of hemoglobin and myoglobin and the shapes of their anaerobic oxidation curves are sensitive indicators of globin alterations surrounding the active site. This report documents concentration-dependent effects of anions on the ease of anaerobic oxidation of representative hemoglobins and myoglobins. Hemoglobin (Hb) oxidation curves reflect the cooperative transition from the T state of deoxyHb to the more readily oxidized R-like conformation of metHb. Shifts in the oxidation curves for Hb A(0) as Cl(-) concentrations are increased to 0.2 m at pH 7.1 indicate preferential anion binding to the T state and destabilization of the R-like state of metHb, leading to reduced cooperativity in the oxidation process. A dramatic reversal of trend occurs above 0.2 m Cl(-) as anions bind to lower affinity sites and shift the conformational equilibrium toward the R state. This pattern has been observed for various hemoglobins with a variety of small anions. Steric rather than electronic effects are invoked to explain the fact that no comparable reversal of oxygen affinity is observed under identical conditions. Evidence is presented to show that increases in hydrophilicity in the distal heme pocket can decrease oxygen affinity via steric hindrance effects while increasing the ease of anaerobic oxidation.

Isohemoglobin differentiation in the bimodal-breathing amazon catfish Hoplosternum littorale

The bimodal gill(water)/gut(air)-breathing Amazonian catfish Hoplosternum littorale that frequents hypoxic habitats uses "mammalian" 2,3-diphosphoglycerate (DPG) in addition to "piscine" ATP and GTP as erythrocytic O(2) affinity modulators. Its electrophoretically distinct anodic and cathodic hemoglobins (Hb(An) and Hb(Ca)) were isolated for functional and molecular characterization. In contrast to Hb(An), phosphate-free Hb(Ca) exhibits a pronounced reverse Bohr effect (increased O(2) affinity with decreasing pH) that is obliterated by ATP, and opposite pH dependences of K(T) (O(2) association constant of low affinity, tense state) and the overall heat of oxygenation. Dose-response curves indicate small chloride effects and pronounced and differentiated phosphate effects, DPG < ATP < GTP < IHP. Hb(Ca)-O(2) equilibria analyzed in terms of the Monod-Wyman-Changeux model show that small T state bond energy differences underlie the differentiated phosphate effects. Synthetic peptides, corresponding to N-terminal fragment of the cytoplasmic domain of trout band 3 protein, undergo oxygenation-linked binding to Hb(Ca), suggesting a metabolic regulatory role for this hemoglobin. The amino acid sequences for the alpha and beta chains of Hb(Ca) obtained by Edman degradation and cDNA sequencing show unusual substitutions at the phosphate-binding site that are discussed in terms of its reverse Bohr effect and anion sensitivities.

Functional characterisation of Eskimo dog hemoglobin: II. The interplay of HCO(3)- and Cl-

Hemoglobin (Hb) from the Eskimo dog (belonging to Canis lupus familiaris) showed similar Bohr effect (delta log P50/delta pH) to human HbA in the presence of 100 mmol l-1 NaCl at 20 degrees C. The presence of 7% carbon dioxide in the desalted condition caused a positive (reversed) Bohr effect in the pH range 7.1-7.5 on Eskimo dog Hb, whereas in human HbA there was no Bohr effect within this pH range. A positive Bohr effect on Eskimo dog Hb in this condition was also observed at 37 degrees C. This could indicate differences in the pK values of the amino terminal residues of the two hemoglobins, with possible pH-dependent binding of both bicarbonate (HCO(3)-) and carbamate. Analysis of the effect of CO2 on oxygen affinity of Eskimo dog Hb in the pH range 6.7-7.6 in the presence of chloride and/or 2,3-diphosphoglycerate (2,3-DPG) support this theory. Our results indicate a competition between HCO(3)- and Cl- in affecting oxygen binding. Thermodynamic analysis reveals that bicarbonate binding lowers the apparent heat of oxygenation in Eskimo dog Hb nearly as much as chloride does in the presence of 2,3-DPG at physiological pH. This safeguards an effective oxygen unloading at lowered red blood cell concentrations of chloride. Moreover, we show that the oxygen affinity at high O2 saturation is less dependent on temperature in the presence than in the absence of CO2-.

Contribution of surface histidyl residues in the alpha-chain to the Bohr effect of human normal adult hemoglobin: roles of global electrostatic effects

We have applied site-directed mutagenesis to our Escherichia coli hemoglobin expression plasmid and constructed five recombinant mutant hemoglobins (r Hbs): r Hb(alpha20His-->Gln or alpha:H20Q); r Hb(alpha:H50Q); r Hb(alpha:H72Q); r Hb(alpha:H89Q); and r Hb(alpha:H112Q). We have constructed these r Hbs to help us assess the contribution of the surface histidyl residues in the alpha-chain to the alkaline Bohr effect of human normal adult hemoglobin (Hb A). In our laboratory, we have monitored the variation of proton nuclear magnetic resonances arising from the C2 protons of the histidyl residues of Hb A as a function of pH and buffer conditions. Several of these resonances have been assigned to the individual histidyl residues on the surface of the hemoglobin molecule using naturally occurring mutant hemoglobins and chemically modified hemoglobins. In the present work, we have identified the C2 proton resonances of five surface histidyl residues of the alpha-chain, alpha20, alpha50, alpha72, alpha89, and alpha112, in both the carbonmonoxy and deoxy forms, by comparing the proton nuclear magnetic resonance spectra of Hb A with those of the r Hbs. For the assignment of the C2 proton resonances of alpha20His and alpha112His, we have used combinations of mutations to compensate for the spectral perturbations resulting from the single mutations, which obscure the resonance assignment. On the basis of the new findings, in solvent containing 0.1 M chloride, the overall contributions from surface histidyl residues of both the alpha- and beta-chain and from other previously identified alkaline Bohr groups account for approximately 75% of the observed Bohr effect at pH 7.3 (the maximum Bohr effect under the prescribed solvent conditions). Our results show that some histidyl residues contribute to the Bohr effect and some oppose the net Bohr effect. In some cases, the addition of anions can diminish or reverse the contributions of specific histidyl residues to the overall Bohr effect. Thus, the Bohr effect, a heterotropic effect, depends on the intricate arrangement and interactions of all hydrogen and anion binding sites in the hemoglobin molecule. It is an excellent example of global electrostatic effects in proteins.

Effects of NaCl on the linkages between O2 binding and subunit assembly in human hemoglobin: titration of the quaternary enhancement effect

Oxygen binding by human hemoglobin (Hb) and the coupled reactions of dimer-tetramer assembly were studied over a range of NaCl concentrations (from 0.08 M to 1.4 M) at pH 7.4 and 21.5 degrees C. A strategy of multi-dimensional analysis was employed [G.K. Ackers and H.R. Halvorson, Proc. Natl. Acad. Sci. U.S.A., 91, (1974) 4312] to optimize the resolution of the contributions to cooperativity and their heterotropic salt linkages at each stoichiometric degree of O2 binding. A wide range of Hb concentration was utilized at each [NaCl] in which O2-linked subunit assembly reactions contributed significantly to the positions and shapes of the binding isotherms. Kinetic determinations yielded forward and reverse rate constants for assembly of the unligated species. Amplitudes for the assembly rate data had concentration dependences in agreement with the independently determined dimer-tetramer assembly constants of oxyhemoglobin. Concentration-dependent binding isotherms were analyzed, in combination with the kinetically determined equilibrium constants, to yield salt-linked components of cooperativity at the four stages of oxygenation. The principal results of this study were as follows. (i) Assembly of fully oxygenated Hb tetramers is opposed by NaCl: the dimer-to-tetramer equilibrium constant becomes two orders of magnitude less favorable over the [NaCl] range 0.08 M to 1.4 M. By contrast, for deoxy-Hb the assembly equilibrium constant is reduced only two-fold. (ii) Oxygen binding to dimers is non-cooperative over the entire salt range, whereas dimer affinity is slightly favored by increasing the NaCl concentration. (iii) Overall affinity of tetramers for O2 is opposed by NaCl, becoming an order of magnitude less favorable over the range employed. Most of this decrease occurs at the fourth binding step, which shows a large, salt-mediated quaternary enhancement effect; i.e., the assembly of dimers into tetramers at 0.08 M NaCl is accompanied by an eight-fold increase in O2 affinity. (iv) The quaternary enhancement effect at the last O2-binding step is titrated progressively by salt until it reaches a negligible value near the highest [NaCl] of this study. The lowest [NaCl] condition (0.08 M) elicits the greatest tetramer cooperativity with the largest maximal Hill coefficient and the greatest suppression of intermediates. Possible origins and mechanistic implications of these phenomena are considered.

Tertiary and quaternary chloride effects of the partially ligated (CN-met) hemoglobin intermediates

Heterotropic effects of NaCl were studied using CN-met hemoglobin, which has been found to follow the same rules of homotropic cooperativity as CO-Hb and O2-Hb [Huang and Ackers, Biochemistry, 35 (1996) 704; Huang et al., Biophys. J., 71 (1996) 2094]. Modulation of heme site cooperativity by NaCl was determined in this study for all partially ligated CN-met intermediates by measuring their dimer-to-tetramer assembly free energies as a function of NaCl concentration (0.08-1.4 M; pH 7.4, T = 21.5 degrees C). Thermodynamic linkage analysis yielded the contributions to heme site binding cooperativity for all 16 reactions of the binding cascade, and also their apparent changes in bound salt. The principal findings were as follows: (i) At each [NaCl] the ten tetrameric species exhibited three discrete cooperative free energy levels; (ii) positional isomers of the doubly ligated tetramers were distributed among two of these levels according to their specific configurations of ligated sites, in conformity with the symmetry rule mechanism of hemoglobin cooperativity [Ackers et al., Science 255 (1992) 54]; (iii) the apparent moles of NaCl release followed the same configuration-specific distribution as that of heme site cooperativity, i.e., this parameter was synchronized according to the same response clusters. The system thus manifests both a "tertiary chloride effect" and a "quaternary chloride effect", which parallel the tertiary and quaternary Bohr effects [Daugherty et al., Biochemistry, 33 (1994) 10345; Perrella et al., Biochemistry, 33 (1994) 10358] and the tertiary and quaternary enthalpy effects [Huang and Ackers, Biochemistry, 34 (1995) 6316]. Comparison with findings on the stoichiometric O2-binding linkages over an identical range of conditions [Doyle et al., Biophys. Chem., 64 (1997)] revealed that the overall NaCl release upon ligating all four hemes is identical for O2 and CN-met, whereas the detailed distributions of apparent chloride release showed variations between the two ligands, i.e., CN-met Hb showed only a negligible quaternary enhancement at all [NaCl] conditions and a larger tertiary chloride effect compared with O2-Hb. Possible origins of these variations are considered.

Evidence of hemoglobin dissociation

Bovine carbonmonoxy hemoglobin investigated with light scattering studies is found to dissociate from its native tetramer structure into dimers and monomers. The values of the hydrated tetramer radius, RT = 32.1 A, and the fractional dissociation vs pH, have been obtained at different ionic strengths from the autocorrelation function of the scattered light. The results suggest that a relevant contribution to Hb dissociation is due to electrostatic effects and, by means of a model derived by Tanford, it has been possible to predict the behavior of dissociation. Among the findings of this approach, we recall the estimates of the electrostatic energy contributions to Hb dissociation, up to congruent to 6RT, and the predicted charge of tetrameric Hb vs pH, which agrees very well with the experimental data.

Random chemical modification of the oxygen-linked chloride-binding sites of hemoglobin: those in the central dyad axis may influence the transition between deoxy- and oxy-hemoglobin

The features of random chemical modification are defined with reference to acetylation of bovine hemoglobin, which has been performed in a random manner so that all of the amino groups that participate in functional chloride binding (i.e., those that are oxygen-linked) could be identified. Random chemical modification, which has objectives different from those of both specific (selective) and extensive chemical modification, has been achieved for bovine hemoglobin with the mild reagent, 14C-methyl acetate phosphate; retention of function was demonstrated by a Hill coefficient of n = 2.2 for the modified hemoglobin. After removal of unmodified Hb chains, the mixture of randomly modified acetylated alpha or beta chains was subjected to tandem treatment with trypsin and chymotrypsin. Peptides were purified by HPLC and identified by amino acid analysis. The amount of radioactivity in the acetylated amino group of a purified peptide was taken as an estimate of the degree of chloride binding. For bovine Hb, two amino groups of the alpha-chain (Val-1 and Lys-99) and three amino groups of the beta-chain (Met-1, Lys-81, and Lys-103) were shown to be oxygen-linked (i.e., to have incorporated significantly more radioactivity in the deoxy conformation compared to the same site in the oxy conformation). Three of these sites were already known chloride-binding sites [i.e., Val-1(alpha), the N-terminus of the alpha-chain, and two sites between the 2 beta-chains of bovine hemoglobin, Met-1(beta) and Lys-81(beta); these findings support the conclusions of the random modification approach. Two other chloride-binding sites, Lys-99(alpha) and Lys-103(beta), align the sides of the central dyad axis connecting the two well-known major chloride-binding sites of bovine Hb. The interrelationship of these five chloride-binding sites was assessed by improved molecular graphics. When viewed through the central dyad axis, the functional chloride-binding sites in the central cavity appear to be symmetrically related and to connect the two major chloride-binding sites. Modifiers or mutants that are directed at these regions in the central dyad axis may favor the deoxy conformation to provide a lower oxygen affinity by preventing the constriction of the central cavity that normally occurs upon oxygenation.

Ionization of clusters. IV. Cooperativity and allosterism derived from shared clusters in macromolecules

Clusters of ionizable groups are examined for conditions that develop cooperativity (1) on the binding of protons, and (2) on the binding of an associated ligand when the clusters are shared between domains or subunits in macromolecules. Cooperative binding isotherms for protons have long been observed (but not emphasized as cooperative binding) when studies have been done on clusters for the evaluation of metal ion complexation [A. E. Martell & M. Calvin (1952) Chemistry of the Metal Chelate Compounds, Prentice-Hall, Englewood Cliffs, New Jersey]. Reactions are formulated in this paper to show that anions, chelating to positively charged clusters, are also capable of developing the cooperative binding of protons. Extension of these simple reactions to those where clusters of ionizable groups are shared between domains of macromolecules provides models for cooperative binding, which include the allosteric, Bohr, anion, and cation effects in proteins.

Effects of anions on the molecular basis of the Bohr effect of hemoglobin

High-resolution 1H-NMR spectroscopy has been used to investigate the molecular basis of the Bohr effect in human normal adult hemoglobin in the presence of anions which serve as heterotropic effectors, i.e., Cl-, Pi, and 2,3-diphosphoglycerate. The individual H+ equilibria of 22-26 histidyl residues of hemoglobin in both deoxy and carbonmonoxy forms have been measured under buffer conditions chosen to demonstrate the effects of anion binding. The results indicate that beta 2His residues are binding sites for Cl- and Pi in both deoxy and carbonmonoxy forms, and that the affinity of this site for these anions is greater in the deoxy form. Recently assigned, the resonance of beta 146His does not show evidence of involvement in anion binding. The results also indicate that the binding of 2,3-diphosphoglycerate at the central cavity between the two beta-chains in deoxyhemoglobin involves the beta 2His residues, and that the 2,3-diphosphoglycerate-binding site in carbonmonoxyhemoglobin may remain similar to that in deoxyhemoglobin. The interactions of Cl-, Pi and 2,3-diphosphoglycerate also result in changes in the pK values for other surface histidyl residues which vary in both magnitude and direction. The array of pK changes is specific for the interaction of each effector. The participation of beta 2His in the Bohr effect demonstrates that this residue can release or capture protons, depending on its protonation properties and its linkage to anion binding, and therefore provides an excellent illustration of the variable roles of a given amino acid. Although beta 146His does not bind anions, its contributions to the Bohr effect are substantially affected by the presence of anions. These results demonstrate that long-range electrostatic and/or conformational effects of anions binding play significant roles in the molecular basis of the Bohr effect of hemoglobin.

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

Chloride plays a key role in activating the photosynethetic oxygen-evolving complex (OEC) of Photosystem II, but the OEC is only one of many enzymes affected by this anion. Some of the mechanistic features of Cl(-) involvement in water-splitting resemble those of other proteins whose structure and chemistry are known in detail. An overview of the similarities and differences between these Cl(-)-binding systems is presented.

Allosteric linkage-induced distortions of the prosthetic group in haem proteins as derived by the theoretical interpretation of the depolarization ratio in resonance Raman scattering

The relationship between functional properties of haem proteins, particularly ligand binding and Bohr effect, and associated variations of the tertiary and quaternary structures is one of the main objectives of haem protein research. In this context one aims to get detailed knowledge of the coupling mechanisms which are involved in the transduction of structural changes from the protein to the functional haem group along distinct pathways.

Functional properties of hemoglobin in human red cells: II. Determination of the Bohr effect

Parameters of the Bohr effect (delta log P50/delta pHi) for human normal red blood cell suspensions have been calculated between intracellular pH (pHi) 5.5 and 8.5, in order to test how these quantities compare with those measured in dilute human hemoglobin solution (Hb). For a precise comparison between red cell and Hb solutions data, the cells were fractionated to reduce cellular heterogeneity and depleted of their 2,3-diphosphoglycerate (DPG) content. The P50 values were related to pHi. From the value of the Donnan equilibrium factor rH+ and the cellular water content at each extracellular pH (pHe), the variations of the intracellular chloride concentration were calculated. This quantity exhibited a three fold change between pHi 5.5 and 8.5, with extracellular chloride concentration [Cl-]e fixed at 140 mM. In the absence of DPG, chloride is the main allosteric effector of Hb and increases the alkaline Bohr effect. When all these interacting factors are accounted for, the oxygen affinity and Bohr parameters for red cell suspensions become identical to those observed for dilute Hb solutions. These results indicate that the hydration of the Hb molecules in the highly concentrated red cell milieu is not much different from that of nearly ideal solutions.

Modulation of oxygen affinity in hemoglobin by solvent components. Interaction of bovine hemoglobin with 2,3-diphosphoglycerate and monatomic anions

In the absence of Cl- in Hepes buffer at pH 7.4, the oxygen affinity of bovine and human hemoglobin is equally sensitive to 2,3-diphosphoglyceric acid. The low oxygen affinity measured for bovine hemoglobin at physiological salt concentration can be explained by the high affinity of Cl- anions for oxygen-linked sites that are absent in human hemoglobin. Bovine hemoglobin can discriminate between the different halogens in the sense that different halide concentrations are necessary to produce the same P50. Competition experiments indicate that the halogens interact with the same oxygen-linked sites. In agreement with the different affinities for halides, the Bohr effect of bovine hemoglobin is larger in the presence of Cl- than in that of Br- and there is good agreement between the number of protons and anions exchanged with the solvent upon oxygenation of bovine hemoglobin.

Properties of carboxymethylated cross-linked hemoglobin A

The selective carboxymethylation of the N-terminal amino groups of hemoglobin A with glyoxylic acid and sodium cyanoborohydride has been studied as a function of the state of ligation of hemoglobin. The N-terminal residues have been established as the primary sites of reaction by peptide mapping of the tryptic digest of each chain and subsequent amino acid analysis of the modified peptides. With oxyhemoglobin, the desired derivatives with a carboxymethyl group at the N-terminal of either or both chains amounted to 55% [Di Donato, A., Fantl, W. J., Acharya, A. S., & Manning, J. M. (1983) J. Biol. Chem. 258, 11890-11895]. In the present study it is shown that with deoxyhemoglobin the amount of the desired derivative is increased to 75%. The oxygen equilibrium curve of hemoglobin A carboxymethylated on its four N-terminal residues [0.5 mM as tetramer in 50 mM [bis(2-hydroxyethyl)amino]tris(hydroxymethyl)methane (Bis-Tris), pH 7.5, 37 degrees C] had a P50 value of 30 mmHg (Hill coefficient n = 2.8, alkaline Bohr value = 0.4) compared to a P50 of 9 mmHg for unmodified hemoglobin under the same conditions (n = 2.5, alkaline Bohr value = 0.5). In carboxymethylated oxyhemoglobin A, cross-linked with the mild agent glycolaldehyde for 3.5 h, there was 85% of Mr 64,000 species and 15% of Mr 128,000 or higher species. For the former, the extent of cross-linking between two subunits was 19%. For the latter, there was 29% of two cross-linked subunits and 13% of three cross-linked subunits. Termination of cross-linking, which may be desirable in some circumstances, can be successfully achieved with isonicotinic acid hydrazide.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of role of beta 146-histidyl and other histidyl residues in the Bohr effect of human normal adult hemoglobin

The contribution of the carboxyl-terminal histidines of the beta chains, beta 146(HC3), to the alkaline Bohr effect of human normal adult hemoglobin has been shown by this laboratory to depend upon the solvent composition. Using high-resolution proton nuclear magnetic resonance spectroscopy, we have found that the pKa value of the beta 146-histidine is 8.0 in the deoxy form, while in the carbonmonoxy form it ranges from 7.1 to 7.85 depending upon the concentration of inorganic phosphate and chloride ions present. These conclusions have been questioned by Perutz and co-workers on the basis of biochemical, structural, and proton nuclear magnetic resonance studies of mutant and enzymatically or chemically modified hemoglobins [Perutz, M. F., Kilmartin, J. V., Nishikura, K., Fogg, J. H., Butler, P. J., & Rollema, H. S. (1980) J. Mol. Biol. 138, 649-670; Kilmartin, J. V., Fogg, J. H., & Perutz, M. F. (1980) Biochemistry 19, 3189-3193; Perutz, M. F., Gronenborn, A. M., Clore, G. M., Fogg, J. H., & Shih, D. T.-b. (1985) J. Mol. Biol. 183, 491-498]. In this work, we use proton nuclear magnetic resonance spectroscopy to assess the effects of structural modifications on the histidyl residues and on the overall conformation of the hemoglobin molecule in solution. The structural perturbations investigated all occur within the tertiary domains around the carboxyl-terminal region of the beta chain as follows: Hb Cowtown (beta 146His----Leu); Hb Wood (beta 97His----Leu); Hb Malmö (beta 97His----Gln); Hb Abruzzo (beta 143His----Arg). Our results demonstrate that the conformational effects of single-site structural modifications upon the conformation and dynamics of hemoglobin depend strongly on their location in the three-dimensional structure of the protein molecule and also on their chemical nature. Furthermore, in normal hemoglobin, the spectral properties of several surface histidyl residues are found to depend, in the ligated state, upon the nature of the ligand. Our present findings do not support the recent spectral assignments proposed by Perutz et al. (1985) for the proton resonances of the beta 146- and beta 97-histidines and their suggestion that the enzymatic removal of the carboxyl-terminal beta 146-histidyl residues induces a conformational equilibrium for the beta 97-histidines in the des-beta 146His hemoglobin molecule in the carbonmonoxy form.

pH-dependent processes in proteins

Recent improvements in the understanding of electrostatic interactions in proteins serve as a focus for the general topic of pH-dependent processes in proteins. The general importance of pH-dependent processes is first set out in terms of hydrogen ion equilibria, stability, ligand interactions, assembly, dynamics, and events in related molecular systems. The development of various theoretical treatments includes various formalisms in addition to the solvent interface model developed by Shire et al. as an extension of the Tanford-Kirkwood treatment. A number of detailed applications of the model are presented and future potentialities are sketched.

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.