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

Temperature independence of the alkaline Bohr effect in pig red cells and pig haemoglobin solutions

The influence of temperature on the oxygen affinity and the alkaline Bohr effect of pig red cells and pig hemoglobin solutions has been compared to that of human adult red cells and human adult hemoglobin. Pig red cells and pig Hb evidence a lower affinity for oxygen in various conditions of pH, temperature and salt concentration, in the presence as well as in the absence of organic phosphates. It has been observed that the alkaline Bohr effect of pig Hb was reduced by 20-25% compared to Hb A0 and independent of changes in temperature, contrary to human Hb A0. Titrations of pig Hb with C1- indicate a lower heterotropic effect of this anion at low concentration of the salt. It is concluded that this may be the origin of the temperature independence of the alkaline Bohr effect in pig Hb. Conversely, the temperature dependence of the alkaline Bohr effect of Hb A0 should be related to the oxygen-linked binding of C1- at the alpha 1-alpha 2 interface.

The apparent buffer value of human blood for CO2 as a function of temperature

The influence of temperature on the acid-base status of normal human deoxygenated whole blood was studied in open systems (variable total CO2 content). (1) When the temperature was raised from 26 degrees C to 42 degrees C, the apparent buffering value of deoxygenated whole blood for CO2 increased by 7% of its value at 26 degrees C; this increase was not statistically significant. (2) Comparing the present data with those obtained previously from oxygenated whole blood in the same temperature range (Castaing & Pocidalo, 1979) indicates that arterial and venous blood have slightly different buffering capacities for CO2 in the 26 to 42 degrees C temperature range. It also suggests that, at physiological SO2 levels (SO2 greater than or equal to 30%), the apparent buffering value of venous blood for CO2 would be increased by at least 10% of its value at 26 degrees C when the temperature is raised to 42 degrees C. (3) It is concluded that pH stability would be reduced upon CO2 uptake within tissues with a high metabolism and therefore a high temperature.

A proton nuclear magnetic resonance investigation of histidyl residues in human normal adult hemoglobin

High-resolution proton nuclear magnetic resonance (NMR) spectroscopy at 250 MHz has been used to titrate 22 individual surface histidyl residues (11 per alpha beta dimer) of human normal adult hemoglobin in both the deoxy and the carbon monoxy forms. The proton resonances of beta 2, beta 143, and beta 146 histidyl residues are assigned by a parallel 1H NMR titration of appropriate mutant and chemically modified hemoglobins. The pK values of the 22 histidyl residues investigated are found to range from 6.35 to 8.07 in the deoxy form and from 6.20 to 7.87 in the carbon monoxy form, in the presence of 0.1 M Bis-Tris or 0.1 M Tris buffer in D2O with chloride ion concentrations varying from 5 to 60 mM at 27 degrees C. Four histidyl residues in the deoxy form and one histidyl residue in the carbon monoxy form are found to have proton nuclear magnetic resonance titration curves that deviate greatly from that predicted by the simple proton dissociation equilibrium of a single ionizable group. The proton nuclear magnetic resonance data are used to ascertain the role of several surface histidyl residues in the Bohr effect of hemoglobin under the above-mentioned experimental conditions. Under these experimental conditions, we have found that (i) the beta 146 histidyl residues do not change their electrostatic environments significantly upon binding of ligand to deoxyhemoglobin and, thus, their contribution to the Bohr effect is negligible, (ii) the beta 2 histidyl residues have a negative contribution to the Bohr effect, and (iii) the total contribution of the 22 histidyl residues investigated here to the Bohr effect is, in magnitude, comparable to the Bohr effect observed experimentally. These results suggest that the molecular mechanism of the Bohr effect proposed by Perutz [Perutz, M.F. (1970) Nature (London) 228, 726-739] is not unique and that the detailed mechanism depends on experimental conditions, such as the solvent composition.

Effect of carbamination on the buffering power of purified human hemoglobin A solutions at two temperatures

The effect of CO2 on hemoglobin buffering power was studied in purified human hemoglobin A solutions, native and specifically carbamoylated on N-terminal valines, at 26 degrees C and 42 degrees C, in the oxy and deoxy states. Titrations were performed by varying Pco2 and by strong acid or base in the absence of CO2. The participation of N-terminal valines to the total buffering power was estimated by subtracting the buffering value measured on carbamoylated hemoglobin solutions from that measured on native hemoglobin solutions. In the absence of CO2 the buffering value of native and modified hemoglobin increased slightly (less than 10%) (a) on going from the deoxy to the oxy state, and (b) on raising the temperature from 26 degrees C to 42 degrees C. In the presence of CO2 the buffering value of Hb increased from 9.1 to 16.6 mol mol Hb4-1 pH-1 and that of HbO2 from 10.1 to 19.6 mol mol Hb4-1 pH-1 when the temperature was raised from 26 degrees C to 42 degrees C. These figures correspond to a rise in the fraction of the total buffering value attributable to N-terminal valines from 11% to 25% for Hb and from 3% to 33% for HbO2. The present results point to a non-specific effect of CO2 within the hemoglobin molecule independent of that of N-terminal valines. This effect nearly doubles the buffering value for CO2 when the temperature is raised, and contributes to pH regulation and CO2 removal in tissues with a high metabolic rate.

Electrostatic contributions to the energetics of dimer-tetramer assembly in human hemoglobin: pH dependence and effect of specifically bound chloride ions

The pH dependence and effects of specifically bound chloride ions on the electrostatic contribution to the energetics of human hemoglobin dimer-tetramer assembly were computed for deoxy- and liganded hemoglobin. In the absence of bound chloride, the electrostatic contribution models the observed contrasting pH dependence of dimer-tetramer assembly for deoxy- and oxyhemoglobin. The effect of specifically bound chloride on the computations depends on the number and placement of the anions. Deoxy assembly shows a greater sensitivity to anion binding, with effects propagating as far as 32 A from the binding site. This sensitivity suggests a mechanism for electronic communication with the heme. At pH 7.4, 24-34% of the experimental value for deoxy and 73-85% for oxy dimer-tetramer assembly stabilization are predicted. Together with the findings of Chu and Ackers [Chu, A. H., & Ackers, G. K. (1981) J. Biol. Chem. 256, 1199] and other recent work, these results suggest that salt bridge formation is not the dominant energetic factor favoring deoxyhemoglobin dimer-tetramer assembly. Results of this work suggest that the marked electrostatic stabilization favoring oxy dimer-tetramer assembly may be a significant contributor to the quaternary enhancement observed in assembly reactions whereas the nonelectrostatic factors favoring deoxy dimer-tetramer assembly may be largely responsible for quaternary constraint.

The effect of pH and ionic strength on the pre-steady-state reaction of cytochrome c and cytochrome aa3

(1) In the pH range between 5.0 and 8.0, the rate constants for the reaction of ferrocytochrome c with both the high- and low-affinity sites on the cytochrome aa3 increased by a factor of approx. 2 per pH unit. (2) The pre-steady-state reaction between ferrocytochrome c and cytochrome aa3 did nt cause a change in the pH of an unbuffered medium. Furthermore, it was found that this reaction and the steady-state reaction are equally fast in H2O and 2H2O. From these results it was concluded that no protons are directly involved in a rate-determining reaction step. (3) Arrhenius plots show that the reaction between ferrocytochrome c and cytochrome aa3 requires a higher enthalpy of activation at temperatures below 20 degrees C (15--16 kcal/mol) as compared to that at higher temperature (9 kcal/mol). We found no effect of ionic strength on the activation enthalpy of the pre-steady-state reaction, nor on that of the steady-state reaction. This suggests that ionic strength does not change the character of these reactions, but merely affects the electrostatic interaction between both cytochromes.

Hemoglobin Cubujuqui (alpha 141 Arg-Ser): functional consequences of the alteration of the C-terminus of the alpha chain of hemoglobin

Hemoglobin Cubujuqui (1) was detected in several members of a Mexican-American family. Structural analysis of this hemoglobin indicated that the carboxyl terminal arginine at position 141 in the alpha chain had been replaced by serine. This residue is critical not only in stabilizing the deoxy or T conformation by electrostatic interactions, but it is also involved in the Bohr effect through its linkage with Val 1 alpha of the opposite alpha chain in the tetramer. The variant exhibits high affinity for oxygen that is associated with destabilization of the deoxy conformation, and reduced cooperativity. The pH and the chloride sensitivity of the variant are also reduced, as compared to Hb A.

The involvement of tertiary conformational changes and the role of the alpha-chain-binding sites on oxygen-linked chloride release from human hemoglobin

Chloride anions, when bound to human hemoglobin, lower the affinity for oxygen and increase the alkaline Bohr effect. These oxygen-linked characteristics are attributed to the preferential binding of Cl- to both alpha- and beta-chains in their deoxy configuration. It is demonstrated that the release of Cl- upon oxygenation is mainly due to tertiary changes, as shown by the effect of the anion on K1, the affinity constant of hemoglobin for oxygen at a very low saturation level (y less than or equal to 1.5%) where the cooperativity is unity. Investigation of the chloride effect on adult hemoglobin specifically carbamylated at the N-terminal valine of the alpha-chains, indicated a large inhibition of the effect of Cl-. The alpha-chain-binding sites appear to be the sites of the greatest affinity for the anion.

Identification of the residues involved in the oxygen-linked chloride-ion binding sites in human deoxyhemoglobin and oxyhemoglobin

The number of Bohr protons released upon oxygenation of human hemoglobin was measured at 25 degrees C and 37 degrees C as a function of the concentration of chloride ions. From the results obtained association constants could be evaluated for the binding of chloride ions to both deoxy and oxyhemoglobin at these two temperatures. Furthermore, pK values could be determined for those protonic groups involved in chloride ion binding to deoxy and oxyhemoglobin. From these data it was inferred that in oxyhemoglobin only imidazole groups are participating in chloride binding, whereas in deoxyhemoglobin the chloride binding site contained the alpha NH2 group of valine-1 alpha. The same conclusions were reached by measuring the pK values of the aminogroups of valine 1 alpha and valine-1 beta at different temperatures and ionic strengths. The pK values were measured by following the rate of reaction of 1-fluoro-2,4-dinitrobenzene with the alpha-NH2 group by spectrophotometric means. We further showed that binding of Cl-, Br- and I- to oxyhemoglobin follows the lyotropic or Hofmeister series, while this effect is much less for deoxyhemoglobin. This result indicates that for the binding of anions to oxyhemoglobin interactions with non-polar groups contribute to the free-energy change of binding.

Role of the beta 146 histidyl residue in the alkaline Bohr effect of hemoglobin

High-resolution proton nuclear magnetic resonance spectroscopy has been used to investigate the effects of inorganic anions, such as phosphate or chloride, on the alkaline Bohr effect of normal human adult hemoglobin. By monitoring the chemical shift of the C2 proton of the beta 146 histidyl residue as a function of pH, we have determined its pK values in both ligated and unligated forms. In the presence of 0.1 M Bis-Tris buffer (with chloride ion concentration ranging from 0.005 to 0.06 M) in D2O at 27 degrees C, the pK value of the beta 146 histidine of deoxyhemoglobin is 7.98 +/- 0.03 and that of (carbon monoxy)hemoglobin is 7.85 +/- 0.03. However, in the presence of 0.2 M phosphate and 0.2 M NaCl in D2O at 27 degrees C, the corresponding pK values are 8.08 and 7.14, as previously reported by this laboratory [Kilmartin, J. V., Breen, J. J., Roberts, G. C. K., & Ho, C. (1973) Proc. Natl. Acad. Sci. U.S.A. 70, 1246-1249]. This large difference in the pK value between the deoxy and carbon monoxy forms in the presence of 0.2 M phosphate and 0.2 M NaCl was interpreted as direct support for (1) the breaking of an intrasubunit salt bridge between beta 146 histidine and beta 94 aspartate when the hemoglobin molecule undergoes the quaternary structural transition as proposed by Perutz [Perutz, M. F. (1970) Nature (London) 228, 726-739] and (2) Perutz's suggestion that the beta 146 histidine is one of the amino acid residues responsible for the alkaline Bohr effect. The absence of a large change in the pK value of the beta 146 histidine in the presence of 0.1 M Bis-Tris buffer implies that (1) the above-mentioned intrasubunit salt bridge is not broken in going from the deoxy to the carbon monoxy form and (2) the beta 146 histidyl residue does not contribute significantly to the alkaline Bohr effect under these conditions. We have also found that in measuring the oxygen affinity of hemoglobin as a function of pH in the presence of 0.1 M Bis-Tris or 0.2 M phosphate plus 0.2 M NaCl (both in D2O), there is no significant difference in the alkaline Bohr effect in these two media. Hence, our results suggest that the detailed molecular mechanism for the Bohr effect depends on the experimental conditions.

Competition in oxygen-linked anion binding to normal and variant human hemoglobins

The two-state model of Monod, Wyman and Changeux has been used extensively in analysis of the functional behavior of human hemoglobin. Within the context of this model, the cooperativity, pH dependence of oxygen binding, and anionic regulation of oxygen affinity are all considered to be due to shifts in the equilibrium between low affinity (T) conformational states and high affinity (R) conformational states. The heterotropic effectors, such as protons, carbon dioxide, inorganic anions, and organic polyphosphates, are considered to be principally active in causing shifts in the allosteric equilibrium constant L. For both simple anions, such as chloride, and strong effectors, such as inositol hexaphosphate, the allosteric equilibrium constant L increases with increasing concentrations of the effector. Detailed examinations of the functional properties of human hemoglobin have revealed in recent years that the two-state model is a beautiful and elegant format for discussing conformational changes within a given condition, but that an infinite number of "T" and "R" states are, in fact, possible. In this paper, the consequences of the occurrence of specific oxygen-linked anion binding sites on the alpha and beta chains of the human hemoglobin tetramer are discussed in terms of the generalized two-state model. In particular, attention is directed to the interesting fact that the allosteric equilibrium constant can be decreased by addition of chloride instead of increased if the ligand binding behavior is analyzed in the presence of inositol hexaphosphate. The structural basis for competitive anionic effects involves the binding sites for anions on both alpha and beta chains. Analysis of the distinct effects of small and large anionic effectors is facilitated through studies of specific human hemoglobin variants and hemoglobins where the C terminal arginines of the alpha chains have been removed by ezymatic digestion.

The binding of lactate and chloride ions to human adult hemoglobin

The effects of sodium lactate (Lact) on the oxygen affinity and the Bohr effect of purified human adult hemoglobin solutions have been compared to the effects of sodium chloride (Cl). Changes in the affinity for oxygen have been estimated from the variations of log(O2)50 with pH and at various salt concentration from 0.005 up to 2.0 mol.l-1. (O2)50 was calculated as alpha.P0.5 where alpha is the solubility coefficient of oxygen in the solutions at various salt concentrations. Variations of log(O2)50 with pH at constant salt concentration and variations of log(O2)50 with anion concentration at constant pH have been studied according to the linked-functions theory (Wyman, 1968). Bohr curves and salt binding curves were calculated from standard iterative curve fitting procedures and various parameters relevant to the effects of salts on hemoglobin function were estimated. It is shown that Lact and Cl increase (O2)50 and the alkaline Bohr effect in a comparable way at low salt concentration. At high concentration the effect of Lact predominated over that of Cl. The amount of oxygen linked Lact was larger than that of Cl. Binding constants for both anions to deoxy and oxy Hb were estimated. Lact and Cl have comparable binding constants to deoxy hemoglobin. By contrast Lact binds to oxy hemoglobin to a lesser extent than Cl. This may account for the differences observed in the effects of Lact and Cl on the function of hemoglobin. The reason for the low affinity of oxy hemoglobin for Lact may be related to steric differences between the two anions.

The binding of chloride ions to ligated and unligated human hemoglobin and its influence on the Bohr effect

The contribution of the interaction of chloride ions with deoxy and oxyhemoglobin to the Bohr effect can be described by a simple binding model. Applying this model to experiment data reveals that at physiological pH and ionic strength about half of the release of Bohr protons is due to a difference in chloride ion binding to deoxy- and oxyhemoglobin. The chloride-independent part of the Bohr effect corresponds with the shift in pK which His-146 beta shows upon oxygenation. The proton absorptioon by hemoglobin observed upon oxygenation below pH 6 is apparently due to a chloride-ion-induced proton uptake, which is larger for oxyhemoglobin than for deoxyhemoglobin. The analysis of the experimental data indicates the existence of only two oxygen-linked chloride ion binding sites in both deoxy and oxyhemoglobin. In deoxyhemoglobin the binding sites most likely consist of Val-1 alpha of one chain and Arg-141 alpha of the partner chain. The sites in oxyhemoglobin consist of groups with a pK value in the neutral pH range; they do not contain lysyl or arginyl residues.

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.

Electrophoretic mobilities and diffusion coefficients of hemoglobin at high pH

Diffusion studies by photon correlation of scattered laser light confirm the dissociation of the tetrameric form of human carboxyhemoglobin to dimers above pH 10 and provide new estimates of the subunit dissociation equilibrium constants in this pH range. Electrophoretic light-scattering experiments under the same conditions reveal that the electrophoretic mobilities of tetramers and dimers are indistinguishable to within instrumental resolution (ca. 7% in these experiments). The data imply an increase of the electrical charge on the dimer of at least 2.8 to 4.4 net negative charges upon dissociation. Mechanisms for the accumulation of negative charge by the dimer upon dissociation of the tetramer are proposed.

Chloride binding and the Bohr effect of human fetal erythrocytes and HbFII solutions

1. We have observed that the alkaline Bohr effect of washed human fetal erythrocytes was larger than in human adult intact red cells, in physiological conditions of pH, PCO2 and temperature. This was also observed independently of the presence of CO2 and of 2,3 diphosphoglycerate (fresh or stored erythrocytes). 2. Experiments performed in purified HbFII and HbA1 solutions and direct titration of protons released upon oxygenation confirmed the larger alkaline Bohr effect of fetal hemoglobin, at physiological ionic strength. 3. At low chloride concentration HbFII solutions had an alkaline Bohr effect identical to that measured in HbA1 solutions. 4. Titration of purified Hb solutions with increasing concentrations of NaCl evidenced a lower O2 linked chloride binding by HbFII and predominantly at acid pH. 5. It is concluded that the larger alkaline Bohr effect of fetal erythrocytes of HbFII solutions is related to a diminished acid Bohr effect, due to the lower affinity of HbFII for chloride anions. 6. The physiological interest of these results for placental O2 transfer (double Bohr effect) and O2 delivery to the foetus is discussed.

Thermodynamic analysis of ion effects on the binding and conformational equilibria of proteins and nucleic acids: the roles of ion association or release, screening, and ion effects on water activity

The purpose of this review is to examine the various effects of low- molecular-weight electrolytes on the associations and interactions of proteins and nucleic acids. Our primary interest is in general electrostatic effects, rather than chemical effects (specific interactions) of particular ions (e.g. transition metals, protons). We consider those interactions in which a variation in salt concentration has a significant effect on the macromolecular equilibrium, and analyse the effects of salt in these situations in terms of (i) direct participation of ions in the biopolymer reaction, (ii) Debye—Hückel screening by salt ions of the charge interactions on the biopolymers, and (iii) the reduction in water activity brought about at high salt concentrations.

The CO and NO Bohr effect of human hemoglobin with and without inositolhexaphosphate

Using NO and CO as ligands the Bohr effect of human hemoglobin has been measured with and without inositolhexophosphate. It appears that in the absence and presence of inositolhexaphosphate hemoglobin shows a distinct ligand specificity with respect to the Bohr effect. Ligation with NO is accompanied by release of a larger number of Bohr effect. It is shown that this latter result is due to the fact that the number of protons taken up upon binding of inositolhexaphosphate to ligated hemoglobin is larger for HbNO than for HbCO. It is suggested that this additional proton uptake is partially due to a restoration of the saltbridge between His 146beta and Asp 94beta upon addition of IHP.

An isolation procedure for the native alpha chain of bovine hemoglobin. A study of the functional properties of this chain and its hybrid with the human beta chain

In this paper we present a procedure for the isolation of the native bovine alpha chain. The method is based on affinity chromatography. The results show that the ligand-binding properties of the bovine alpha chain are almost identical to those of the human alpha chain. The hybrid alphaB2 betaH2 prepared by mixing bovine alpha chains and human beta chains shows ligand binding properties similar to those of human hemoglobin and different from those of bovine hemoglobin.

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

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