Binding characteristics of a fluorescent analogue of diphosphoglyceric acid to human, amphibian and fish hemoglobins

An attempt was made to establish the binding of N-(2,4-diphosphobenzyl)-1-amino-5-naphthalenesulfonic acid, DIPANS, as an estimator of conformation in the carbonmonoxy (CO)-hemoglobins (Hbs) of several vertebrates. DIPANS failed to bind menhaden I, trout I or tuna Hbs which are ligand insensitive. Below a pH of 7.0, DIPANS bound menhaden II, Bufo, Xenopus, and human Hbs with a binding stoichiometry greater than one. The charge of the DIPANS molecule does not control its binding to these Hbs. The binding to human CO-Hb can not be due to Hb conformation. For Xenopus Hbs and menhaden II, conformation predominates DIPANS binding. The binding to CO-Hb of DIPANS, can not be unambiguously attributed to the Hb's quaternary conformation.     

The relation between carbon monoxide binding and the conformational change of hemoglobin.

The spectral difference between normal and rapidly reacting deoxyhemoglobin (Sawicki and Gibson (1976), J. Biol Chem. 251:1533-1542) is used to study the relationship between CO binding to hemoglobin and the conformational changes to the rapidly reacting form in a combined flow-laser flash experiment. In both pH 7 phosphate buffer and pH 7 bis(2-hydroxy-ethyl)imino-tris (hydroxymethyl)methane buffer (bis-Tris) with 500 muM 2,3-diphosphoglycerate (DPG), the conformational change lags far behind CO binding; rapidly reacting hemoglobin is not observed until more than 10% of the hemoglobin is liganded. In pH 9 borate buffer the formation of rapidly reacting hemoglobin leads CO binding by a significant amount. A simple two-state allosteric model (Monod et. al. (1965), J. Mol. Biol. 12:88-118) which assumed equivalence of the hemoglobin subunits in their reaction with CO was used to simulate the experimental results. In terms of the model, the conformational change lead observed at pH 9 suggests that significant conformational change has occurred after binding of only one CO molecule per tetramer. In the presence of phosphates good agreement between experimental results and simulations is obtained using parameter values suggested by previous experimental studies. The simulations suggest that the conformational change occurs after binding of three CO molecules.     

Analysis of the acid and alkaline dissociation of earthworm hemoglobin, Lumbricus terrestris, by front-face fluorescence spectroscopy

The steady-state fluorescence properties of the multisubunit hemoglobin isolated from the earthworm, Lumbricus terrestris, were studied by front-face fluorometry. Acid and alkaline dissociation of this high-molecular-weight hemoglobin were examined over the pH range 3.7-12.5 using different liganded states (oxy, CO, met). The relative intensity of the emission maximum at 320 nm (exc. 280 nm) is ligand-dependent increasing as follows: oxy less than deoxy less than CO less than met at pH 7.0. The intensity of the emission maximum of oxyhemoglobin at the alkaline acid end point, pH 10.5 (333 nm), is significantly greater than that observed at the acid end point, pH 4.18 (320 nm), suggesting different subunit dissociation. The spectra of oxyhemoglobin at pH 4.18 and the spectrum of carbonmonoxy hemoglobin at pH 7.0 in the presence of 1 M magnesium chloride were almost identical, indicating similar subunit dissociation. Difference spectrum (pH 9.0-7.2) of fluorescence emission (exc. 305) resulted in a maximum at 341 nm, indicative of tyrosinate formation. This suggests that tyrosine(s) may also be located at the subunit interface(s) of this hemoglobin. These studies indicate that several aromatic amino acid residues are associated with the critical sites of subunit interactions within this molecule. Analysis of the fluorescence spectra also suggests that the formation of different subunit species resulting from acid and alkaline dissociation cannot be ruled out.     

Effect of bezafibrate and clofibric acid on the spectroscopic properties of the nitric oxide derivative of ferrous human hemoglobin.

The effect of bezafibrate (BZF) and clofibric acid (CFA) on the spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous human hemoglobin (HbNO) has been investigated quantitatively. In the presence of BZF and CFA, the X-band EPR spectra and the absorption spectra in the Soret region of HbNO display the same basic characteristics described in the presence of inositol hexakisphosphate (IHP) and 2, 3-diphosphoglycerate (2,3-DPG). Next, in the presence of these allosteric effectors, the oxygen affinity for ferrous human hemoglobin (Hb) is reduced. These findings indicate that BZF and CFA, as already reported for IHP and 2, 3-DPG, induce the stabilization of a low affinity conformation of the ligated hemoprotein (i.e., HbNO). Values of the apparent equilibrium constant for BZF and CFA binding to HbNO (K) are 1.5(+/- 0.2) x 10(-2) M and 2.8(+/- 0.3) x 10(-2) M, respectively, at pH 7.0 (in 0.1 M N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid]/NaOH buffer system plus 0.1 M NaCl) and 20 degrees C. The results reported here represent clearcut evidence for BZF and CFA specific (i.e., functionally relevant) binding to a ligated derivative of Hb (i.e., HbNO).     

Use of ionic and zwitterionic (Tris/BisTris and HEPES) buffers in studies on hemoglobin function.

The functional characteristics of hemoglobin (Hb) depend on oxygenation-linked proton and anion binding and thus on solvent buffer groups and ionic composition. This study compares the oxygenation properties of human Hb in ionic [tris(hydroxymethyl)aminomethane (Tris) and BisTris] buffers with those in zwitterionic N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer under strictly controlled chloride concentrations at different pH values, two temperatures, and in the absence and presence of the erythrocytic cofactor, 2,3-diphosphoglycerate (DPG). In contrast to earlier studies (carried out at the same or different chloride concentrations) it shows only small buffer effects that are manifested at low chloride concentration and high pH. These observations suggest chloride binding to the Tris buffers, which reduces the interaction with specific chloride binding sites in the Hb. The findings indicate that HEPES allows for more accurate assessment of Hb-oxygen affinity and its anion and temperature sensitivities than ionic buffers and advocates standard use of HEPES in studies on Hb function. Precise oxygen affinities of Hb dissolved in both buffers are defined under standard conditions.     

Thermodynamical studies of oxygen equilibrium of hemoglobin. Nonuniform heats and entropy changes for the individual oxygenation steps and enthalpy-entropy compensation.

Precise oxygen equilibrium curves of human adult hemoglobin were determined by the automatic recording method at several temperatures in the presence and absence of 2,3-diphosphoglycerate (DPG) or inositol hexaphosphate (IHP) with 0.05 M 2,2-bis(hydroxymethyl)-2,2',2'-nitrolotriethanol (bis-tris) buffers (pH 7.4) containing 0.1 M Cl-. The equilibrium data were analyzed according to the Adair scheme, and the heats, deltaHi (i = 1,2,3,4) and the entropy changes, deltaSi (i = 1,2,3,4), for the individual oxygenation steps were obtained. The shape of the equilibrium curve varies on temperature changes whether DPG or IHP is present or absent. In consequence, the deltaHi value depends on i and on the presence of DPG and IHP. Behavior of deltaSi is similar to that of deltaHi. The similar behavior of deltaHi and deltaSi resulted in a compensation phenomenon. The contribution of T cdeltaSi to the free energy change is compensated by the contribution of deltaHi at the first three oxygenation steps but not at the fourth step, and for i = 1,2, and 3 changes of T cdeltaSi value upon the addition of DPG and IHP are compensated by accompanied changes of deltaHi value, where T c (= 260 K) is the compensation temperature. A major part of both the enthalpy-entropy compensation and nonuniformity of deltaHi and deltaSi appears to be attributable to contributions of the oxygen-linked binding of Cl-, DPG and IHP, by hemoglobin. The present results do not necessarily support the earlier idea of Wyman that the cooperative oxygenbinding is essentially an entropy effect.     

Characterization of the hemoglobins of the adult brushtailed possum, Trichosurus vulpecula (Kerr) reveals non-genetic heterogeneity

The hemoglobins contained within the red blood cells of the adult brushtail possum exhibited cooperative (n=2.6) oxygen binding curves with an associated p50 of 38 mm Hg at pH 7.4 and a large Bohr effect (-0.60). Stripped hemolysate showed a Bohr effect of -0.27, and was sensitive to added DPG (K=56 micromol L(-1)), ATP (K=130 micromol L(-1)), and chloride ions. Four isoforms of hemoglobin were identified using isoelectric focusing. Mass spectrometry indicated that all four isoforms most likely represent the same gene products which have differentially undergone post-translational deamidation and glutathionylation. The oxygen binding characteristics of three isolated isohemoglobins have been determined.     

Hemoglobins of the tadpole of the bullfrog, Rana catesbeiana. Temperature dependence of oxygen binding and pH dependence of subunit dissociation.

The temperature dependence of the oxygen equilibrium of tadpole hemoglobin has been determined between 0 degrees and 32 degrees for the unfractionated but phosphate-free lysate and between 12 degrees and 32 degrees for each of the four isolated components between pH 6 and 10 in 0.05 M cacodylate, Tris, or glycine buffers containing 0.1 M NaCl and 1 mM EDTA. Under these conditions the Bohr effect (defined as deltalog p50/deltapH) of the unfractionated lysate is positive at low temperatures between pH 6 and 8.5 and is negative above pH 8.5 to 8.8 at any temperature. As the temperature rises the Bohr effect below pH 8.5 changes greatly. In the interval pH 7.0 to 7.5, the magnitude of the Bohr effect decreases from + 0.28 at 0 degrees to zero at about 24 degrees and becomes negative, as in mammalian hemoglobins, above this temperature. Measurements with the isolated components show that the temperature dependence of oxygen binding for Components I and II and for Components III and IV is very similar. For both sets of components the apparent overall enthalpy of oxygenation at pH 7.5 is about -16.4 kcal/mol and -12.6 kcal/mol at pH 9.5. The measured enthalpies include contributions from the active Bohr groups, the buffer ions themselves, the hemoglobin groups contributing buffering, and any pH-dependent, oxygenation-dependent binding of ions such as chloride by the hemoglobin. The apportioning of the total enthalpy among these various processes remains to be determined. Between pH 8 and 10.5 tadpole oxyhemoglobin undergoes a pH-dependent dissociation from tetramer to dimer. The pH dependence of the apparent tetramer-dimer dissociation constant indicates that at pH 9.5 the dissociation of each tetramer is accompanied by the release of approximately 2 protons. In this pH range the oxygen equilibrium measurements indicate that about 0.5 proton is released for each oxygen molecule bound. The results are consistent with the conclusion that one acid group per alphabeta dimer changes its pK from about 10 to 8 or below upon dissociation of the tetramer.     

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

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

Rate of allosteric change in hemoglobin measured by modulated excitation using fluorescence detection.

We have measured the forward and reverse rates of the allosteric transition of hemoglobin A with three CO molecules bound by using modulated excitation coupled with fluorescence quenching of the DPG analogue, PTS (8-hydroxy-1,3,6 pyrene trisulfonic acid). This dye is observed to bind to the T state with significantly larger affinity than to the R state, and thus provides an unequivocal marker for the molecule's conformational change. The allosteric rates obtained with the fluorescent dye (pH 7.0, bis-Tris buffer) are (3.4 +/- 1.0) x 10(3)s-1 for the R to T transition and (2.1 +/- 0.5) x 10(4)s-1 for the T to R transition. This gives an equilibrium constant L3 of 0.16 +/- 0.06. These results provide good agreement with modulated difference spectra calibrated from model compounds, arguing that there is little if any difference in the kinetics observed by the heme spectra and the kinetics of the full subunit motion. The equilibrium constant between structures (L3) is smaller in the absence of phosphates than observed in phosphate buffer (0.33). However, the rates of the allosteric transition increase in the absence of phosphates as compared with the corresponding rates in phosphate buffer of 1.0 x 10(3)s-1 and 3.0 x 10(3)s-1. The effects of inorganic phosphates on the equilibrium can be separated from the effects on kinetics. We find that phosphates also affect the dynamic behavior of hemoglobin, and the presence of 0.15 M phosphate can be viewed as raising the transition state energy between R and T conformations by approximately 0.5 kcal/mol exclusive of the T state stabilization. Dissociation constants for the dye were measured to be 104 +/- 25 microM for unligated T state and 930 +/- 300 microM for the fully ligated R state. The best fit equilibrium constant (125 +/- 40 microM) for three ligands bound does not differ significantly from that measured without ligands bound. Incidental to the measurement technique is the determination of the rates of binding and release of the dye. The association rate for binding to the T state is large, (at least 4 x 10(9) M-1 s-1) and may be diffusion limited, while the association and dissociation rates for R state binding, while not determined with precision, are clearly much smaller, of the scale of 10(5) M-1 s-1 for association.     

The effects of inositol hexaphosphate on the allosteric properties of two beta-99-substituted abnormal hemoglobins, hemoglobin Yakima and hemoglobin Kempsey.

Hemoglobins (Hb) Yakima and Kempsey were purified from patients' blood with diethylaminoethyl cellulose column chromatography. The oxygen equilibrium curves of the two hemoglobins and the effects of organic phosphates on the function were investigated. In 0.1 M phosphate buffer, Hill's constants n for Hb Yakima and Hb Kempsey were 1.0 to 1.1 at the pH range for 6.5 to 8.0 and the oxygen affinities of both the mutant hemoglobins were about 15 to 20 times that of Hb A at pH 7.0. The Bohr effect was normal in Hb Yakima and one-fourth normal in Hb Kempsey. In the presence of inositol hexaphosphate, the oxygen affinities to Hb Yakima and Hb Kempsey were greatly decreased, and an interesting result revealed that these hemoglobins showed clear cooperativity in oxygen binding. Hill's constant n in the presence of inositol hexaphosphate was 1.9 for Hb Kempsey and 2.3 for Hb Yakima at pH 7.0. The cooperativities of these mutant hemoglobins were pH-dependent, and Hb Kempsey showed high cooperativity at low pH (n equal 2.1 at pH 6.6) and low cooperativity at high pH (n equal 1.0 at pH 8.0). Hb Yakima showed similar pH dependence in cooperativity. In the presence of inositol hexaphosphate, Hb A showed a pH-dependent cooperativity different from those of Hb Yakima and Hb Kempsey, namely, Hill's n was the highest in alkaline pH (n equal 3.0 at pH 8.0) and decreased at lower pH (n equal 1.5 at pH 6.5). 2,3Diphosphoglycerate bound with the deoxygenated Hb Yakima and Hb Kempsey, however, had no effect on the oxygen binding of these abnormal hemoglobin. The pH-dependent cooperativity of alpha1beta2 contact anomalous hemoglobin and normal hemoglobin was explained by the shifts in the equilibrium between the high and low ligand affinity forms.     

Apparent oxidation-reduction potential of Clostridium acidi-urici ferredoxin. Effect of pH, ionic strength, and amino acid replacements.

The effects of pH and ionic strength on the midpoint reduction potential (Emp) of Clostridium acidi-urici ferredoxin were determined using hydrogen gas and hydrogenase. The Emp of native ferredoxin at 24-25 degrees in 0.1 M Tris-chloride buffer, pH 7.0, is--0.434 V. In the pH range examined, the Emp becomes approximately 13 mv more negative per each pH unit increase. A plot of the log of ionic strength versus the apparent Emp of ferredoxin in 0.1 M Tris-chloride buffer, pH 7.5, Was linear over the range of 1.0 to 0.01 ionic strength with Emp values of--0.414 and--0.475 V, respectively, at these extremes. This effect is the same with sodium chloride, sodium bromide, or ammonium sulfate. Potassium phosphate buffer caused a similar change, but the absolute values of Emp differed from those obtained in the presence of the other salts. This effect of pH and ionic strength on Emp may be general for clostridial-type (Fe4S4)2-ferredoxins, since the apparent Emp of Clostridium pasteurianum ferredoxin is affected in a similar manner by these two variables. The Emp of this ferredoxin in 0.1 M Tris-chloride buffer pH 7.0, is--0.405 V. Since the NH2-terminal amino acid residue, Ala1, and Tyr2 of C. acidi urici ferredoxin are near an (Fe4S4)2-cluster in the protein, the apparent Emp of derivatives that contained amino acid replacements in these two positions were determined. Under similar conditions, the Emp of most of the 13 derivatives examined, including those of [Leu2]- and[3-NH2-Tyr30]ferredoxin, is approximately the same as that of native ferredoxin. However, the Emp of [His2]ferredoxin is approximately 15 mv more positive, whereas that of [Trp2]ferredoxin is 22 mv more negative than that of native C. acidi-urici ferredoxin. Variations in sodium chloride concentration and pH also affected the apparent Emp of the derivatives. It is suggested that the changes observed in the Emp of C. acidi-urici ferredoxin are caused by protein conformational changes.     

Microbial desulfonation of substituted naphthalenesulfonic acids and benzenesulfonic acids

Sulfur-limited batch enrichment cultures containing one of nine multisubstituted naphthalenesulfonates and an inoculum from sewage yielded several taxa of bacteria which could quantitatively utilize 19 sulfonated aromatic compounds as the sole sulfur source for growth. Growth yields were about 4 kg of protein per mol of sulfur. Specific degradation rates were about 4 to 14 mu kat/kg of protein. A Pseudomonas sp., an Arthrobacter sp., and an unidentified bacterium were examined. Each desulfonated at least 16 aromatic compounds, none of which served as a carbon source. Pseudomonas sp. strain S-313 converted 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 5-amino-1-naphthalenesulfonic acid, benzenesulfonic acid, and 3-aminobenzenesulfonic acid to 1-naphthol, 2-naphthol, 5-amino-1-naphthol, phenol, and 3-aminophenol, respectively. Experiments with 18O2 showed that the hydroxyl group was derived from molecular oxygen.     

Microbial desulfonation of substituted naphthalenesulfonic acids and benzenesulfonic acids.

Sulfur-limited batch enrichment cultures containing one of nine multisubstituted naphthalenesulfonates and an inoculum from sewage yielded several taxa of bacteria which could quantitatively utilize 19 sulfonated aromatic compounds as the sole sulfur source for growth. Growth yields were about 4 kg of protein per mol of sulfur. Specific degradation rates were about 4 to 14 mu kat/kg of protein. A Pseudomonas sp., an Arthrobacter sp., and an unidentified bacterium were examined. Each desulfonated at least 16 aromatic compounds, none of which served as a carbon source. Pseudomonas sp. strain S-313 converted 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, 5-amino-1-naphthalenesulfonic acid, benzenesulfonic acid, and 3-aminobenzenesulfonic acid to 1-naphthol, 2-naphthol, 5-amino-1-naphthol, phenol, and 3-aminophenol, respectively. Experiments with 18O2 showed that the hydroxyl group was derived from molecular oxygen.     

Functional and subunit assembly properties of hemoglobin Alberta (alpha 2 beta 2(101) Glu----Gly)

Hemoglobin Alberta has an amino acid substitution at position 101 (Glu----Gly), a residue involved in the alpha 1 beta 2 contact region of both the deoxy and oxy conformers of normal adult hemoglobin. Oxygen equilibrium measurements of stripped hemoglobin Alberta at 20 degrees C in the absence of phosphate revealed a high affinity (P50 = 0.75 mm Hg at pH 7), co-operative hemoglobin variant (n = 2.3 at pH 7) with a normal Bohr effect (- delta log P50/delta pH(7-8) = 0.65). The addition of inositol hexaphosphate resulted in a decrease in oxygen affinity (P50 = 8.2 mm Hg at pH 7), a slight increase in the value of n and an enhanced Bohr effect. Rapid mixing experiments reflected the equilibrium results. A rapid rate of carbon monoxide binding (l' = 7.0 X 10(5) M-1 S-1) and a slow rate of overall oxygen dissociation (k = 15 s-1) was seen at pH7 and 20 degrees C in the absence of phosphate. Under these experimental conditions the tetramer stability of liganded and unliganded hemoglobin Alberta was investigated by spectrophotometric kinetic techniques. The 4K4 value (the liganded tetramer-dimer equilibrium dissociation constant) for hemoglobin Alberta was found to be 0.83 X 10(-6) M compared to a 4K4 value for hemoglobin A of 2.3 X 10(-6) M, indicating that the Alberta tetramer was less dissociated into dimers than the tetramer of hemoglobin A. The values of 0K4 (the unliganded tetramer-dimer equilibrium dissociation constant) for hemoglobin Alberta and hemoglobin A were also measured and found to be 2.5 X 10(-8) M and 1.5 X 10(-10) M, respectively, demonstrating a greatly destabilized deoxyhemoglobin tetramer for hemoglobin Alberta compared to deoxyhemoglobin A. The functional and subunit dissociation properties of hemoglobin Alberta appear to be directly related to the dual role of the beta 101 residue in stabilizing the tetrameric form of the liganded structure, while concurrently destabilizing the unliganded tetramer molecule.     

Studies on Foliar Penetration: VII. FACTORS CONTROLLING THE PENETRATION OF CHLORIDE IONS INTO THE LEAVES OF PHASEOLUS VULGARIS

The pattern of penetration of chloride ions into the abaxialsurfaces of the primary leaves of Phaseolus vulgaris has severalfeatures in common with those previously recorded for 2,4-dichlorophenoxyaceticacid (2,4-D), 2,2-dichloropropionic acid (dalapon), and 4-amino-3,5,6-trichloropicolinicacid (picloram). In the dark the rate of entry of chloride ionsup to 24 h is constant, but in the light entry is at first slowand then more rapid. This acceleration does not occur at lowtemperature or when the tissue is treated with 3-(3,4-dichlorophenyl)-l,l-dimethylureaor phenylmercuric chloride. Neither does it take place at theadaxial surface or when the leaves are more mature. The most distinguishing feature of the pattern of penetrationof chloride ions is its dependence on external pH. In darknessentry is unaffected by changes in pH but in the light the rateof entry is increased as the pH falls, but this response isrestricted to young leaves. No binding of chloride seems tooccur within the tissue. These findings support the view that penetration of the abaxialsurface in young leaves of Phaseolus is largely determined bya membrane system. This system decreases in importance as theleaf matures and the overlying cuticle thickens. At the adaxialsurface the thicker cuticle is seemingly a major barrier topenetration even in very young leaves.     

Effect of 2,3-diphosphoglycerate on the Bohr effect of human adult hemoglobin

The effect of 2,3-diphosphoglycerate (DPG) on the Bohr effect of human hemoglobin has been studied by means of hydrogen ion titration techniques. The results indicate a) that both the acid and the alkaline Bohr effect are equally affected, b) that the DPG binding to deoxyhemoglobin (Hb) is much stronger than to carboxyhemoglobin (HbCO) and c) that Hb binds effectively one DPG molecule. The effect on the Bohr effect can roughly be described by assuming that upon binding two groups per tetramer change their pK from 6.8 to 7.8 and two others from 6.8 to 5.8. These groups very probably are the imidazole groups of the two histidines H21 (143)β and the two phosphate groups of DPG (second dissociation). From the experiments a value for the dissociation constant K of the Hb-DPG complex of about 10⁻⁵ M⁻¹ could be estimated at pH 6.2 and pH 7.5.     

Influence of allosteric effectors and temperature on oxygen binding properties and the Bohr effect of bovine hemoglobin

The O2 binding properties of bovine Hb were examined. The increase in Cl- and DPG concentration enhanced P50. A reduction in n(max) was observed at high Cl- concentration, while DPG had little effect on n(max). An increase in Cl- concentration enhanced the Bohr effect, the magnitude of which reached a maximum at 0.1 M Cl- and 20 degrees C. This concentration is nearly equal to that at the highest slope of the log P50 vs. log [Cl-] plot, and also equal to the physiological Cl- concentration (0.1 M) of bovine blood. Furthermore, the influence of Cl- concentration on the Bohr effect is independent of temperature. On the other hand, in the absence of Cl-, bovine Hb is sensitive to DPG; an increase in DPG concentration enhanced the Bohr effect, which reached a maximum at 3 mM DPG and 20 degrees C. This concentration is nearly equal to that at the highest slope of the log P50 vs. log [DPG] plot. At low DPG concentrations, the DPG effect on the Bohr effect became small with increasing temperature, whereas at high DPG concentrations, the DPG effect was insensitive to temperature changes. At the physiological concentration of DPG (0.5 mM), increases in both Cl- concentration and temperature diminished the DPG effect. At the physiological concentrations of Cl- and DPG, the Bohr effect was -0.36 at 37 degrees C. The deltaH value at the physiological concentrations of Cl- and DPG was approximately -5.8 kcal/mol at pH 7.4. These results indicate that Cl- and temperature are important determinants of the O2 binding properties of bovine Hb.     

Transposing sequences between fetal and adult hemoglobins indicates which subunits and regulatory molecule interfaces are functionally related

To correlate amino acid sequence changes with hemoglobin function we are carrying out a detailed recombinant analysis of the adult hemoglobin/fetal hemoglobin (HbA/HbF) systems. The important physiological differences between these two tetramers lie at unspecified sites in the 39 sequence substitutions of the 146 amino acids in their beta and gamma chains. In this paper, significant differences in the tetramer-dimer dissociation constants (referred to as tetramer "strength" or "stability") of adult (HbA) and fetal (HbF) hemoglobin tetramers have been used to probe the relationship between the allosteric, sliding interface and the effects of the allosteric regulator, 2,3-DPG, in promoting oxygen release. The single amino acid difference at the allosteric interfaces of these two hemoglobins, Glu-43(beta) --> Asp-43(gamma), which is not near the DPG binding site, leads to a significantly lower DPG response, approaching that of HbF. The results are inconsistent with the long-held idea that the replacement of His-143(beta) in HbA to Ser-143(gamma) in HbF is solely responsible for the lowered DPG response in HbF. On the other hand, the Val-1(beta) --> Gly-1(gamma) replacement near the DPG binding site has no effect on the DPG response. The replacement of His-116(beta) by the hydrophobic Ile-116(gamma) at the rigid alpha(1)beta(1) interface has a marginal yet detectable effect on the allosteric alpha(1)beta(2) interface. The results, overall, are interpreted using a model involving electrostatic coupling between certain side chains and extend the concept of a long-range relationship between some distant regions of the tetramer that are likely mediated through the central cavity.     

Thermodynamics of 2,3-diphosphoglycerate association with human oxy- and deoxyhemoglobin

The association of 2,3-diphosphoglycerate with oxy- and deoxyhemoglobin was studied by means of ultrafiltration and microcalorimetry. It was found that in addition to parameters that are known to influence the binding of 2,3-diphosphoglycerate to both species of hemoglobin (such as pH, temperature and concentration of competing anion), the association is also strongly dependent on the hemoglobin concentration. The difference between the apparent association constants for the formation of the complex of the organic phosphate with oxy- and deoxyhemoglobin is relatively small. At pH 7.3, 25° C and 0.154 M chloride this difference is only 0.6 kcal/mole of free energy favoring the Hb·DPG complex. This free energy difference increases with decreasing pH but is not strongly affected by hemoglobin concentration. The enthalpy change for the formation of the 2,3-diphosphoglycerate complex with deoxyhemoglobin is 8–10 kcal/mole more exothermic than the complex with oxyhemoglobin.