The magnitude of the Bohr effect profoundly influences the shape and position of the blood oxygen equilibrium curve

For a century, the influence of the Bohr effect on the utilization of blood-borne oxygen has been deemed secondary to its influence on the uptake of carbon dioxide by the blood. Here, we show that the opposite is the case. Using a simple two-ligand, two-state formulation, we modeled the simultaneous oxygen and proton binding to hemoglobin, as well as the resulting acid-base changes of the surrounding solution. Blocking of the Bohr effect in this model system results in a dramatic increase in the oxygen affinity, as expressed by the oxygen partial pressure at half saturation, the P50. It also becomes clear that the P50 and the Bohr factor (a measure of the size of the Bohr effect) are not independent but directly related. Thus, everything else being equal, varying the number of Bohr groups from 0 to 8 per tetramer results in an increase in the Bohr factor from 0 to −0.9 and an increase in P50 from 6 to 46 mmHg at a constant Pco2 of 40 mmHg. Therefore, changes in hemoglobin structure that lead to changes in the Bohr factor will inevitably also change hemoglobin oxygen affinity. NEW & NOTEWORTHY Using a mathematical model, we show that the Bohr effect has a more profound effect on gas exchange than is evident when comparing oxygen equilibrium curves measured in the laboratory at different constant values of Pco2 or pH. Protons preloaded on the Bohr groups, as well as the protons taken up during oxygen unloading, dramatically decrease oxygen affinity of the physiological oxygen equilibrium curve. Therefore, the Bohr effect is instrumental in setting the oxygen affinity.

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Effect of haemoglobin concentration on the oxygen affinity of intact lamprey erythrocytes

Journal of Experimental Biology ◽

10.1242/jeb.198.11.2393 ◽

1995 ◽

Vol 198 (11) ◽

pp. 2393-2396 ◽

Cited By ~ 2

Author(s):

S Airaksinen ◽

M Nikinmaa

Keyword(s):

Blood Cell ◽

Red Blood Cell ◽

Haemoglobin Concentration ◽

Oxygen Affinity ◽

Equilibrium Curve ◽

Volume Changes ◽

Oxygen Equilibrium ◽

The Mean ◽

The Right ◽

Oxygen Equilibrium Curve

We investigated whether the oxygen affinity of lamprey haemoglobin decreases with increasing oxygen concentration at the high (1025 mmol l-1 monomeric) haemoglobin concentrations prevailing within the erythrocytes. The intracellular concentration of haemoglobin was experimentally adjusted by shrinking the cells osmotically: the osmolality of the equilibration medium was increased from approximately 250 mosmol kg-1 by 90 mosmol kg-1 to 340 mosmol kg-1 or by 180 mosmol kg-1 to 430 mosmol kg-1 by adding sucrose in the medium. This increased the mean cellular haemoglobin concentration from 16.9±0.23 mmol l-1 (monomeric haemoglobin) to 20.0±0.20 mmol l-1 (monomeric haemoglobin) and to 23.0±0.36 mmol l-1 (monomeric haemoglobin), respectively (means ± s.e.m., N=3540; all the samples from 78 different pools of blood at each osmolality combined). The oxygen equilibrium curves at each osmolality were determined by Tucker's method. An increase in haemoglobin concentration shifted the oxygen equilibrium curve to the right as indicated by the P50 values, which were 4.26±0.07 kPa at the lowest, 4.64±0.13 kPa at the intermediate and 5.64±0.40 kPa (means ± s.e.m., N=78) at the highest haemoglobin concentrations. The decrease in haemoglobin oxygen-affinity was attributed to the volume changes, since the intracellular pH did not decrease with increasing mean cellular haemoglobin concentration. Thus, the variations in red blood cell volume commonly observed during hypoxia may play a role in the regulation of haemoglobin function.

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A study of the properties of hybrids of oxyhaemoglobin and deoxyhaemoglobin with two porphyringlobin species

Biochemical Journal ◽

10.1042/bj1370339 ◽

1974 ◽

Vol 137 (2) ◽

pp. 339-348 ◽

Cited By ~ 2

Author(s):

Amyra Treffry ◽

Stanley Ainsworth

Keyword(s):

Fast Moving ◽

Dissociation Constants ◽

Initial Increase ◽

Equilibrium Curve ◽

Thiol Groups ◽

Oxygen Equilibrium ◽

Partial Pressures ◽

Thiol Reagent ◽

Slow Moving ◽

Oxygen Equilibrium Curve

The fluorescence of porphyringlobin is quenched on adding haemoglobin to its solutions. It is suggested that this result indicates the formation of hybrids (comprising a dimer of porphyringlobin and a dimer of haemoglobin) in which quenching occurs by energy transfer from the porphyrin to the haem groups of the protein. From an analysis of fluorescence quenching, dissociation constants were calculated for the hybrids of oxy- and deoxyhaemoglobin with the fast- and slow-moving porphyringlobin species isolated by chromatography on CM-Sephadex (Treffry & Ainsworth, 1974). The values obtained are: deoxyhaemoglobin–fast-moving porphyringlobin, 0.8X10−9m; deoxyhaemoglobin–slow-moving porphyringlobin, 5X10−10m; oxyhaemoglobin–fast-moving porphyringlobin, 0.8X10−6m; oxyhaemoglobin–slow-moving porphyringlobin, 1.2X10−7m. The rates of reactions of solutions of haemoglobin and porphyringlobin, containing hybrids, with the thiol reagent 4,4′-dithiodipyridine showed that the thiol groups of the hybrids deoxyhaemoglobin–fast-moving porphyringlobin and oxyhaemoglobin–slow-moving porphyringlobin react more slowly than expected on the basis of composition alone: this result indicates that the deoxy and slow-moving conformations are the more stable, imposing themselves partially on to the fast-moving or oxy dimer of the hybrid. Also the rate of the reaction of CO with deoxyhaemoglobin is decreased when slow-moving porphyringlobin is added to its solutions: this is reflected in a movement of the oxygen equilibrium curve of such a mixture to higher oxygen partial pressures. Similar experiments with deoxyhaemoglobin solutions containing fast-moving porphyringlobin, showed an initial increase in the rate of CO uptake. Correspondingly, the oxygen equilibrium curve of the mixture showed an increased affinity for oxygen. Approximate calculations to determine the oxygen equilibria of the hybrids indicate that a functional dimer retains co-operative characteristics even when the dimer accompanying it within the tetramer has the reacted conformation.

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