Bohr effect

Haemoglobin Dissociation Curve. Dotted red line corresponds with shift to the right caused by Bohr effect

The Bohr effect is a physiological phenomenon first described in 1904 by the Danish physiologist Christian Bohr, stating that haemoglobin's oxygen binding affinity (see Oxygen–haemoglobin dissociation curve) is inversely related both to acidity and to the concentration of carbon dioxide.^[1] That is, an increase in blood CO₂ concentration, which leads to a decrease in blood pH, will result in haemoglobin proteins releasing their load of oxygen. Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in haemoglobin picking up more oxygen. Since carbon dioxide reacts with water to form carbonic acid, an increase in CO₂ results in a decrease in blood pH.

Mechanism

In deoxyhaemoglobin, the N-terminal amino groups of the α-subunits and the C-terminal histidine of the β-subunits participate in ion pairs. The formation of ion pairs causes them to decrease in acidity. Thus, deoxyhaemoglobin binds one proton (H⁺) for every two O₂ released. In oxyhaemoglobin, these ion pairings are absent and these groups increase in acidity. Consequentially, a proton is released for every two O₂ bound. Specifically, this reciprocal coupling of protons and oxygen is the Bohr effect.^[2]

Additionally, carbon dioxide reacts with the N-terminal amino groups of α-subunits to form carbamates:^[3]

R−NH₂ + CO₂ $\rightleftharpoons$ R−NH−COO⁻ + H⁺

Deoxyhaemoglobin binds to CO₂ more readily to form a carbamate than oxyhaemoglobin. When CO₂ concentration is high (as in the capillaries), the protons released by carbamate formation further promotes oxygen release. Although the difference in CO₂ binding between the oxy and deoxy states of haemoglobin accounts for only 5% of the total blood CO₂, it is responsible for half of the CO₂ transported by blood. This is because 10% of the total blood CO₂ is lost through the lungs in each circulatory cycle.^[4]

Physiological role

This effect facilitates oxygen transport as haemoglobin binds to oxygen in the lungs, but then releases it in the tissues, particularly those tissues in most need of oxygen. When a tissue's metabolic rate increases, its carbon dioxide production increases. Carbon dioxide forms bicarbonate through the following reaction:

CO₂ + H₂O $\rightleftharpoons$ H₂CO₃ $\rightleftharpoons$ H⁺ + HCO₃⁻

Although the reaction usually proceeds very slowly, the enzyme family of carbonic anhydrase, which is present in red blood cells, accelerates the formation of bicarbonate and protons.^[4] This causes the pH of tissues to decrease, and so, promotes the dissociation of oxygen from haemoglobin to the tissue, allowing the tissue to obtain enough oxygen to meet its demands. Conversely, in the lungs, where oxygen concentration is high, binding of oxygen causes haemoglobin to release protons, which combine with bicarbonate to drive off carbon dioxide in exhalation. Since these two reactions are closely matched, there is little change in blood pH.

The dissociation curve shifts to the right when carbon dioxide or hydrogen ion concentration is increased. This facilitates increased oxygen dumping. This mechanism allows for the body to adapt the problem of supplying more oxygen to tissues that need it the most. When muscles are undergoing strenuous activity, they generate CO₂ and lactic acid as products of cellular respiration and lactic acid fermentation. In fact, muscles generate lactic acid so quickly that pH of the blood passing through the muscles will drop to around 7.2. As lactic acid releases its protons, pH decreases, which causes haemoglobin to release ~10% more oxygen.^[4]

Effects of allostery

The Bohr effect is dependent on allosteric interactions between the hemes of the haemoglobin tetramer. This is evidenced by the fact that myoglobin, a monomer with no allostery, does not exhibit the Bohr effect. Haemoglobin mutants with weaker allostery may exhibit a reduced Bohr effect.

In the Hiroshima variant haemoglobinopathy, allostery in haemoglobin is reduced, and the Bohr effect is diminished. During periods of exercise, the mutant haemoglobin has a higher affinity for oxygen and tissue may suffer minor oxygen starvation.^[5]

See also

• Allosteric regulation
• Haldane Effect
• Root effect

References

1. Bohr; Hasselbalch, Krogh. "Concerning a Biologically Important Relationship - The Influence of the Carbon Dioxide Content of Blood on its Oxygen Binding".
2. Murray, Robert K.; Darryl K. Granner; Peter A. Mayes; Victor W. Rodwell (2003). Harper’s Illustrated Biochemistry (LANGE Basic Science) (26th ed.). McGraw-Hill Medical. pp. 44–45. ISBN 0-07-138901-6.
3. Lehninger, Albert L.; Nelson, David L.; Cox, Michael M. (2008). Principles of Biochemistry (5th ed.). New York, NY: W.H. Freeman and Company. p. 166. ISBN 978-0-7167-7108-1.
4. Voet, Donald; Judith G. Voet; Charlotte W. Pratt (2013). Fundamentals of Biochemistry: Life at the Molecular Level (4th ed.). John Wiley & Sons, Inc. p. 189.
5. Olson, JS; Gibson QH; Nagel RL; Hamilton HB (December 1972). "The ligand-binding properties of hemoglobin Hiroshima ( 2 2 146asp )". The Journal of Biological Chemistry 247 (23): 7485–93. PMID 4636319.

External links

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