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Bohr Effect in Hemoglobin the Bohr Effect Was First Discovered by a Physiologist Christian Bohr in 1904

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Bohr Effect in Hemoglobin the Bohr Effect Was First Discovered by a Physiologist Christian Bohr in 1904 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) Bohr Effect in Hemoglobin The Bohr Effect was first discovered by a physiologist Christian Bohr in 1904. This effect explains how hydrogen ions and carbon dioxide affect the affinity of oxygen in Hemoglobin. If pH was lower than it normally was (normal physiological pH is 7.4), then the hemoglobin does not bind oxygen effectively. + In other words, the lower the pH, the more the H ion concentration, the higher the CO2 level and the LESS affinity Hemoglobin has for oxygen. The opposite explains: the higher the pH, the lower the H+ ion concentration, the lower the CO2 level, and the GREATER affinity hemoglobin has for oxygen. The binding of oxygen to hemoglobin in the lungs is not affected by changing the pH and the oxygen will continue to be loaded normally. This does not prove to be true in tissues however, and a change in the pH results in a lower percent saturation of hemoglobin. More oxygen is delivered to tissues at a lower pH even when the amount of oxygen available remains unchanged. Levels of Oxygen in a Tissue When a tissue is more active, the amount of carbon dioxide produced will be increased. Carbon dioxide reacts with water as is shown in the following equation: + 3- CO2+ H2O <---------> H + HCO This shows that as the amount of carbon dioxide increases, more H+ is formed and the pH will decrease. In other words, more the CO2 is present, the more H+ is formed (so the lower the pH; remember pH is inversely related to the H+ concentration by the equation pH = -log [H+]) According to Bohr, the lower pH will cause hemoglobin to deliver more oxygen. The Bohr Effect is dependent upon Cooperativity * between the hemoglobin tetramer and the Heme group; it is key to note that although myoglobin and hemoglobin are very similar, myoglobin does not exhibit this effect because Myoglobin is a monomer and does not exhibit any cooperative interactions. If the hemoglobin's cooperativity is weak, then the Bohr Effect will in turn be low. Figure Showing: Binding of Oxygen Molecules in the four subunits of Hemoglobin. Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 1 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) * Cooperativity Effect When a substrate binds to one enzymatic subunit, the rest of the subunits are stimulated and become active. An example of cooperativity is the binding of oxygen to hemoglobin. One oxygen molecule can bind to the ferrous iron of a heme molecule in each of the four chains of a hemoglobin molecule. Deoxy-hemoglobin has a relatively low affinity for oxygen, but when one molecule binds to a single heme, the oxygen affinity increases, allowing the second molecule to bind more easily, and the third and fourth even more easily. The oxygen affinity of 3-oxy-hemoglobin is ~300 times greater than that of deoxy- hemoglobin. This behavior leads the affinity curve of hemoglobin to be sigmoidal, rather than hyperbolic as with the monomeric myoglobin. By the same process, the ability for hemoglobin to lose oxygen increases as fewer oxygen molecules are bound. This phenomenon explains why Hemoglobin can readily release oxygen in human tissue. The pH of the tissue is much lower than in the human lungs, so the blood wants to release the oxygen creating hemoglobin in its t-state. Once the blood travels back to the lungs, where the pH is higher, blood will pick up more oxygen for transport. NOTE: Myoglobin holds onto its oxygen in the tissue because it is not influenced by the Bohr Effect. NOTE: On average, the hemoglobin can release 66% of its oxygen, whereas myoglobin only releases about 7%. Temperature Another factor that will also affect the binding of oxygen to hemoglobin is temperature, which may be affected due to physical activity among many other factors. A more active tissue will be producing more heat and will be warmer. This increased temperature may lead to changes in hemoglobin's affinity to oxygen in a similar fashion as would be expected from a decrease in pH. pH The affinity that hemoglobin has for oxygen is decreased when the pH of the solution is decreased. When the solution is at a lower pH, hemoglobin tends to release more oxygen because as affinity of oxygen for heme group decreases. The main reason for this is shown by what occurs in deoxyhemoglobin. If the pH is lowered the imidazole group on histidine can be protonated. This triggers salt bridges to form bond between the protonated and positively charged imidazole group on the histidine with the negatively charged carboxylate group on a nearby aspartate. Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 2 EWING CHRISTIAN COLLEGE, PRAYAGRAJ. B.Sc. Semester VI Department of Chemistry Paper – 1 (Inorganic Chemistry) Unit – 5 (Chemistry of Hemoglobin) This causes the stabilization of deoxyhemoglobin or T state. This causes the T state, which has less affinity for oxygen, to be more prominent which pushes for oxygen to be released from hemoglobin. Carbon Dioxide The presence of Carbon dioxide gives rise to the release of oxygen from hemoglobin. 1. By formation of Carbonic acid and reducing the pH: At high concentrations the carbon dioxide reduces the pH. This occurs due to the fact that carbon dioxide reacts with water and forms carbonic acid, and carbonic acid dissociate to release proton (H+) and bicarbonate ion, so it will decrease pH. This reaction is accelerated with an enzyme present in red blood cells, Carbonic anhydrase. Carbonic acid is a relatively strong acid, so it tends to dissociate causing an increase in hydrogen ion presence. This results in a decrease in pH. 2. By reaction with deoxyhemoglobin: The second way it helps in releasing oxygen from hemoglobin is by directly reacting with hemoglobin itself. What occurs is that carbon dioxide stabilizes deoxyhemoglobin form by reacting with the terminal amino groups. It forms a carbamate group which is negatively charged. These negatively charged groups participate in salt bridges. Due to this deoxyhemoglobin or T state is stabilized which results in release of oxygen from hemoglobin. Abhinav Lal, Dept. of Chemistry, Ewing Christian College,Prayagraj. Page 3 .
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