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The Bohr Effect and Hemoglobin
Carbon dioxide and hydrogen ions are two allosteric effectors of hemoglobin. They bind to different sites on the hemoglobin molecule, stabilize the T-state of hemoglobin and lower its affinity for oxygen. This in turn shifts the oxygen-binding curve to the right side and allows hemoglobin to unload more oxygen to the exercising tissue. Hydrogen ions bind to several different groups such as the amino group of the terminal amino acid and histidine amino acids such beta-146 and alpha-122. By binding, they promote the formation of salt bridges that stabilize the T-state structure of hemoglobin, thereby lowering its affinity for oxygen and shifting the curve to the right side. Carbon dioxide on the other hand binds onto the terminal amino residue to form a carbamate ion. The carbamate ion can also participate in forming stabilizing salt bridges. Exercising tissue produce many carbon dioxide molecules, which are then transferred into the blood plasma and eventually enter the red blood cells. Inside the red blood cells, the majority of the carbon dioxide is transformed into carbonic acid, which dissociates into bicarbonate ions and hydrogen ions. Therefore, an increase in carbon dioxide concentration also means there will be more hydrogen ions and therefore a lower pH. Together the effect of hydrogen ions and carbon dioxide on hemoglobin is known as the Bohr effect.
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