Hemoglobin is a multimeric respiratory protein. It displays cooperative binding, which means that its affinity for oxygen changes with the amount of oxygen already bound. Hemoglobins are composed of two alpha and two beta subunits that form two dimers that interact loosely with one another. When a hemoglobin molecule is fully deoxygenated, it is in the rigid T state. It is stabilized by hydrogen bonds, binding of allosteric effectors, and salt bridges between the subunits. In this state, it has a relatively low affinity for oxygen. But when oxygen binds to one of the heme groups, the hemoglobin changes to an R state, with an increased affinity for oxygen. According to the Bohr effect, increasing the concentration of protons and/or carbon dioxide will reduce the oxygen affinity of hemoglobin. So when hemoglobin passes by tissues that have produced CO2 as a byproduct of cellular respiration, oxygen is released and CO2 is carried off to the lungs. Increasing blood carbon dioxide levels can lead to a decrease in pH (more acidic) because of the chemical equilibrium between protons and carbon dioxide.
The decrease in pH may be due to an increase in CO2 and lactic acid during vigorous physical activity, or to metabolic acidosis. The Bohr effect is beneficial, because the unloading of oxygen from hemoglobin increases under such conditions. If the decrease in pH is due to exercise, then the skeletal muscles have an increased demand for oxygen, and thus increased unloading is desirable.
H+(aq) + HCO3-(aq)
H2CO3(aq)
H2O(l) + CO2(g)
Basically, CO2 becomes dissolved in water forming carbonic acid, which can release H+ or HCO3- ions to raise or lower pH. These three equations are maintained by the exchange of CO2 from the lungs which raises pH by increasing HCO3- concentration, while the blood releases H+ ions when CO2 levels get high in order to lower it. Hemoglobin actually binds H+ ions and releases them when oxygen becomes available, when CO2 levels rise, the brain senses a lack of oxygen and H+ ions are released increasing the satuaration ability of hemoglobin.
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