Oxygen Transport by Hemoglobin and Myoglobin
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Hemoglobin, the essential component of red blood cells (erythrocytes), transports oxygen () through the bloodstream from the lungs to all the tissues of the body. Hemoglobin also carries carbon dioxide () back to the lungs to complete the process of respiration. Vertebrate hemoglobin is a nearly spherical protein molecule consisting of an array of four globin polypeptide chains, each containing a heme group, which is the oxygen-binding site. Its molecular weight is approximately 64,500 daltons. The oxygen uptake of hemoglobin exhibits cooperativity, such that each successively increases the affinity of the molecule for adding another , up to four. The saturation is a measure of the fractional occupancy of the oxygen-binding sites. It increases as a sigmoid-shaped function of the partial pressure of in its immediate environment. In the alveoli of the lungs, is approximately 100 torr.
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Contributed by: S. M. Blinder (April 2011)
Open content licensed under CC BY-NC-SA
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The binding of oxygen to myoglobin is described by the equilibrium . The saturation can be defined by , where is the partial pressure for 50% saturation, about 1 torr for myoglobin. For hemoglobin, the equilibrium takes the form , to . Here , where to account for the cooperativity of binding and at pH 7.4 but slightly higher as the pH decreases.
The central part of the heme group has this structure: . The complexes with the iron atom.
Reference: L. Stryer, Chap. 7, Molecular Design of Life, New York: W. H. Freeman, 1989.
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