Oxygen Transport by Hemoglobin and Myoglobin
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.[more]
Myoglobin, which is located in muscle cells, serves as a reserve supply of oxygen for muscle functioning. This molecule consists of a single globin unit (MW ≈ 16,700 Da) with just one oxygen binding site. In contrast to hemoglobin, myoglobin will absorb or release an molecule at a much lower partial pressure. The skeletal muscles of aquatic mammals, such as whales and dolphins, are particularly rich in myoglobin, which allows them to be submerged for long periods of time.
A phenomenon known as the Bohr effect was discovered in 1904 by Danish physiologist Christian Bohr (father of physicist Niels Bohr). This constitutes a reduction in the oxygen affinity of hemoglobin as blood acidity increases, with pH decreasing from its normal value of 7.4, usually as a consequence of an increase of concentration in the blood. Myoglobin does not exhibit the Bohr effect.
This Demonstration plots the dependence of oxygen saturation on and pH. The average number of bound molecules is shown in the top illustrations. These are highly schematic, with the molecules being greatly magnified.[less]
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.