Red blood cells contain enormous numbers of hemoglobin molecules. Hemoglobin is a protein consisting of four polypeptide subunits (see Figure 3.8). Each of these polypeptides surrounds a heme group—an iron-containing ring structure that can reversibly bind a molecule of O2. Thus, each molecule of hemoglobin can bind to four molecules of O2.
As O2 diffuses into the red blood cells, it binds to hemoglobin. Once O2 is bound, it cannot diffuse back across the red cell plasma membrane. By binding O2 molecules as they enter the red blood cells, hemoglobin maximizes the partial pressure gradient driving the diffusion of O2 into the cells. In addition, it enables the red blood cells to carry a large amount of O2 to the tissues of the body.
The ability of hemoglobin to pick up or release O2 depends on the Po2 of its environment. When the Po2 of the blood plasma is high, as it usually is in the lung capillaries, each molecule of hemoglobin can carry its maximum load of four molecules of O2. As the blood circulates through the rest of the body, it encounters lower Po2 values. At these lower Po2 values, the hemoglobin releases some of the O2 it is carrying (Figure 48.12).
The relation between Po2 and the amount of O2 bound to hemoglobin is not linear, but S-shaped (sig-moidal). The sigmoidal hemoglobin-O2 binding curve in Figure 48.12 reflects interactions between the four subunits of the hemoglobin molecule. At low Po2 values, only one subunit will bind an O2 molecule. When it does so, the shape of that subunit changes, causing an alteration in the quaternary structure of the whole hemoglobin molecule. That structural change makes it easier for the other subunits to bind a molecule of O2; that is, their O2 affinity is increased. Therefore, a smaller increase in Po2 is necessary to get most of the hemoglobin molecules to bind two O2 molecules (that is, to become 50% saturated) than was necessary to get them to bind one O2 molecule (to become 25% saturated). This influence of the binding of O2 by one subunit on the O2 affinity of the other subunits is called positive cooperativity.
Once the third molecule of O2 is bound, the relationship seems to change, as a larger increase in Po2 is required for the hemoglobin to reach 100 percent saturation. This upper bend of the sigmoid curve is due to a probability phenomenon: The closer we get to having all subunits occupied, the less likely it is that any particular O2 molecule will find a place to bind. Therefore, it takes a relatively greater Po2 to achieve 100 percent saturation.
This is a good place to mention the danger posed by carbon monoxide (CO), which can come from a faulty furnace or from burning a fuel such as charcoal or kerosene without adequate ventilation. CO binds to hemoglobin with a higher affinity (240 times higher!) than O2. Thus, CO prevents hemoglobin from transporting and releasing O2 to the tissues of the body. The victim loses consciousness and can die because the brain lacks O2.
The O2-binding properties of hemoglobin help get O2 to the tissues that need it most. In the lungs, where the Po2 is about 100 mm Hg, hemoglobin is 100 percent saturated. The Po2 in blood returning to the heart from the body is usually about 40 mm Hg. You can see from Figure 48.12 that at this Po2, the hemoglobin is still about 75 percent saturated. This means that as the blood circulates around the body, only about one in four of the O2 molecules it carries is released to the tissues. This system seems inefficient, but it is really quite
48.12 The Binding of O2to Hemoglobin Depends on Po2 Hemoglobin in blood leaving the lungs is 100 percent saturated (four molecules of O2 are bound to each hemoglobin molecule). Most hemoglobin molecules will drop only one of their four O2 molecules as they circulate through the body, and are still 75 percent saturated when the blood returns to the lungs.The steep portion of this oxygen-binding curve comes into play when tissue Po2 falls below the normal 40 mm Hg,at which point the hemoglobin will "unload" its O2 reserves.
adaptive, because the hemoglobin keeps 75 percent of its O2 in reserve to meet peak demands.
When a tissue becomes starved of O2 and its local Po2 falls below 40 mm Hg, the hemoglobin flowing through that tissue is on the steep portion of its sigmoid binding curve. That means that relatively small decreases in Po2 below 40 mm Hg will result in the release of lots of O2 to the tissue. Thus the positive cooperativity of O2 binding by hemoglobin is very effective in making O2 available to the tissues precisely when and where it is needed most.
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