Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Globular Proteins: Structure and Function of Hemoglobin
Cooperative oxygen binding makes hemoglobin a more efficient oxygen carrier

In the air-filled Lungs, the partial pressure of oxygen is approximately 100 mm Hg; at this pressure, Hemoglobin is about 96% saturated with oxygen. In the Cells of an active Muscle, however, the partial pressure of oxygen is only about 26 mm Hg, as muscle cells consume oxygen very rapidly, and its local concentration decreases accordingly. As Blood flows through the capillaries in the Muscles, oxygen from the nearly fully saturated hemoglobin in erythrocytes enters the Blood Plasma and then the muscle cells. The hemoglobin oxygen saturation curve shown in Fig. 8-16 shows that as blood passes through the muscle capillaries, about one-third of the bound oxygen is released, so that the hemoglobin leaving the Muscle tissue is only 64% saturated. When the blood returns to the lungs, where the partial pressure of oxygen is much higher (100 mm Hg), hemoglobin rapidly binds additional oxygen and is again saturated to 96%.

Now let us imagine that the hemoglobin in erythrocytes is replaced by Myoglobin. The hyperbolic oxygen saturation curve of myoglobin (Fig. 8-16) shows that a decrease in partial pressure from 100 mm Hg in the lungs to 26 mm Hg in the muscles would result in the release of only 1-2% of the bound oxygen from myoglobin. Therefore, myoglobin is poorly suited for transporting oxygen from the lungs to Tissues: its oxygen affinity is much higher than that of hemoglobin, and at the partial pressures of oxygen existing in muscles and other peripheral tissues, it releases very little oxygen to them. Hemoglobin, on the contrary, performs this function very efficiently, since its characteristic oxygen-binding curve allows it to release a significant portion of its bound oxygen at the low partial pressures of oxygen present in the tissues.

This comparison by no means implies that myoglobin is an inefficient or poorly designed protein. For the biological function it performs in muscle tissue (storing oxygen and supplying it to Cell/35.html">Mitochondria), myoglobin is far better adapted than hemoglobin, as its high oxygen affinity at low partial pressures of oxygen enables it to bind and store oxygen more effectively. Thus, hemoglobin and myoglobin are specifically adapted to perform different oxygen-binding Functions. At the same time, as we will see shortly, hemoglobin also possesses another function.