The hyperbolic relationship between the concentration, or partial pressure, of O2 (Po2 ) and the quantity of O2 bound, expressed as an O2 saturation isotherm (Figure1), reveals why myoglobin is more suitable for O2 storage than for O2 transport. Myoglobin loads O2 readily at the Po2 of the lung capillary bed (100 mm Hg). However, since myoglobin releases only a small fraction of its bound O2 at the Po2 values typically encountered in active muscle (20 mm Hg) or other tissues (40 mm Hg), it represents an ineffective vehicle for delivery of O2. However, when strenuous exercise lowers the Po2 of muscle tissue to about 5 mm of Hg, dissociation of O2 from myoglobin permits mitochondrial synthesis of ATP, and hence muscular activity, to continue.

Fig1. Oxygen-binding curves of both hemoglobin and myoglobin. Arterial oxygen tension is about 100 mm Hg; mixed venous oxygen tension is about 40 mm Hg; capillary (active muscle) oxygen tension is about 20 mm Hg; and the minimum oxygen tension required for cytochrome oxidase is about 5 mm Hg. Association of chains into a tetrameric structure (hemoglobin) results in much greater oxygen delivery than would be possible with single chains. (Modified with permission from Scriver CR, Beaudet AL, Sly WS: The Molecular and Metabolic Bases of Inherited Disease, 7th ed. New York, NY: McGraw Hill; 1995.)

By contrast, hemoglobin behaves as if it were two proteins. At the high Po2 of the lungs, 100 mm Hg and above, it displays a high affinity for oxygen that enables it to bind oxygen to nearly every available heme iron. This form of the protein is commonly referred to as R, for relaxed, state hemoglobin. At the lower Po2 values encountered in peripheral tissues, 40 mm Hg and below, hemoglobin exhibits a much lower apparent affinity for oxygen. Transition of hemoglobin to this low affinity, taut or T state enables it to release a large proportion of the oxygen previously picked up in the lungs. This dynamic interchange between the high and low affinity R and T states serves as the foundation for hemoglobin’s sigmoidal O2-binding curve.

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