Hemoglobins are tetramers comprised of pairs of two different polypeptide subunits. Greek letters are used to designate each subunit type. The subunit composition of the principal hemoglobins are a2P2 (HbA; normal adult hemoglobin), a2y2 (HbF; fetal hemoglobin), a2S2 (HbS; sickle cell hemoglobin), and a262 (HbA2; a minor adult hemoglobin). The primary structures of the P, Y, and 6 chains of human hemoglobin are highly conserved.
0 20 40 60 80 100 120 140
Gaseous pressure of oxygen (mm Hg)
Figure 6-4. 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 et al [editors]: The Molecular and Metabolic Bases of Inherited Disease, 7th ed. McGraw-Hill, 1995.)
binding curve for myoglobin is hyperbolic. Myoglobin therefore 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 Hg, myoglobin releases O2 for mitochondrial synthesis of ATP, permitting continued muscular activity.
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