Hemoglobin

Hemoglobin

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Hemoglobin is an iron-containing protein found in the blood of nearly all vertebrates and many invertebrates . It transports oxygen from the lungs or gills of an animal to the tissues. Four polypeptide chains, each wrapped in a specific way around its own heme group , make up the hemoglobin molecule. An iron atom in the ferrous state (Fe²⁺) sits in the middle of the heme. Ferric hemoglobin (Fe³⁺) cannot bind oxygen.

The polypeptide chains of hemoglobin are of two kinds; in the hemoglobin of an adult human, HbA, they are designated as α and β, and the subunit structure of HbA is written α ₂β ₂. The α chain contains 141 amino acid residues, the β chain 146. Throughout most of the animal kingdom the subunit structure of hemoglobin remains the same: two polypeptides of one kind with 141 amino acid residues and two of another with 146 residues. The amino acid sequences (the primary structure) of the polypeptides, however, are species dependent. Closely related animals display fewer differences in these amino acid sequences. The α and β chains of human hemoglobin, for example, differ from chicken hemoglobin in 35 and 45 places, respectively. Between human and horse α and β chains these differences drop to 18 and 25. As for chimpanzee and human, their hemoglobins are identical.

Often several kinds of hemoglobin exist in a given animal. In the adult human, for example, HbA (α ₂δ ₂) makes up 98 percent of the total hemoglobin, and HbA₂ (α ₂δ ₂), the remaining 2 percent. The polypeptide chain δ has the same number of amino acids as the β chain, but the sequences differ in 10 places. A third kind of human hemoglobin, fetal hemoglobin or HbF (α ₂γ ₂), constitutes over 80 percent of the total hemoglobin of a newborn, but vanishes rapidly during the first year of life. The amino acid sequences of the γ and β chains differ in 39 of the 146 residues. Additional types of human hemoglobins exist at embryonic stages of life.

Each polypeptide chain of a hemoglobin molecule coils into several helical segments (the secondary structure of the polypeptide chain) that are linked with nonhelical segments. Intertwining sets of helices wrap tightly around the heme group and produce a compact subunit (the tertiary structure). The four subunits are packed into a nearly spherical package (the quaternary structure) of 55-angstrom (2.17 ×10⁻⁷-inch) diameter.

Each ferrous iron within hemoglobin provides one binding site for O₂. Thus a single hemoglobin molecule has the capacity to combine with four molecules of oxygen. Hemoglobin binds oxygen in a cooperative fashion; occupation of one binding site enhances the affinity of another binding site for oxygen in the molecule. Consequently, the oxygenation curve of hemoglobin (see Figure 1), in which the fractional saturation of hemoglobin with oxygen is displayed as a function of the oxygen pressure in the alveoli of the lungs, rises slowly at first, then more steeply, until it levels off and approaches unity (100% saturation). The steep rise in the oxygenation curve over a relatively small interval of oxygen pressure allows hemoglobin to serve as an efficient transporter of oxygen.

see also Primary Structure; Quaternary Structure; Secondary Structure; Tertiary Structure; Transport Protein.

N. M. Senozan

Bibliography

Dickerson, R. E., and Geis, I. (1983). Hemoglobin: Structure, Function and Evolution. Menlo Park, CA: Benjamin/Cummings.

Horton, H. R.; Moran, L. A.; Ochs, R. S.; Rawn, J. D.; and Scrimgeour, K. G. (2002). Principles of Biochemistry, 3rd edition. Upper Saddle River, NJ: Prentice Hall.