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Hemoglobin Structure

Hemoglobin is a complex molecule containing four heme groups and four globin chains. Each heme group consists of a protoporphyrin ring with an iron molecule at its center, attached by four nitrogen bonds. The iron molecule reversibly binds oxygen and carbon dioxide, allowing hemoglobin to carry oxygen to tissues and return carbon dioxide to the lungs. Each heme group is surrounded by a globular polypeptide molecule called a globin chain. There are four types of globin chains—α, β, δ, and γ—and they always appear in pairs (two of one type of chain and two of another).

How is the structure of hemoglobin important? It turns out that hemoglobin always has one pair of α chains, but the other pair varies with age. In adults, most hemoglobin is comprised of two α chains and two β chains (each, of course, with its own heme molecule). This is called adult hemoglobin, or hemoglobin A1. But in the fetus, hemoglobin is composed of two α chains and two γ chains (fetal hemoglobin, or hemoglobin F). As one might expect, hemoglobin F is much better at binding oxygen than hemoglobin A1 so that the fetus can extract oxygen from the mother’s bloodstream.

The switch from hemoglobin F to hemoglobin A1 occurs at around 6 months of age. At around 4 months of age, the infant begins to make β chains, and the production of γ chains begins to diminish. The swap is virtually complete by around 8 months of age. Production of γ chains never entirely ceases, however; a small amount (less than 1%) of total hemoglobin in the adult is composed of hemoglobin F. In addition, there is always a small amount of δ chain production, both in the fetus and in the adult. Hemoglobin with two α chains and two δ chains is called hemoglobin A2, and it always comprises a small (1%-2%) amount of total hemoglobin. Its function is unknown.

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