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menstruating woman has about 40 mg/kg of iron because of her smaller erythrocyte mass and iron store (Bothwell et al., 1979).

Function

Iron can exist in oxidation states ranging from –2 to +6. In biological systems, these oxidation states occur primarily as the ferrous (+2), ferric (+3), and ferryl (+4) states. The interconversion of iron oxidation states is a mechanism whereby iron participates in electron transfer, as well as a mechanism whereby iron can reversibly bind ligands. The common biological ligands for iron are oxygen, nitrogen, and sulfur atoms.

Four major classes of iron-containing proteins exist in the mammalian system: iron-containing heme proteins (hemoglobin, myoglobin, cytochromes), iron-sulfur enzymes (flavoproteins, hemeflavoproteins), proteins for iron storage and transport (transferrin, lactoferrin, ferritin, hemosiderin), and other iron-containing or activated enzymes (sulfur, nonheme enzymes). In iron sulfur enzymes, iron is bound to sulfur in one of four possible arrangements (Fe-S, 2Fe-2S, 4Fe-4S, 3Fe-4S proteins). In heme proteins, iron is bound to porphyrin ring structures with various side chains. In humans, the predominant form of heme is protoporphyrin-IX.

Hemoglobin

The movement of oxygen from the environment to the tissues is one of the key functions of iron. Oxygen is bound to an iron-containing porphyrin ring, either as part of the prosthetic group of hemoglobin within erythrocytes or as part of myoglobin as the facilitator of oxygen diffusion in tissues.

Myoglobin

Myoglobin is located in the cytoplasm of muscle cells and increases the rate of diffusion of oxygen from capillary erythrocytes to the cytoplasm and mitochondria. The concentration of myoglobin in muscle is drastically reduced in tissue iron deficiency, thus limiting the rate of diffusion of oxygen from erythrocytes to mitochondria (Dallman, 1986a).

Cytochromes

The cytochromes contain heme as the active site with the iron-



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