Body Content Forms and Function

Iron, the 26th element of the periodic table, has a molecular weight of 55.85. Two common aqueous oxidation states, ferrous (Fe2+) and ferric (Fe3+), enable iron to participate in oxidation/reduction reactions that are essential to energy metabolism by accepting or donating electrons. However, this property also enables free iron to catalyze oxidative reactions, resulting in reactive and damaging free radicals. Accordingly, body iron must be chemically bound to facilitate appropriate physiological function, transport, and storage, with minimal opportunity for free ionic iron to catalyze harmful oxidative reactions.

Most of the body's iron functions in heme protein complexes that transport oxygen as hemoglobin and myoglobin. Approximately two-thirds of the body iron is in hemoglobin, a 68,000 MW structure containing four subunits of heme, a protoporphyrin ring with iron in the center (Figure 1), and four polypeptide chains (two chains each of a- and /3-globin). For transport by hemoglobin, oxygen bonds directly to the iron atom, stabilized in a Fe2+ oxidation state surrounded by the protoporphyrin ring and histidine residues. Hemoglobin iron easily binds and releases oxygen, circulating in blood erythrocytes. Myoglo-bin, consisting of a single heme molecule and globin, enables oxygen transfer from erythrocytes to cellular mitochondria in muscle cytoplasm.

Smaller quantities of iron in the heme form function in mitrochondrial cytochromes involved with electron transfer, oxygen utilization, and the production of ATP. A small fraction of body iron functions in heme-containing hydrogen peroxidases such as catalase that protect against excessive hydrogen peroxide accumulation by catalyzing its conversion to hydrogen and oxygen.

Iron also functions in non-heme proteins that contain an iron-sulfur complex, a cubical arrangement of four iron and four sulfur atoms. This is the principal form of iron in mitochondria, functioning in enzymes of energy metabolism such as aconitase, NADH dehydrogenase, and succinate dehydrogen-ase. In both mitochondria and cytosol, aconitase

h2c ch2

OOC COO-

Figure 1 Heme (ferroprotoporphyrin 9).

is sensitive to iron concentrations. When iron is abundant, the aconitase enzyme assumes the full iron-sulfur cubic structure that is associated with carbohydrate metabolism. However, when iron concentrations are reduced, the protein loses aconitase activity and functions as an iron binding protein (IRP). IRPs interact with iron response elements (IREs) of the mRNA to regulate the synthesis of proteins involved with iron transport, storage, and use, in response to changes in cellular iron concentrations.

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