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Iron performs many important functions in the body. It is primarily involved in the transfer of oxygen from the lungs to tissues. However, iron also plays a role in metabolism as a component of some proteins and enzymes.
Iron is toxic to the body in its free state. It is associated with proteins either through ligand binding or by being incorporated into a porphyrin group - a ring-shaped molecule. A complex of the ferrous form of iron and protoporphyrin IX is known as heme. Heme iron is found in proteins connected with oxygen transport, including hemoglobin and myoglobin. Non-heme iron can be found in proteins connected with oxidative phosphorylation and in iron storage proteins like transferrin and ferritin.
About 70 percent of iron in the body is found in hemoglobin and myoglobin. Hemoglobin is the protein in red blood cells responsible for carrying oxygen to the tissues from the lungs. Myoglobin is a protein found in muscles that is used for storage of oxygen.
Hemoglobin is the oxygen transport system found in the red blood cells of all vertebrates and some invertebrates. In humans, hemoglobin is made up of four globular protein subunits. The four subunits form a pocket that binds a heme group.
Oxygen binds to the iron atom within the hemoglobin molecule in the lungs to form oxyhemoglobin. This occurs in the capillaries of the lung alveoli. It is released at its destination in the cells. Hemoglobin carries CO2 back to the lungs to be exhaled as waste, but CO2 binds to the protein portion of the hemoglobin molecule, not to the bound iron in the heme group.
Like hemoglobin, myoglobin binds iron within a heme group. However, structurally, it is much simpler, consisting of a single polypeptide chain of 154 amino acids. It is found only in cardiac myocytes and oxidative skeletal muscle. Myoglobin is an oxygen storage protein. In marine mammals, it provides an oxygen supply for extended periods when the animal is diving under water. At those times, myoglobin releases oxygen to sustain aerobic metabolism in the muscle. In humans, myoglobin levels have been shown to be increased at high altitudes.
A large number of enzymes require iron as a cofactor for their functions. Among the most significant of these are enzymes involved in oxidative phosphorylation, the metabolic pathway that converts nutrients to energy. The cytochrome enzymes bind heme iron, and some protein complexes in the oxidative phosphorylation process have iron-sulfur centers that are crucial to their function.
Dietary iron is stored within a protein complex called ferritin. Ferritin has 24 subunits that form a capsule around the bound iron atoms. Each complex binds 2000 to 45000 iron atoms. Another protein, transferrin, made in the liver, transports iron within the blood to other locations for storage. The main locations of iron storage in the body include the liver, skeletal muscle, and reticuloendothelial cells. If the storage capacity of these cells is exceeded, iron is deposited near the ferritin-iron complexes in the cells. These deposits are called hemosiderin. Iron in hemosiderin is not available to the cell. Hemosiderin deposits may be found in the body following a hemorrhage.
The overall utilization of iron in the body is regulated by ferritin and transferrin mRNAs that contain iron responsive elements (IREs). Iron homeostasis requires ascorbic acid (vitamin C), which stimulates the absorption of dietary iron, and assists with uptake of transferrin-bound iron in the plasma. Ascorbic acid also stimulates synthesis of ferritin, while inhibiting ferritin degradation and flow of iron out of the cell.