About 70 percent of the iron in the body is found in hemoglobin and myoglobin. Hemoglobin is the protein in red blood cells that is responsible for carrying oxygen from the lungs to the tissues. Myoglobin is a protein in muscles that is used to store oxygen.
Hemoglobin is the oxygen transport system found in the red blood cells of all vertebrates and some invertebrates. In humans, hemoglobin is composed of four spherical 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, forming oxyhemoglobin. This happens in the capillaries of the 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 the bound iron in the heme group.
Like hemoglobin, myoglobin also binds iron within a heme group. However, structurally it is much simpler and consists 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 oxygenation for prolonged periods when the animal is underwater. At these times, myoglobin releases oxygen to maintain aerobic metabolism in the muscle. In humans, myoglobin levels have been shown to be elevated at high altitudes
Iron-dependent enzymes involved in iron metabolism
A large number of enzymes require iron as a cofactor for their functions. Among the most important are the enzymes involved in oxidative phosphorylation, the metabolic pathway that converts nutrients into energy. The cytochrome enzymes bind heme-iron and some protein complexes in the oxidative phosphorylation process have iron-sulfur centers that are crucial for their function.
Function of ferritin and transferrin in iron metabolism
Dietary iron is stored in a protein complex called ferritin. Ferritin has 24 subunits that form a capsule around the bound iron atoms. Each complex binds 2,000 to 45,000 iron atoms. Another protein, transferrin, made in the liver transports iron in the blood to other locations for storage. The main sites of iron storage in the body include the liver, skeletal muscle and reticuloendothelial cells. When 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 the hemosiderin is not available to the cell. Hemosiderin deposits can be found in the body after bleeding.
The body’s overall utilization of iron is regulated by ferritin and transferrin mRNAs, which contain iron-responsive elements (IREs). Iron homeostasis requires ascorbic acid (vitamin C), which stimulates the uptake of dietary iron and aids in the uptake of transferrin-bound plasma iron. Ascorbic acid also stimulates the synthesis of ferritin while inhibiting ferritin breakdown and the flow of iron out of the cell.