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Ferritin and Transferrin
Transporting Iron Ions
Iron ions are delivered in the blood by the protein transferrin, shown here at the top in blue from PDB entry 1h76. Each transferrin molecule can carry two iron ions, shown here in brown, with each ion coupled with a carbonate ion, shown in red and white. The protein contains an array of amino acids that are perfectly arranged to form four bonds to the iron ion, which locks it in place. Once it finds its iron atoms, transferrin flows through the blood until it finds a transferrin receptor on the surface of a cell, shown here at the bottom from PDB entry 1cx8 (that PDB file contains coordinates for the part of the receptor that is outside the cell--the rest is shown as a schematic here). Transferrin binds tightly to the receptor and is drawn into the cell in a small vesicle. The cell then acidifies the inside of this little pocket, which causes transferrin to release its iron. Then, the receptor and empty transferrin are recycled back to the outside of the cell. Triggered by the neutral pH of the blood, the receptor releases the empty transferrin, and it continues its job of gathering iron.If you look through the PDB, you will find several molecules called lactoferrin and ovotransferrin that are similar to transferrin. These molecules, found in milk and egg whites respectively, also have strong binding sites for iron. However, their main function is not delivery. Instead, they serve to protect cells from bacteria. Since they mop up any free iron ions, they starve bacteria of a vital resource, slowing the growth of an infection.
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Last changed by: A.Honegger,