Introduction

Iron is found everywhere on the Earth, so it is no surprise that living cells use iron ions in many ways. We use iron throughout our body, for many tasks. Iron ions bind strongly and specifically to small molecules such as oxygen, making it an essential tool for manipulating these elusive molecules. Iron ions also cycle easily between the ferrous and ferric forms, providing a handy tool for manipulating individual electrons. Iron ions, however, pose a great challenge in our modern biological environment. The water filling cells and the oxygen in the air together conspire to convert iron ions to the ferric state, which is highly insoluble, forming rust-like oxides. The cell must somehow shelter iron ions so that they may be stored and delivered in the necessary quantities. This is the job of ferritin and transferrin.

Iron Storage

Inside cells, extra iron ions are locked safely in the protein shell of ferritin, shown here from PDB entry 1fha. Ferritin is composed of 24 identical protein subunits that form a hollow shell. The bottom illustration shows the hollow shell cut in half, showing the chamber inside and a few of the pores that lead inside the shell. After entering the ferritin shell, iron ions are converted into the ferric state, where they form small crystallites along with phosphate and hydroxide ions. There is room to pack about 4500 iron ions inside.

Rich in Iron

We have about 3.7 grams of iron in our body, painstakingly gathered from iron in our diet. About 2.5 grams are locked inside the hemoglobin in our blood, where they assist in the transport of oxygen. This is a valuable and essential resource, so special mechanisms for the recycling of this iron have been developed. Another few tenths of a gram are found in myoglobin, which also assists in oxygen management. A remarkably small amount--about 0.02 g--is distributed between the many different proteins that transfer electrons, such as the proteins of the electron transport chain that create most of our cellular ATP supplies. The rest, about a gram, is stored inside ferritin to fulfill future needs.

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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Exploring the Structure

The way that iron atoms get into and out of ferritin is still something of a mystery. Many researchers believe that they enter through the small pores between the 24 subunits. A few of the pores at four-fold axes are shown on the first page, and the pore at one of the three-fold axes is shown in the middle here. Once inside, iron atoms form crystallites that are too large to get back out of the pores. So, in order to release the iron atoms, the entire complex must be disassembled.

Here is a hint for when you look at these structures yourself. The structure shown here has all 24 chains, but PDB entries for the human and horse forms only contain one subunit, such as in PDB entry 1aew. The Structure Explorer page for each entry (e.g. 1aew) contains an image that toggles between the biological molecule and the asymmetric unit. It also contains download links for both the coordinates (under the "File" menu) as well as the images (under the "Visualize" menu) of the biological molecule. Using the coordinates, you can create pictures similar to this one, using programs such as RasMol or the other programs found under "View Structure."

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Information on ferritin and transferrin

D. R. Richardson and P. Ponka (1997): The molecular mechanisms of the metabolism and transport of iron in normal and neoplastic cells. Biochimica et Biophysica Acta 1331, pp. 1-40.

E. C. Theil (1987): Ferritin: structure, gene regulation, and cellular function in animals, plants and microorganisms. Annual Review of Biochemistry 56, pp. 289-315.

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© 2013 David Goodsell & RCSB Protein Data Bank