Ferritin (iron transport protein) H-chain mutant (LYS 86 replaced by GLN)
Homo sapiens (human)
Iron is an important element for the human body. While proteins or DNA themselves contain no iron, many prosthetic groups associated with proteins for instance the heme groups in oxygen storage or other proteins, contain an iron at their center and it is the atom that binds the oxygen. Iron storage and iron transport inside the body are therefore important functions carried out by proteins like transferrin or ferritin. These iron carrier proteins are, despite just having to carry a simple atom, very large proteins. Their building blocks may however be small as is the case with ferritin.
This protein consists of twenty-four chains of two different chain types, the H type and the L type. The structure here shows a detailed 3-dimensional atomic model of one of these two, of the H type. The structure is very simple, four helices almost parallel to each other. This motif is called a four-helix bundle, first found as a structural motif by Wayne Hendrickson in the 1970s. Within these four helices, almost at the center of the molecule, sits an iron atom. It had been observed that the amount of H and L chains tend to vary in a ferritin molecule, depending on whether the ferritin was more involved in storing iron or more involved in processing iron. It was not clear why that was so and people were looking for a detailed answer at the atomic level. However, that proved difficult. L chains, with the help of cadmium sulfate, were easy to crystallize, but all attempts with H chains failed. By looking at L chain molecules and how they make contact with each other inside a crystal it was possible to understand the cause of this failure. This information was used to modify H chains by genetic engineering, changing one lysine residue to a glutamine residue. As a result very nice crystals could be obtained and used to find the structure of the ferritin H chain that you see here.
Protein Data Bank (PDB)
author: Arno Paehler