Iron transport protein Serotransferrin (serum transferrin)
Sus scrofa (pig)
Iron is a very essential element for biological systems. It is prominent in many prosthetic groups like the heme groups of oxygen carrier and storage proteins. Iron is transported through the body to places where it is needed by large proteins called transferrins. Three different kinds of transferrins exist: ovotransferrins in egg whites, lactoferrins in extracellular body fluids and serum transferrins. All these transferrins are very similar, both in amino acid sequences and in their structures. They are monomeric proteins.
From the structure here, showing pig serum transferrin it may seem as if there were two molecules. That is not true; just one part of the chain in the middle of the structure is missing. They are however clearly divided into two separate domains, connected only by a short loop of 10 residues which is missing here. Each of these two domains forms a rather deep cleft in which an iron atom is bound, so there are two bound ions per molecule of transferrin. In prosthetic groups iron is often located at the center of a fairly large ring system. Here it is coordinated to five oxygen and one nitrogen atom. The nitrogen comes from a histidine sidechain while three of the oxygens come from two tyrosine sidechains and one aspartic acid sidechain. The remaining two oxygens come from carbonate (CO3), also bound here.
Protein Data Bank (PDB)
author: Arno Paehler