Homo sapiens (human)
In the early days of protein crystallography essentially all of the determined structure were what is called globular proteins, that is they were rather round in overall shape. The zoo of protein structures has since grown considerably to include many other shapes. When its structure was first determined, calmodulin drew some attention because of its at that time unusual shape. Calmodulin is a protein which binds to calcium ion (Ca2+), and works for intercellular signaling.
It is made of two small, fairly compact domains connected by a long alpha-helix, looking somewhat like a dumbbell, a device that you use to train your biceps with. The structure of the globular domains, basically two helices connected by a loop, is often called an EF hand motif. This kind of structure can bind calcium ions and is a typical motif for this purpose. Calmodulin thus has two such calcium binding sites. It is assumed to change its shape considerably between the states with and without calcium bound and interacts with and modulates the activity of a variety of other proteins: that is what has given it its name, a modulator protein controlled by calcium = calmodulin. It is a small, but evolutionary highly conserved protein, being almost identical in all organisms found. The conclusion about the change of its shape is based on comparison with a related structure, the first (N-terminal) part of the protein troponin C, which is involved in the control of muscle action. The structure here includes two calcium ions. They are bound by coordination with oxygen from six glutamic or aspartic acid side-chains and one water molecule, and in one case the peptide nitrogen of a threonine residue instead. The two binding sites are not symmetrical, as each site involves different kind of coordination partners to hold the calcium. The changing shape of calmodulin under the influence of calcium and its interaction with other proteins is one of the ways in which nature uses calcium to control biological processes.
Protein Data Bank (PDB)
author: Arno Paehler