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Calmodulin.

This page shows some interactive JSmol views of calmodulin, an abbreviation for calcium-modulated protein. Calmodulin is a calcium binding trigger protein, and mediates a wide range of essential processes.

The first figure, [apocalmodulin 1], shows one molecule of apocalmodulin (from Rattus norvegicus). Note that there are a number of short α-helices and β-sheets, also that the overall shape is quite condensed.
In fact the protein crystallises as a dimer - the second figure [apocalmodulin 2] shows the two closely-associated chains.

The next two figures show the calcium-bound form of (human) calmodulin.
In [Ca calmodulin 1], note that the conformation of the protein has changed in two major ways:

• It has wrapped around the calcium ions, forming two "dumbell" or "EF hand" assemblies
• Two of the short helices from the apoportein have joined to form a single long and exposed central helix

The second view [Ca calmodulin 2] zooms in to one of the calcium binding sites. Note that:

• All of the donor atoms are oxygen (1 bidentate Glu, 3 Asp, a backbone C=O and a water)
• The Ca²⁺ion is 7-coordinate
• The geometry at the metal ion is irregular

These features make the site specific for Ca²⁺ ions.

[Calmodulin & target] shows calmodulin bound to a target α-helix (shown in maroon). Note that the long helix seen in the calcium calmodulin complex is folded again to wrap the calmodulin around the target helix.

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