Fil:Calmodulin 1CLL.png

The protein Calmodulin with Calcium binding
Calcium: blue
alpha-Helices: orange
beta-sheets: green.
One can clearly see the helix-turn(sheet)-helix-structure of the four Ca²⁺-binding sites.
The picture is derived from Protein 1CLL in [The Protein Data Base]. The official citation is:
H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne: The Protein Data Bank. Nucleic Acids Research, 28 pp. 235-242 (2000).

Picture created with the Public-Domain-Program RasMol by Mathias Klode (w:de:Benutzer:Lode).

This image has been (or is hereby) released into the public domain by its author, Lode at the de project. This applies worldwide. In case this is not legally possible: Lode grants anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.