Calmodulin is a small calcium binding protein found in all eukaryotic cells. It is composed of 148 amino acids and its sequence is virtually identical in all organisms in which it is found. There are four Ca2+ binding sites, two at each globular end of the molecule. Calmodulin is only active when all four sites are filled. The left image below is that of calmodulin from the frog Xenopus (1CFD) without any Ca2+ bound. On the right is the Paramecuim form of calmodulin with Ca2+ bound (1CLM). Despite coming from different organisms, differences between the two images are more likely due to the presence of Ca2+, not differences in sequence. Notice that the alpha helix connecting the two globular domains is incomplete in the form without Ca2+ and that the positions of the ends of the protein change after binding Ca2+.
Active calmodulin can activate many other proteins including Ca2+ dependent protein kinases, cAMP-phosphodiesterase, glycogen phosphorylase, and Ca2+ ATPase. The latter enzyme pumps Ca2+ out of the cytosol thus leading to the eventual inactivation of calmodulin. The binding of the four Ca2+ ions is cooperative. This means that the binding of one Ca2+ facilitates the binding of the next ion and so forth until the four sites are filled. This has a practical consequence since a small change in the concentration of Ca2+ can have a large effect on the level of active calmodulin.
8/22/04 Copyright (C) 2004,
Jonathan Monroe, firstname.lastname@example.org.
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