text: Calmodulin is a small, highly conserved protein that is 148 amino acids long (16.7 kDa). The protein has two approximately symmetrical globular domains (the N- and C- domains) each containing a pair of EF hand motifs separated by a flexible linker region for a total of four Ca binding sites, two in each globular domain.  In the Ca-free state, the helices that form the four EF-hands are collapsed in a compact orientation, and the central linker is disordered;  in the Ca-saturated state, the EF-hand helices adopt an open orientation roughly perpendicular to one another, and the central linker forms an extended alpha-helix in the crystal structure, but remains largely disordered in solution.  The C-domain has a higher binding affinity for Ca than the N-domain.
  Calmodulin is structurally quite similar to troponin C, another Ca-binding protein containing four EF-hand motifs.  However, troponin C contains an additional alpha-helix at its N-terminus, and is constitutively bound to its target, troponin I.  It therefore does not exhibit the same diversity of target recognition as does calmodulin.