Calmodulin binds to a helices. (A) An α helix (purple) in CaM kinase I is a target for calmodulin. (B) On Ca2+ binding to the apo, or calcium-free, form of calmodulin (1), the two halves of calmodulin clamp down around the target helix (2), binding it through hydrophobic and ionic interactions. In CaM kinase I, this interaction allows the enzyme to adopt an active conformation.
[Drawn from 1A06, 1CFD, 1CLL, and 1CM1.pdb.]