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Helix-Loop-Helix in a Calcium-binding Protein Calmodulin is an alpha-helical protein that binds 4 Calcium ions (Green spheres). Each Calcium is bound by a helix-loop-helix motif. The helices are perpendicular to each other. Only the loop contributes oxygens to ligate the Calcium ion: 3 aspartate side chains and one CO from the backbone. See how the protein swallows up the Calcium in spacefilling view. Chattopadhyaya R, Meador WE, Means AR, Quiocho FA. 1cll (PDB) |