English: The helix-loop-helix EF-hand Ca2+ binding motif. (left) Cartoon illustration of the canonical EF-hand Ca²⁺-binding motif. The EF-hand motif contains a 29-residue helix-loop-helix topology, much like the spread thumb and forefinger of the human hand. Ca²⁺ is coordinated by ligands within the 12-residue loop, including seven oxygen atoms from the sidechain carboxyl or hydroxyl groups (loop sequence positions 1, 3, 5, 12), a main chain carbonyl group (position 7), and a bridged water (via position 9). Residue at position 12 serves as a bidentate ligand. (middle) The Ca²⁺ binding pocket adopts a pentagonal bipyramidal geometry. n stands for hydrophobic residue. (right) 3D structure of a typical canonical EF-hand motif from calmodulin (PDB code: 3cln). Ca3D structure of a typical canonical EF-hand motif from calmodulin (PDB code: 3cln). Ca3D structure of a typical canonical EF-hand motif from calmodulin (PDB code: 3cln). Calcium is chelated by ligands from a 12-residue loop.