Details of the EF hand loops. All loops are shown in approximately the same orientation, with entering and exiting helices ghosted for clarity. Residues of note are coloured in orange. cEF1 is shown in its apo state. cEF2 is shown in its apo, Ca2+‐bound and Mg2+‐bound states. In the cEF2 apo state, E354 is stabilized by nearby loop residue R344. In the Ca2+‐bound state, cEF2 loop residues coordinate Ca2+ in a pentagonal, bipyramidal arrangement, with E354 as a bidentate ligand. An ordered water molecule at the apex of one of the pyramids is represented as a small red sphere. In the Mg2+‐bound state, the coordination is octahedral with E354 as a monodentate ligand. The hEF1 and hEF2 loops adopt a conformation quite different from the cEF hands, and also lack the acidic residues that typically coordinate cations. Note the conserved hEF1 272VTL274 and hEF2 386VTL388 residues, which overlay identically in Fig 2A. cEF, canonical EF hand; hEF, hidden EF hand. 