According to their structural features, Ca 2+ -binding sites in proteins are classified as either non-continuous or continuous. In non-continuous sites the Ca 2+ ligand residues are located remotely from one another in the protein sequence. Most of the Ca 2+ binding proteins, such as cadherins, C 2 domains, site I of thermitase, phospholipase A 2 , and D- galactose binding protein (GBP) belong to this family. Continuous Ca 2+ -binding sites have binding pockets formed by a stretch of contiguous amino acids in the primary sequence (e.g. EF-hand proteins) ( Figure 2 ). EF-hand proteins have a conserved Ca 2+ binding loop flanked by two helices [18, 19] . Based on the conserved features of the Ca 2+ -binding loop, EF-hand proteins have been divided into two major groups: the canonical EF-hands as seen in CaM and the pseudo EF-hands exclusively found in the N-termini of S100 and S100-like proteins [18] . Their major difference lies in the Ca 2+ binding loop: the 12-residue canonical EF-hand loop binds Ca 2+ mainly via sidechains (loop positions 1, 3, 5, 12), whereas the 14-residue pseudo EF-hand loop chelates Ca 2+ mostly via backbone carbonyls (positions 1, 4, 6, 9) ( Figure  2 ). Each type of EF-hand loop has a bidentate Ca 2+ ligand (Glu or Asp) that functions as an anchor at the C-terminal of the binding loop. Among all the structures reported to date, the majority of EF-hand sites have been found to be paired either within multiple canonical EF-hand motifs or through the interaction of one pseudo EF-hand motif with one canonical motif [18] (Figure 2 ). For proteins with odd numbers of EF-hands, such as the penta-EF-hand calpain, EF-hand motifs are coupled through homodimerization or heterodimerization [20] [21] [22] .