The calcium ion is coordinated in a pentagonal bipyramidal configuration.
The six residues involved in the binding are in positions 1, 3, 5, 7, 9 and 12; these residues are denoted by X, Y, Z, -Y, -X and -Z. The invariant Glu or Asp at position 12 provides two oxygens for liganding Ca (bidentate ligand).

The calcium ion is bound by both protein backbone atoms and by amino acid side chains, specifically those of the acidic amino acid residues aspartate and glutamate.
These residues are negatively charged and will make a charge-interaction with the positively charged calcium ion.
The EF hand motif was among the first structural motifs whose sequence requirements were analyzed in detail.
Five of the loop residues bind calcium and thus have a strong preference for oxygen-containing side chains, especially aspartate and glutamate.
The sixth residue in the loop is necessarily glycine due to the conformational requirements of the backbone.
The remaining residues are typically hydrophobic and form a hydrophobic core that binds and stabilizes the two helices.

Upon binding to Ca²⁺, this motif may undergo conformational changes that enable Ca²⁺-regulated functions as seen in Ca²⁺ effectors such as calmodulin (CaM) and troponin C (TnC) and Ca²⁺ buffers such as calreticulin and calbindin D9k.
While the majority of the known EF-hand Calcium-binding proteins (CaBPs) contain paired EF-hand motifs, CaBP’s with single EF hands have also been discovered in both bacteria and eukaryotes.
In addition, "EF-hand-like motifs" have been found in a number of bacteria.
Although the coordination properties remain similar with the canonical 29-residue helix-loop-helix EF-hand motif, the EF-hand-like motifs differ from EF-hands in that they contain deviations in the secondary structure of the flanking sequences and/or variation in the length of the Ca²⁺-coordinating loop.

EF hands have very high selectivity for calcium.
For example, the dissociation constant of alpha parvalbumin for Ca²⁺ is ~1000 times lower than that for the similar ion Mg²⁺. This high selectivity is due to the relatively rigid coordination geometry, the presence of multiple charged amino acid side chains in the binding site, as well as the ion solvation properties.
The calcium ion is coordinated in a pentagonal bipyramidal configuration. The six residues involved in the binding are in positions 1, 3, 5, 7, 9 and 12; these residues are denoted by X, Y, Z, -Y, -X and -Z. The invariant Glu or Asp at position 12 provides two oxygens for liganding Ca (bidentate ligand).
The calcium ion is bound by both protein backbone atoms and by amino acid side chains, specifically those of the acidic amino acid residues aspartate and glutamate. These residues are negatively charged and will make a charge-interaction with the positively charged calcium ion. The EF hand motif was among the first structural motifs whose sequence requirements were analyzed in detail. Five of the loop residues bind calcium and thus have a strong preference for oxygen-containing side chains, especially aspartate and glutamate. The sixth residue in the loop is necessarily glycine due to the conformational requirements of the backbone. The remaining residues are typically hydrophobic and form a hydrophobic core that binds and stabilizes the two helices.
Upon binding to Ca²⁺, this motif may undergo conformational changes that enable Ca²⁺-regulated functions as seen in Ca²⁺ effectors such as calmodulin (CaM) and troponin C (TnC) and Ca²⁺ buffers such as calreticulin and calbindin D9k. While the majority of the known EF-hand Calcium-binding proteins (CaBPs) contain paired EF-hand motifs, CaBP’s with single EF hands have also been discovered in both bacteria and eukaryotes. In addition, "EF-hand-like motifs" have been found in a number of bacteria. Although the coordination properties remain similar with the canonical 29-residue helix-loop-helix EF-hand motif, the EF-hand-like motifs differ from EF-hands in that they contain deviations in the secondary structure of the flanking sequences and/or variation in the length of the Ca²⁺-coordinating loop.
EF hands have very high selectivity for calcium. For example, the dissociation constant of alpha parvalbumin for Ca²⁺ is ~1000 times lower than that for the similar ion Mg²⁺. This high selectivity is due to the relatively rigid coordination geometry, the presence of multiple charged amino acid side chains in the binding site, as well as the ion solvation properties.
EF hand