{c}

An \b[EF hand] is a <protein structural motif> in which two <alpha helices> flank a loop. In a canonical calcium-binding EF hand, the loop contains 12 <amino acid residues> whose oxygen-containing groups organize a <calcium ion> site. PROSITE identifies coordinating positions 1, 3, 5, 7, 9 and 12; the final <glutamate> or <aspartate> can contribute two oxygen atoms. The motif is an arrangement of a chain, not a literal separate hand-shaped molecule.<EF-hand calcium-binding domain signature and profile in PROSITE>[PDOC00018, Description: loop organization and coordinating positions]{ref}

Structural evidence makes the arrangement concrete. The deposited <calmodulin> structure PDB 1CLL was determined by <X-ray crystallography> at $1.70\,\text{Å}$ resolution. Its model contains four calcium ions and two EF-hand binding loops in each lobe. A resolved ion and surrounding coordinates support a structural binding-site interpretation; they do not by themselves specify the rate or affinity of binding in every solution condition.<Calmodulin structure 1CLL in the Protein Data Bank>[Experimental Data Snapshot and Primary Citation abstract]{ref}

An original one-site model illustrates the distinction. If a site binds calcium independently with <dissociation constant> $K_d$ and free calcium concentration $c$, the equilibrium occupied fraction is
$$
\theta=\frac{c}{K_d+c}.
$$
At $c=K_d$, half the sites are occupied; at $c=10K_d$, the fraction is $10/11$. These values follow from the stipulated model and are not affinity measurements for PDB 1CLL.

EF hands commonly occur as interacting pairs, and atypical or altered relatives need not retain the same binding properties.<EF-hand calcium-binding domain signature and profile in PROSITE>[Description: paired motifs, cooperativity and excluded nonbinding/atypical regions]{ref} Counting motifs alone therefore cannot justify treating every site as an identical independent binder. Structural arrangement, sequence chemistry and binding measurements provide complementary information.


 EF hand (source code)