page_title: Metalloprotein
section_title: Calmodulin
hierarchical_section_title: Metalloprotein / Signal-transduction metalloproteins / Calmodulin
caption_attribution_description: English: The helix-loop-helix EF-hand Ca2+ binding motif. (A), 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. The Ca2+ binding pocket adopts a pentagonal bipyramidal geometry. n stands for hydrophobic residue. (B), HMM logo for the canonical EF hand motif (http://pfam.sanger.ac.uk/family?acc=PF00036). The conservation of amino acids at several positions makes it possible to predict EF-hand motifs from genomic sequences. (C), 3D structure of a typical canonical EF-hand motif from calmodulin (PDB code: 3cln). Ca2+ is chelated by ligands from a 12-residue loop. (D), 3D structure of an EF-hand-like motif from a soluble fragment of lytic transglycosylase B of Escherichia coli (PDB code: 1qut). This motif contains a 15-residue (instead of 12-residue) Ca2+-binding loop flanked by two helices.
context_page_description: Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins contain zinc-binding protein domains although there may be up to 3000 human zinc metalloproteins.
context_section_description: Calmodulin is an example of a signal-transduction protein. It is a small protein that contains four EF-hand motifs, each of which is able to bind a Ca²⁺ ion.
  In an EF-hand loop the calcium ion is coordinated in a pentagonal bipyramidal configuration. Six glutamic acid and aspartic acid residues involved in the binding are in positions 1, 3, 5, 7 and 9 of the polypeptide chain. At position 12, there is a glutamate or aspartate ligand that behaves as a (bidentate ligand), providing two oxygen atoms. The ninth residue in the loop is necessarily glycine due to the conformational requirements of the backbone. The coordination sphere of the calcium ion contains only carboxylate oxygen atoms and no nitrogen atoms. This is consistent with the hard nature of the calcium ion.
  The protein has two approximately symmetrical domains, separated by a flexible "hinge" region. Binding of calcium causes a conformational change to occur in the protein. Calmodulin participates in an intracellular signaling system by acting as a diffusible second messenger to the initial stimuli.
caption: EF-hand motif
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