Metalloproteins are proteins containing a metal ion cofactor that is required for the protein's biological activity. It is estimated that approximately half of all proteins contain a metal. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. In metalloproteins, metal ions are usually coordinated by nitrogen, oxygen or sulfur centres belonging to amino acid residues of the protein. In addition to donor groups that are provided by amino acid residues, a large number of organic cofactors function as ligands.