Definition: A metalloprotein is a protein that contains a metal cofactor. The metal may be an isolated ion or may be coordinated with an nonprotein organic compound, such as the porphyrin group found in hemoproteins.
In some cases, the metal is coordinated with both a side chain of the protein and an inorganic nonmetallic ion. This type of protein-metal-nonmetal structure is found in iron-sulfur clusters.
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