These are metal complexes of proteins. In many cases, the metal ion is coordinated directly to functional groups on amino acid residues. In some cases, the protein contains a bound metallo-cofactor such as heme. In metalloproteins with more than one metal-binding site, the metal ions may be found in clusters. Examples include ferredoxins, which contain iron-sulfur clusters (Fe2S2 or Fe4S4), and nitrogenase, which contains both Fe4S4 units and a novel MoFe7S8 cluster. See also Protein.
Some metalloproteins are designed for the storage and transport of the metal ions themselves—for example, ferritin and transferrin for iron and metallothionein for zinc. Others, such as the yeast protein Atx1, act as metallochaperones that aid in the insertion of the appropriate metal ion into a metalloenzyme. Still others function as transport agents. Cytochromes and ferredoxins facilitate the transfer of electrons in various metabolic processes.
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