Metalloenzymes
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large number of all proteins are part of this category.
It is estimated that approximately half of all proteins contain a metal. In another estimate, about one quarter to one third of all proteins are proposed to require metals to carry out their functions. Thus, metalloproteins have many different functions in cells, such as storage and transport of proteins, enzymes and signal transduction proteins. The role of metal ions in infectious diseases has been reviewed.
In metalloproteins, metal ions are usually coordinated by nitrogen, oxygen or sulfur centers belonging to amino acid residues of the protein. These donor groups are often provided by side-chains on the amino acid residues. Especially important are the imidazole substituent in histidine residues, thiolate substituents in cysteine residues, and carboxylate groups provided by aspartate. Given the diversity of the metalloproteome, virtually all amino acid residues have been shown to bind metal centers. The peptide backbone also provides donor groups; these include deprotonated amides and the amide carbonyl oxygen centers. Lead(II) binding in natural and artificial proteins has been reviewed.
In addition to donor groups that are provided by amino acid residues, a large number of organic cofactors function as ligands. Perhaps most famous are the tetradentate N4macrocyclic ligands incorporated into the heme protein. Inorganic ligands such as sulfide and oxide are also common.
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