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The heme group, Fe-protoporphyrin IX, is one of nature's most widely used metal cofactors. Its roles are in transport of small molecules, electron transfer and catalysis. Different classes of heme proteins containing variants of the original heme group are known today, and among these our focus is on the cytochrome c family. Here, the cofactor is covalently linked to the protein chain via thioether bonds to the cysteine residues of a conserved binding motif, and this has a series of important structural and functional consequences. For once, the cofactor cannot easily be lost, so that c-type cytochromes are ideal extracellular proteins, or are located in 'extracytoplasmic' compartments such as the periplasm of Gram-negative bacteria or the intermembrane space of mitochondria or chloroplasts. Second, the attachment of heme groups to the peptide chains occurs before the protein folds, allowing for a very high heme:protein ratio. This gives rise to cytochrome with many hemes attached to a rather short peptide: multiheme cytochromes c.

Multiheme cytochromes c act as electron carriers or as redox enzymes that can catalze multi-electron transfer reactions.  


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