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Heme molecule
Heme molecule. Heme-containing proteins have a diverse set of functions, including small-molecule transport, catalysis for bioorganic synthesis and electron transfer. Heme consists of a macrocyclic porphyrin ligand with an iron atom (orange) embedded in the porphyrin cavity. The metal ion is held in place by coordinate covalent bonds to the four nitrogen atoms (blue) of the macrocyclic ring. Heme is bonded to its protein by at least one axial histidine residue. The sixth metal site most often is either a small ligand (oxygen, water, etc.) or a second amino acid side chain.
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