Protoporphyrin complexes containing metal ions have a large role in living organisms. The metal central atom, usually in the oxidation state (2+), lies between four pyrrole rings. Metal complexes derived from porphyrins occur naturally. One of the best-known families of porphyrin complexes is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. What kind of metal ion is contained in the heme protoporphyrin complex?

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^Hemoglobin

O2 transport from the lungs to the cells