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Porphyrins are not complexed with metal ions.

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Porphyrins are a macrocyclic compounds containing four pyrole rings. Can you identify them in the scene to the right? ) The four pyrole rings are connected together by four . Porphyrins are distinguished from each other by the substituents attached to the pyrole rings. Can you identify the pyrole substituents on this porphyrin (protoporphyrin IX)? They are , and groups. An important function of porphyrins is to form by complexing Fe2+ or Fe3+ with the lone pairs of electrons on the four nitrogens. Heme displayed as . Since Fe2+ and Fe3+ prefer to complex with six ligands, there are two positions open for nucleophilic groups to bond whenever the heme associates with a protein. Both of these groups may be residues of a protein, or one of them may be a small substrate molecule. In one of these two ways the heme becomes covalently connected to the protein, and therefore can be called a prosthetic group. Residues that are commonly found as the fifth or sixth ligands are His, Cys and Met. Examples of the fifth ligand being His and the sixth ligand being a small molecule are oxymyoglobin and oxyhemoglobin.

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