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Porphyrins are tetrapyrroles

    They consist of four pyrrole rings (which are weakly aromatic) joined by methene bridges.

The names of the porphyrins of interest consist of a word and a number,
uroporphyrin III

    The word denotes the kinds of substituents found on the ring,

    The number denotes how the substituents are arranged.

    Iron can be added to protoporphyrin IX. The oxidation state of the iron ion and the identity of the negatively charged counterion determine the name of the product.

    If iron (II), or ferrous iron, is added the product is heme:
If iron (III) - or ferric iron - is added, the product is
  • hemin if the counterion is chloride.
  • hematin if the counterion is hydroxide.

    Porphyrinogens are more reduced than the corresponding porphyrins, so oxidation of a porphyrinogen (pronounce) produces the corresponding porphyrin.

Here is a reaction in which the enzyme, protoporphyrinogen IX oxidase, converts protoporphyrinogen (pronounce) IX to protoporphyrin (pronounce) IX. The porphyrins contain a system of conjugated double bonds all around the tetrapyrrole ring. Resonance of this system makes the porphyrins more stable than the corresponding porphyrinogens:

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