Heme

A molecule found in red blood cells, the part of hemoglobin that carries oxygen. Hundreds of hemes are attatched to proteins known as Globins, hence "hemoglobin". The iron atom at a heme's center gives oxygenated blood its characteristic red color. The folks on the other side of the pond spell the word as haem.

Bilirubin is the principal breakdown product of a heme.

      H-C-H        H
        "          |
        C-H      H-C-H
        |          |
   H   //\   //\  //\     H
   |  //  \ //  \//  \    |
 H-C-<'    |    |    />-C=C
   |  \    |    |   //  | |
   H   >---N    N---<   H H
      /     \  /     \
     <\      Fe      />
      \\    /  \    //
   H   >---N    N---<   H
   |  /    |    ||   \  |
 H-C-<     |    ||    >-C-H
   |  \\  / \\  /\  //  |
   H   \\/   \\/  \//   H
        |          |
      H-C-H      H-C-H
        |          |
      H-C-H      H-C-H
        |          |
      O=C-O-H  H-O-C=O
 
 

The iron-porphyrin group in heme proteins such as hemoglobin.

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From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.