II. Definitions

  1. Porphyrin
    1. Porphyrins are cyclic molecule composed of four pyrrole rings connected by methine bridges
    2. Porphyrins have a central mineral (e.g. iron in heme, cobalt in Vitamin B12, Magnesium in chlorophyll)
    3. Porphyrins vary by their side chains and central mineral
    4. Porphyrins are formed from the single pyrrol ringed molecule, Porphobilinogen
  2. Protoporphyrins
    1. Protoporphyrins are Porphyrins that have four methyl, two vinyl, and two propionic acid side chains attached to their pyrrole rings
  3. Protoporphyrin 9
    1. Protoporphyrin 9 occurs in heme, which is a the prosthetic group in Hemoglobin, myoglobin, and most cytochromes
    2. Protopoprphyrin 9 is activated when Protoporphyrinogen's methylene bridge is cleaved by the enzyme Protoporphyrinogen oxidase
  4. Heme
    1. hemoglobin.jpg
    2. Heme is a Protoporphyrin 9 based prosthetic group, with a central ferrous 2+ ion (iron)
    3. Heme in combination with globin Protein, forms Hemoglobin
    4. Heme is also the the prosthetic group for myoglobin and most cytochromes (combined with different globin Proteins)
      1. Unlike Hemoglobin which contains 4 polypeptide chains, myoglobin contains only 1 polypeptide chain
    5. While heme in Hemoglobin transports oxygen, the heme in cytochromes transport electrons (Electron Transport Chain)

III. Physiology: Heme Metabolism

  1. Hemoglobin breakdown occurs when Red Blood Cells are destroyed (e.g. in Spleen at the end of their roughly 120 day life cycle)
  2. Hemoglobin is converted into Biliverdin and ultimately into Bilirubin for excretion
  3. See Bilirubin

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