A group of compounds called porphyrins are widely distributed in plants, animals and bacteria. Porphyrins associate with metals to form metalloporphyrins which form a variety of compounds. For example chlorophyll about which is a magnesium-porphyrin complex and cytochromes which contain porphyrin group play an important role in intracellular oxidation. When ferrous iron Fe++ is added to a porphyrin protoporphyrin IX, ferrous porphyrin or haem is formed. The molecular structure is shown in figure.
Figure: Chemical structure of haem group (a), schematic representative of a single submit of haemoglobin (b).
The ferrous iron atom is bound to the four nitrogens of the protoporphyrin ring. The ferrous iron makes two more links, one with the oxygen atom to form oxyhaemoglobin and the other to the globin portion of the molecule at a single amino acid site.