The heme portion of the hemoglobin molecule consists of:
Structurally, porphyrin consists of four pyrrole rings (five-membered closed structures containing one nitrogen and four carbon atoms) linked to each other by methine groups (−CH=).
The iron atom is kept in the centre of the porphyrin ring by interaction with the four nitrogen atoms.
Porphyrin ring with a molecule of Fe in the center
A polypeptide chain containing Fe
A pyrole ring with four molecules of Fe in the center
Four porphyrin rings, each containing a molecule of Fe in the center