Haemoglobin and haem

The haemoglobin molecule is formed by four polypeptide chains, or subunits, called alpha₁, alpha₂, beta₁ and beta₂ globins, non-covalently linked.

Each of the globins, like myoglobin, presents a high content of alpha helix.

Each globin molecule contains a haem molecule (rendered in red to remind it is responsible for the red colour of haemoglobin and blood).

The haem group consists of a protoporphyrin IX molecule complexed with an Fe²⁺ ion (now rendered in CPK colours, according to the chemical element; hydrogen atoms have been omitted).

Finally, let's see the location of the haem molecule (with a bound oxygen molecule) in one of the four chains of haemoglobin, rendered using a spacefilling model (spheres). Propionate groups in haem (hydrophilic) are oriented toward the protein surface, while the remainder of the molecule (hydrophobic) is buried among the nonpolar amino acids of the protein and the two histidines (proximal and distal).

Colour the hydrophobic and hydrophilic regions of haem.