Quaternary structure

This is the whole three-dimensional structure of a protein formed by several polypeptide chains (subunits).

A typical example is hemoglobin, which is formed by 4 globin subunits:
alpha₁, alpha₂, beta₁ and beta₂.

You can change rendering to or . Observe that most of the molecule has a secondary structure in alpha helix, and there are no beta strands.

Each globin subunit has an heme group bound (a protoporphyrin IX molecule complexed with an Fe²⁺ ion), which gives hemoglobin and blood their red color.

Click in this order:
1) for a close view of a group.
2) Let's put the iron atom in its . Observe the flat shape of heme.
3) In the lungs, where oxygen is abundant, an oxygen molecule binds to Fe²⁺ in each heme group. Later, it will be released in the tissues, where oxygen is needed for metabolism.
4) Iron is also bound to the N in the sidechain of an histidine in globin.
5) model.