How proteins fold will be one of the main topics of Chapter 22, but the general idea is outlined here.

In hemoglobin in blood, and in the closely related oxygen-storage protein myoglobin (right), several regions along the polypeptide chain fold spontaneously into a helical structure known as an helix (shown in blue).

These rodlike helices then fold against one another, with "hinges" at nonhelical parts of the chain between them (shown in white), to build a three-dimensional molecule.

The correct folding together of helices is guided by having hydrocarbon side chains spaced along the main chain so one side of each cylindrical helix is covered with hydrocarbon groups, and the opposite side is polar and charged.

When the helices spontaneously fold against one another in such a way as to bury the hydrocarbon side chains away from the aqueous surroundings, the proper three-dimensional structure is formed.