Figure 1. Phosphatidylinositol.

An inositol headgroup is bound to phosphatidic acid via its 1′-hydroxyl group. Phosphatidic acid is composed of a glycerol backbone with fatty acids bound to carbons-1 and -2 (represented by waves in this graphic) and a phosphate group bound to carbon-3. The 4′- and 5′-hydroxyl groups are phosphorylated in PIP₂. PI3K generates PIP₃ by phosphorylating the 3′-hydroxyl group.