Phosphatidylinositol

Phosphatidylinositol (abbreviated PtdIns, or PI) is a negatively charged phospholipid and a minor component in the cytosolic side of eukaryotic cell membranes.

The inositol can be phosphorylated to form phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate (PIP2) and phosphatidylinositol trisphosphate (PIP3). PIP, PIP2 and PIP3 are collectively called phosphoinositides.

Biosynthesis
The synthesis of phosphatidylinositol is catalyzed by phosphatidylinositol synthase and involves CDP-diacylglycerol and L-myo-inositol. .

Chemistry
PI has a polar and non-polar region, making the lipid an amphiphile. Amphiphatic lipids demonstrate polymorphic behavior, a current academic research topic. Phosphatidylinositol is classified as a glycerophospholipid that contains a glycerol backbone, two non-polar fatty acid tails, a phosphate group substituted with an inositol polar head group.

The most common fatty acids of phosphoinositides are stearic acid in the SN1 position and arachidonic acid, in the SN2 position. Hydrolysis of phosphoinositides yield one mole of glycerol, two moles of fatty acids, one mole of inositol and one, two, or three moles of phosphoric acids, depending on the number of phosphates on the inositol rings. Phosphoinositides are regarded as the most acidic phospholipid.

Phosphoinositides
Phosphorylated forms of phosphatidylinositol are called phosphoinositides and play important roles in lipid signaling, cell signaling and membrane trafficking. The inositol ring can be phosphorylated by a variety of kinases on the three, four and five hydroxyl groups in seven different combinations. However, the two and six hydroxyl group is typically not phosphorylated due to steric hindrance.

All seven variations of the following phosphoinositides have been found in animals:

Phosphatidylinositol monophosphates:
 * Phosphatidylinositol 3-phosphate, also known as PtdIns3P or PI(3)P
 * Phosphatidylinositol 4-phosphate, also known as PtdIns4P or PI(4)P
 * Phosphatidylinositol 5-phosphate, also known as PtdIns5P or PI(5)P

Phosphatidylinositol bisphophosphates:
 * Phosphatidylinositol 3,4-bisphosphate, also known as PtdIns(3,4)P or PI(3,4)P2
 * Phosphatidylinositol 3,5-bisphosphate, also known as PtdIns(3,5)P or PI(3,5)P2
 * Phosphatidylinositol 4,5-bisphosphate, also known as PtdIns(4,5)P or PI(4,5)P2

Phosphatidylinositol trisphophosphate:
 * Phosphatidylinositol 3,4,5-trisphosphate, also known as PtdIns(3,4,5)P or PI(3,4,5)P3

These phosphoinositides are also found in plant cells, with the exception of PIP3.