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Phosphatidylinositol
Other names	PI, PtdIns
Properties
Molecular formula	C47H83O13P
Molar mass	886.56 g/mol, neutral with fatty acid composition - 18:0, 20:4
	Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
	Infobox references

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.

[edit] Biosynthesis

Biosynthesis of phosphatidylinositol catalyzed by phosphatidylinositol synthase. Figure adapted from Christopher, K.; van Holde, K.E.; Ahern, Kevin G. Biochemistry Third Edition. Pearson Education, Inc: Singapore, 2005; p 678.^[1].

The synthesis of phosphatidylinositol is catalyzed by phosphatidylinositol synthase and involves CDP-diacylglycerol and L-myo-inositol.^[1].

[edit] 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 the 18:0 in the SN₁ position and 20:4, 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.

[edit] 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^[2].

[edit] References

^ ^a ^b Mathews, Chrisotphe K.; van Holde, K.E.; Ahern, Kevin G., (2005). Biochemistry Third Edition.
^ Muller-Roeber B, Pical C; , (2002). Inositol Phospholipid Metabolism in Arabidopsis. Characterized and Putative Isoforms of Inositol Phospholipid Kinase and Phosphoinositide-Specific Phospholipase C.