Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the phosphatidylglycerides. In such molecules the isomer of the

inositol Inositol, or more precisely ''myo''-inositol, is a carbocyclic sugar that is abundant in the brain and other mammalian tissues; it mediates cell signal transduction in response to a variety of hormones, neurotransmitters, and growth factors and ...

group is assumed to be the myo- conformer unless otherwise stated. Typically phosphatidylinositols form a minor component on the cytosolic side of eukaryotic

cell membranes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. The

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

group gives the molecules a negative charge at physiological pH. The form of phosphatidylinositol comprising the isomer ''muco''-inositol acts as a sensory receptor in the taste function of the sensory system. In this context it is often referred to as PtdIns, but that does not imply any molecular difference from phosphatidylinositols comprising the myo- conformers of inositol. The phosphatidylinositol can be phosphorylated to form phosphatidylinositol phosphate (PI-4-P, referred to as PIP in close context or informally), phosphatidylinositol bisphosphate (PIP₂) and phosphatidylinositol trisphosphate (PIP₃). All lipids based on phosphatidylinositol are known as inositides, or sometimes phosphoinositides.

Biosynthesis

The synthesis of phosphatidylinositol in the laboratory 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 An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...

. Phosphatidylinositol is classified as a glycerophospholipid that contains a

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

backbone, two non-polar

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

tails, a

group substituted with an

polar head group. The most common fatty acids of phosphoinositides are

stearic acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means tallow. ...

in the SN₁ position and

arachidonic acid Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6), or 20:4(5,8,11,14). It is structurally related to the saturated arachidic acid found in cupuaçu butter. Its name derives from the New Latin word ''arachi ...

, 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 phospholipids. The specific fatty acids of PtdIns, and their conformation, employed in the sensory neurons has not been elucidated.

Phosphoinositides

Phosphorylated forms of phosphatidylinositol (PI) are called phosphoinositides and play important roles in

lipid signaling Lipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular r ...

cell signaling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...

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 groups are typically not phosphorylated due to steric hindrance. All seven variations of the following phosphoinositides have been found in animals: Phosphatidylinositol monophosphates: *

Phosphatidylinositol 3-phosphate Phosphatidylinositol 3-phosphate (PtdIns3''P'') is a phospholipid found in cell membranes that helps to recruit a range of proteins, many of which are involved in protein trafficking, to the membranes. It is the product of both the class II and II ...

, also known as PtdIns3''P'' or PI(3)P *

Phosphatidylinositol 4-phosphate Phosphatidylinositol-4-phosphate (PtdIns4''P'', PI-4-P, PI4P, or PIP) is a precursor of phosphatidylinositol (4,5)-bisphosphate. PtdIns4''P'' is prevalent in the membrane of the Golgi apparatus. In the Golgi apparatus, PtdIns4''P'' binds to the GT ...

, also known as PtdIns4''P'' or PI(4)P *

Phosphatidylinositol 5-phosphate Phosphatidylinositol 5-phosphate (PtdIns5P) is a phosphoinositide, one of the phosphorylated derivatives of phosphatidylinositol (PtdIns), that are well-established membrane-anchored regulatory molecules. Phosphoinositides participate in signaling ...

, also known as PtdIns5''P'' or PI(5)P Phosphatidylinositol bisphosphates: *

Phosphatidylinositol 3,4-bisphosphate Phosphatidylinositol (3,4)-bisphosphate (PtdIns(3,4)''P''2) is a minor phospholipid component of cell membranes, yet an important second messenger. The generation of PtdIns(3,4)''P''2 at the plasma membrane activates a number of important cell sig ...

, also known as PtdIns(3,4)''P''₂ or PI(3,4)P₂ *

Phosphatidylinositol 3,5-bisphosphate Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) is one of the seven phosphoinositides found in eukaryotic cell membranes. In quiescent cells, the PtdIns(3,5)P2 levels, typically quantified by HPLC, are the lowest amongst the constitutively pr ...

, also known as PtdIns(3,5)''P''₂ or PI(3,5)P₂ *

Phosphatidylinositol 4,5-bisphosphate Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)''P''2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)''P''2 is enriched at the plasma membrane where it is a substrate for a number of ...

, also known as PtdIns(4,5)''P''₂, PI(4,5)P₂ or often simply referred to as PIP₂ Phosphatidylinositol trisphosphate: *

Phosphatidylinositol 3,4,5-trisphosphate Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)''P''3), abbreviated PIP3, is the product of the class I phosphoinositide 3-kinases (PI 3-kinases) phosphorylation of phosphatidylinositol (4,5)-bisphosphate (PIP2). It is a phospholipid t ...

, also known as PtdIns(3,4,5)''P''₃ or PI(3,4,5)P₃ These phosphoinositides are also found in plant cells, with the exception of PIP₃.

References

Additional images

Image:Membrane_Lipids.svg, Membrane lipids Image:Phosphatidylinositol_core_skeletal.svg, Phosphatidyl-inositol Image:Myo-inositol structure.svg,

Inositol Inositol, or more precisely ''myo''-inositol, is a carbocyclic sugar that is abundant in the brain and other mammalian tissues; it mediates cell signal transduction in response to a variety of hormones, neurotransmitters, and growth factors and ...

Image:Glycerine chemical structure.png,

Glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

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