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 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
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 phosph ...
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
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
to form
phosphatidylinositol phosphate (PI-4-P, referred to as PIP in close context or informally),
phosphatidylinositol bisphosphate (PIP
2) and
phosphatidylinositol trisphosphate (PIP
3). 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. Phosphatidylinositol is classified as a
glycerophospholipid
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.
Structures
The t ...
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, ...
tails, a
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 phosph ...
group substituted with an
inositol polar head group.
The most common fatty acids of phosphoinositides are
stearic acid in the SN
1 position and
arachidonic acid, in the SN
2 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 acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
s, 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,
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 Membrane vesicle trafficking in eukaryotic animal cells involves movement of biochemical signal molecules from synthesis-and-packaging locations in the Golgi body to specific release locations on the inside of the plasma membrane of the secretory ...
. The inositol ring can be
phosphorylated
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
by a variety of
kinases
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
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
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
.
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 ...
, also known as PtdIns3''P'' or PI(3)P
*
Phosphatidylinositol 4-phosphate, also known as PtdIns4''P'' or PI(4)P
*
Phosphatidylinositol 5-phosphate, also known as PtdIns5''P'' or PI(5)P
Phosphatidylinositol bisphosphates:
*
Phosphatidylinositol 3,4-bisphosphate, also known as PtdIns(3,4)''P''
2 or PI(3,4)P
2
*
Phosphatidylinositol 3,5-bisphosphate, also known as PtdIns(3,5)''P''
2 or PI(3,5)P
2
*
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 o ...
, also known as PtdIns(4,5)''P''
2, PI(4,5)P
2 or often simply referred to as PIP
2
Phosphatidylinositol trisphosphate:
*
Phosphatidylinositol 3,4,5-trisphosphate, also known as PtdIns(3,4,5)''P''
3 or PI(3,4,5)P
3
These phosphoinositides are also found in plant cells, with the exception of PIP
3.
See also
*
PI 3-kinase
*
Inositol phosphate
Inositol phosphates are a group of mono- to hexaphosphorylated inositols. They play crucial roles in diverse cellular functions, such as cell growth, apoptosis, cell migration, endocytosis, and cell differentiation.
The group comprises:
* inosito ...
*
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 ...
*
Phosphatidylinositol 4-phosphate
*
Phosphatidylinositol 5-phosphate
*
Phosphatidylinositol (3,4)-bisphosphate
*
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 p ...
*
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 ...
*
Inositol 1,4,5-triphosphate
*
Phosphatidylinositol (3,4,5)-trisphosphate
*
inositol pentakisphosphate
*
inositol hexaphosphate
Phytic acid is a six-fold dihydrogenphosphate ester of inositol (specifically, of the ''myo'' isomer), also called inositol hexakisphosphate (IP6) or inositol polyphosphate. At physiological pH, the phosphates are partially ionized, resulting in ...
*
inositol triphosphate receptor
References
Additional images
Image:Membrane_Lipids.svg, Membrane lipids
Membrane lipids are a group of compounds (structurally similar to fats and oils) which form the double-layered surface of all cells (lipid bilayer). The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. Lipid ...
Image:Phosphatidylinositol_core_skeletal.svg, Phosphatidyl-inositol
Image:Myo-inositol structure.svg, Inositol
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 ...
External links
*
Phosphatidylinositol at Lipid Library
{{Phospholipids
Phospholipids
Membrane biology