phospholipid

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A lipid that contains one or more phosphate groups. Phospholipids are amphipathic in nature; that is, each molecule consists of a hydrophilic (water-loving) portion and a hydrophobic (water-hating) portion. Due to the amphipathic nature and insolubility in water, phospholipids are ideal compounds for forming the biological membrane. See also Lipid.

There are two classes of phospholipids: those that have a glycerol backbone and those that contain sphingosine. Both classes are present in the biological membrane. Phospholipids that contain a glycerol backbone are called phosphoglycerides (or glycerophospholipids), which are the most abundant class of phospholipid found in nature. The most abundant types of naturally occurring phosphoglyceride are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, and cardiolipin. The structural diversity within each type of phosphoglyceride is due to the variability of the chain length and degree of saturation of the fatty acid ester groups.

Sphingomyelin is the major sphingosine-containing phospholipid. Its general structure consists of a fatty acid attached to sphingosine by an amide linkage.

A bilayer membrane is formed spontaneously when phospholipids are dispersed in an aqueous solution. In this bilayer structure, phospholipids are arranged in two leaflets with the hydrophobic tails facing each other, and the hydrophilic ends exposed to the aqueous medium. Differences in the head group, the chain length, and the degree of saturation of fatty acids in the hydrophobic end are important factors in determining the shape of the bilayer. Individual phospholipid molecules are able to move freely in the lateral plane of the bilayer but not in the transverse plane (flip-flop). Small uncharged molecules are able to diffuse through the bilayer structure, but the permeability of larger or charged molecules is restricted. The arrangement of phospholipid molecules into a bilayer in an aqueous medium follows the laws of thermodynamics and represents the structural basis for the formation of all biological membranes.

For a long time, phospholipids were regarded as merely building blocks for the biological membrane. It was discovered in the mid-1970s, however, that phospholipids participate in the transduction of biological signals across the membrane. For example, when the hormone vasopressin is bound to its receptor on the plasma membrane of a liver cell, the binding sets off a cascade of reactions which result in enhanced breakdown of glycogen in the liver cell, thus producing more glucose. See also Second messengers.

A special form of phosphoglyceride, 1-alkyl-2-acetyl-glycero-3-phosphocholine, acts as a very powerful biological mediator. It causes the aggregation and degranulation of blood platelets, and is known as platelet-activating factor (PAF).

Phospholipases are responsible for the degradation of phosphoglycerides. These enzymes are found in all tissues and in the pancreatic juice. A number of toxins and venoms have very high phospholipase activity, and several pathogenic bacteria produce phospholipases that dissolve cell membrane and allow the spread of infection. There are very few inherited diseases associated with the metabolism of phosphoglycerides; presumably, such genetic defects would be lethal during the early stage of cellular development.

Sphingomyelinase, a lysosomal enzyme, hydrolytically degrades sphingomyelin. A genetic disorder caused by a defect in the production of sphingomyelinase, called Niemann-Pick disease, leaves the cell with no or limited or ability to degrade sphingomyelin. In a severe form (type A) of this disease, the liver and spleen are sites of lipid deposits and are therefore tremendously enlarged. The lipid deposits consist primarily of the sphingomyelin that cannot be degraded. See also Lipid metabolism.

>Glycerol esterified to two molecules of fatty acid, one of which is commonly polyunsaturated. The third hydroxyl group is esterified to phosphate and one of a number of water-soluble compounds, including serine (phosphatidylserine), ethanolamine (phosphatidylethanolamine), choline (phosphatidylcholine, also known as lecithin), and inositol (phosphatidylinositol).

Cell membranes are a double layer of phospholipids with the fatty acid side-chains on the inside and the water-soluble compound esterified to the phosphate interacts with water. This is why phospholipids can be used to emulsify oils and fats in water and are commonly used in food manufacture as emulsifiers.

From the energy point of view they can be regarded as being equivalent to simple fats (triacylglycerols); they also provide a dietary source of choline and inositol, neither of which is a dietary essential.

An organic compound made from a combination of glycerol, two fatty acids, and a phosphate group. Phospholipids are major components of all cell membranes. They are involved in the transport of fat in the blood and lymph, and also take part in many other metabolic reactions throughout the body.

One of a class of compounds, widely distributed in living cells, containing phosphoric acid, fatty acids, and a nitrogenous base.

An organic compound consisting of a hydrophobic tail of two chains of fatty acid, and a hydrophilic head of glycerol and phosphate. Phospholipid is a major component of all cell membranes, and is involved in fat transport in blood and lymph. Phospholipids are also involved in many metabolic reactions.

Any lipid that contains phosphorus, including those with a glycerol backbone (phosphoglycerides and plasmalogens) or a backbone of sphingosine or a related substance (sphingomyelins). They are the major lipids in cell membranes.

• p. bilayer — a layer containing two phospholipid molecules which is the basic structural unit of all biological membranes.

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Phospholipid

Polar group of the molecule, highlighted in red.
The U indicates the uncharged hydrophobic portion of the molecule, highlighted in blue.

Phosphatidyl choline is the major component of lecithin. It is also a source for choline in the synthesis of acetylcholine in cholinergic neurons.

Phospholipids are a class of lipids and are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from sphingosine instead of glycerol.

Contents 1 Amphipathic character 2 Types of phospholipid 2.1 Diacylglyceride structures 2.2 Other structures 3 Simulations 4 Phospholipid synthesis 5 In signal transduction 6 Food technology 7 See also 8 References

Amphipathic character

The 'head' of a phospholipid is hydrophilic (attracted to water), while the hydrophobic 'tails' repel water. The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups. The hydrophobic tail usually consists of long fatty acid hydrocarbon chains. When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid. These specific properties allow phospholipids to play an important role in the phospholipid bilayer. In biological systems, the phospholipids often occur with other molecules (e.g., proteins, glycolipids, cholesterol) in a bilayer such as a cell membrane.^[1] Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane with hydrophilic heads on both sides facing the water.

This type of membrane is partially permeable, capable of elastic movement, and has fluid properties, in which embedded proteins (integral or peripheral proteins) and phospholipid molecules are able to move laterally. Such movement can be described by the Fluid Mosaic Model, that describes the membrane as a mosaic of lipid molecules that act as a solvent for all the substances and proteins within it, so proteins and lipid molecules are then free to diffuse laterally through the lipid matrix and migrate over the membrane. Cholesterol contributes to membrane fluidity by hindering the packing together of phospholipids. However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple.

Types of phospholipid

Diacylglyceride structures

See: Glycerophospholipid

Phosphatidic acid (phosphatidate)

Phosphatidylethanolamine (cephalin)

Phosphatidylcholine (lecithin)

Phosphatidylserine

Phosphoinositides: Phosphatidylinositol, phosphatidylinositol phosphate, phosphatidylinositol bisphosphate and phosphatidylinositol triphosphate.

Other structures

Sphingomyelin

Simulations

Modeling of phospholipids is usually made by the method of molecular dynamics in force fields such as GROMACS, CHARMM and their modifications[1].

Phospholipid synthesis

Phospholipid synthesis occurs in the cytosol adjacent to ER membrane that is studded with proteins that act in synthesis (GPAT and LPAAT acyl transferases, phosphatase and choline phosphotransferase) and allocation (flippase and floppase). Eventually a vesicle will bud off from the ER containing phospholipids destined for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids destined for the exoplasmic cellular membrane on its inner leaflet.^[2]

In signal transduction

Some types of phospholipid can be split to produce products that function as second messengers in signal transduction. Examples include phosphatidylinositol (4,5)-bisphosphate (PIP₂), that can be split into inositol triphosphate (IP₃) and diacylglycerol (DAG), which both carry out the functions of the G_q type of G protein in response to various stimuli.

Food technology

Phospholipids can also act as an emulsifiers, enabling oils to dissolve in water. Phospholipids called lecithin are extracted out of cooking oil and then used as food additives in many things such as bread and can also be purchased separately in a health food store.

See also

Lipids

References

1. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html. 
2. ^ Lodish, Harvey; Berk, Krieger, Kaiser, Scott, Bretsher, Ploegh, Matsuaira (2008). Molecular Cell Biology. W.H. Freeman and Company. ISBN 0716776014. http://bcs.whfreeman.com/lodish6e/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0. 

• J.M.Berg, J.L. Tymoczko, and L. Stryer, Biochemistry. 5th ed. 2002, New York: W.H. Freeman. viii, 974, [976] (various pages)

v • d • e Lipids: membrane lipids Lipid bilayer - Phospholipids - Proteolipids - Sphingolipids - Sterols

v • d • e Lipids: phospholipids Glycerol backbone (Glycerophospholipids/ Phosphoglycerides) Phosphatidyl-: -ethanolamine/cephalin - -choline/lechithin (dipalmitoyl-) - -serine - -glycerol - -inositol (glyco-) Phosphoinositides: PIP (PI(3)P, PI(4)P, PI(5)P) - PIP2 (PI(3,4)P2, PI(3,5)P2, PI(4,5)P2) - PIP3 Cardiolipin Ether lipids: Plasmalogen (Platelet-activating factor) Sphingosine backbone Sphingomyelin

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