Glycerophospholipids or phosphoglycerides are

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

-based

phospholipid Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s. They are the main component of

biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of th ...

s in

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

cells. They are a type of

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

, of which its composition affects membrane structure and properties. Two major classes are known: those for

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s and a separate family for

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

Structures

Glycerophospholipids are derived from glycerol-3-phosphate in a de novo pathway. The term glycerophospholipid signifies any derivative of glycerophosphoric acid that contains at least one ''O''- acyl, or ''O''-

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

, or ''O''-alk-1'-enyl residue attached to the glycerol moiety. The phosphate group forms an ester linkage to the glycerol. The long-chained hydrocarbons are typically attached through ester linkages in bacteria/eukaryotes and by ether linkages in archaea. In bacteria and procaryotes, the lipids consist of diesters commonly of C16 or C18 fatty acids. These acids are straight-chained and, especially for the C18 members, can be unsaturated. For archaea, the hydrocarbon chains have chain lengths of C10, C15, C20 etc. since they are derived from isoprene units. These chains are branched, with one methyl substituent per C5 subunit. These chains are linked to the glycerol phosphate by ether linkages. The two hydrocarbon chains attached to the glycerol are hydrophobic while the polar head, which mainly consists of the phosphate group attached to the third carbon of the glycerol backbone, is hydrophilic. This dual characteristic leads to the amphipathic nature of glycerophospholipids. They are usually organized into a bilayer in membranes with the polar hydrophilic heads sticking outwards to the aqueous environment and the non-polar hydrophobic tails pointing inwards. Glycerophospholipids consist of various diverse species which usually differ slightly in structure. The most basic structure is a phosphatidate. This species is an important intermediate in the synthesis of many phosphoglycerides. The presence of an additional group attached to the phosphate allows for many different phosphoglycerides. By convention, structures of these compounds show the 3 glycerol carbon atoms vertically with the phosphate attached to carbon atom number three (at the bottom). Plasmalogens and phosphatidates are examples.

Nomenclature and stereochemistry

In general, glycerophospholipids use an "sn" notation, which stands for stereospecific numbering. When the letters ''"sn"'' appear in the nomenclature, by convention the

hydroxyl group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

of the second carbon of glycerol (2-''sn'') is on the left on a Fischer projection. The numbering follows the one of Fischer's projections, being 1-''sn'' the carbon at the top and 3-''sn'' the one at the bottom. The advantage of this particular notation is that the spatial configuration (D or L) of the glycero-molecule is determined intuitively by the residues on the positions ''sn''-1 and ''sn''-3. For example ''sn''-glycero-3-

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, ...

and ''sn''-glycero-1-phosphoric acid are

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s. Most vegetable oils have unsaturated fatty acids in the ''sn''-2 position, with saturated fatty acids in the 1-''sn'' and/or 3-''sn'' position. Animal fats more often have saturated fatty acids in the 2-''sn'', with unsaturated fatty acids in the 1-''sn'' and/or 3-''sn'' position.

Examples

;Plasmalogens

Plasmalogen Plasmalogens are a class of glycerophospholipid with a plasmenyl group linked to a lipid at the sn-1 position of the glycerol backbone. Plasmalogens are found in multiple domains of life, including Mammal, mammals, Invertebrate, invertebrates, p ...

s are a type of phosphoglyceride. The first carbon of glycerol has a hydrocarbon chain attached via an ether, not ester, linkage. The linkages are more resistant to chemical attack than ester linkages are. The second (central) carbon atom has a fatty acid linked by an ester. The third carbon links to an ethanolamine or choline by means of a phosphate ester. These compounds are key components of the membranes of muscles and nerves. ;Phosphatidates Phosphatidates are lipids in which the first two carbon atoms of the glycerol are fatty acid esters, and the 3 is a phosphate ester. The phosphate serves as a link to another alcohol-usually ethanolamine, choline, serine, or a carbohydrate. The identity of the alcohol determines the subcategory of the phosphatidate. There is a negative charge on the phosphate and, in the case of choline or serine, a positive quaternary ammonium ion. (Serine also has a negative carboxylate group.) The presence of charges give a "head" with an overall charge. The phosphate ester portion ("head") is hydrophilic, whereas the remainder of the molecule, the fatty acid "tail", is hydrophobic. These are important components for the formation of lipid bilayers. Phosphatidylethanolamines, phosphatidylcholines, and other phospholipids are examples of phosphatidates. ;Phosphatidylcholines Phosphatidylcholines are

lecithin Lecithin ( ; from the Ancient Greek "yolk") is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues which are amphiphilic – they attract both water and fatty substances (and so ar ...

s. Choline is the alcohol, with a positively charged quaternary ammonium, bound to the phosphate, with a negative charge. Lecithins are present in all living organisms. An egg yolk has a high concentration of lecithins, which are commercially important as an emulsifying agent in products such as mayonnaise. Lecithins are also present in brain and nerve tissue. ;Phosphatidylinositol

Phosphatidylinositol Phosphatidylinositol or inositol phospholipid is a biomolecule. It was initially called "inosite" when it was discovered by Léon Maquenne and Johann Joseph von Scherer in the late 19th century. It was discovered in bacteria but later also found ...

makes up a small component of the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

in eukaryotic cell membranes and gives molecules a negative charge. Its importance relies in its role in activating sensory receptors that correlate with taste functions. ;Phosphatidylserine Phosphatidylserine is important in

cell signaling In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...

, specifically

apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

. Cells will use this phosphatidylserine to enter cells via apoptotic mimicry. The structure of this lipid differs in plants and animals, regarding fatty acid composition. In addition, phosphatidylserine plays an important role in the human brain content, as it makes up 13–15% of the phospholipids in the human cerebral cortex. This lipid is found in a wide range of places. For example, in the human diet, about 130 mg are derived from phosphatidylserine. This has been said to have a positive impact on the brain, as it helps with reduced stress and improved memory. ;Sphingomyelin Sphingomyelin is a type of

sphingolipid Sphingolipids are a class of lipids containing a backbone of sphingoid bases, which are a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mythological sp ...

, which contains a backbone of sphingoid bases. It can be found in the

myelin sheath Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

of nerve cell axons in animal cell membranes. Sphingomyelin can be found in eggs or bovine brain. This sphingolipid is synthesized at the

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

and is enriched at the

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

with a larger concentration on the outside. ;Other phospholipids There are many other phospholipids, some of which are glycolipids. The glycolipids include phosphatidyl sugars where the alcohol functional group is part of a carbohydrate. Phosphatidyl sugars are present in plants and certain microorganisms. A carbohydrate is very hydrophilic due to the large number of hydroxyl groups present.

Uses

Functions and use in membranes

Glycerophospholipids are the main structural component of biological membranes. Their amphipathic nature drives the formation of the lipid bilayer structure of membranes. The cell membrane seen under the

electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...

consists of two identifiable layers, or "leaflets", each of which is made up of an ordered row of glycerophospholipid molecules. The composition of each layer can vary widely depending on the type of cell. * For example, in human

erythrocyte Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood ce ...

s the '' cytosolic'' side (the side facing the

) of the

consists mainly of phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. * By contrast, the ''exoplasmic'' side (the side on the exterior of the cell) consists mainly of phosphatidylcholine and sphingomyelin, a type of

. Each glycerophospholipid molecule consists of a small polar head group and two long

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

chains. In the cell membrane, the two layers of phospholipids are arranged as follows: * the

tails point to each other and form a fatty, hydrophobic center * the ionic head groups are placed at the inner and outer surfaces of the cell membrane Apart from their function in cell membranes, they function in other cellular processes such as signal induction and transport. In regards to signaling, they provide the precursors for prostanglandins and other

leukotriene Leukotrienes are a family of eicosanoid inflammation, inflammatory mediators produced in leukocytes by the redox, oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxyg ...

s. It is their specific distribution and catabolism that enables them carry out the biological response processes listed above. Their roles as storage centers for secondary messengers in the membrane is also a contributing factor to their ability to act as transporters. They also influence protein function. For example, they are important constituents of lipoproteins (soluble proteins that transport fat in the blood) hence affect their metabolism and function.

Use in emulsification

Glycerophospholipids can also act as an emulsifying agent to promote dispersal of one substance into another. This is sometimes used in

candy Candy, alternatively called sweets or lollies, is a Confectionery, confection that features sugar as a principal ingredient. The category, also called ''sugar confectionery'', encompasses any sweet confection, including chocolate, chewing gum ...

making and ice-cream making.

Presence in the brain

Neural membranes contain several classes of glycerophospholipids which turnover at different rates with respect to their structure and localization in different cells and membranes. There are three major classes namely; 1-alkyl-2-acyl glycerophospholipid, 1,2-diacyl glycerophospholipid and plasmalogen. The main function of these classes of glycerophospholipids in the neural membranes is to provide stability, permeability and fluidity through specific alterations in their compositions. The glycerophospholipid composition of neural membranes greatly alters their functional efficacy. The length of glycerophospholipid acyl chain and the degree of saturation are important determinants of many membrane characteristics including the formation of lateral domains that are rich in polyunsaturated fatty acids. Receptor-mediated degradation of glycerophospholipids by phospholipases A(l), A(2), C, and D results in generation of second messengers, such as prostaglandins, eicosanoids, platelet activating factor and diacylglycerol. Thus, neural membrane phospholipids are a reservoir for second messengers. They are also involved in

, modulation of activities of transporters, and membrane-bound enzymes. Marked alterations in neural membrane glycerophospholipid composition have been reported to occur in neurological disorders. These alterations result in changes in membrane fluidity and permeability. These processes along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for the neurodegeneration observed in neurological disorders.

Metabolism

The metabolism of glycerophospholipids is different in eukaryotes, tumor cells, and prokaryotes. Synthesis in prokaryotes involves the synthesis of glycerophospholipids phosphatidic acid and polar head groups. Phosphatidic acid synthesis in eukaryotes is different, there are two routes, one to the other toward phosphatidylcholine and phosphatidylethanolamine. Glycerophospholipids are generally metabolized in several steps with different intermediates. The very first step in this metabolism involves the addition or transfer of the fatty acid chains to the glycerol backbone to form the first intermediate, lysophosphatidic acid (LPA). LPA then becomes acylated to form the next intermediate phosphatidic acid (PA). PA can be dephosphorylated leading to the formation of diacylglycerol which is essential in the synthesis of phosphatidylcholine (PC). PC is one of the many species of glycerophospholipids. In a pathway called the Kennedy pathway, the polar heads are added to complete the formation of the entire structure consisting of the polar head regions, the two fatty acid chains and the phosphate group attached to the glycerol backbone. In this Kennedy pathway, Choline is converted to CDP-Choline which drives the transfer of the polar head groups to complete the formation of PC. PC can then be further converted to other species of glycerophospholipids such as phosphatidylserine (PS) and phosphatidylethanolamine (PE).

References

External links

*
Diagram at uca.edu
{{Phospholipids Phospholipids Membrane biology Glycerol esters