Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes.
StructuresThe term glycerophospholipid signifies any derivative of glycerophosphoric acid that contains at least one O- acyl, or O- alkyl, or O-alk-1'- enyl residue attached to the glycerol moiety.IUPAC Gold Book. http://goldbook.iupac.org/G02648.html The alcohol here is glycerol, to which two fatty acids and a phosphoric acid are attached as esters.The two fatty acid chains attached to the molecule of glycerol are nonpolar hence hydrophobic while the polar heads which mainly consists of the phosphate group attached to the third carbon of the glycerol molecule is hydrophillic. This dual characteristic leads to the amphipathic nature of glycerophospholipids. They are usually organized into a bilayer in memberanes with the polar hydrophillic heads sticking outwards to the aqueous environment and the non polar hydrophobic tails pointing inwards. Glycerophospholipids consists 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 stereochemistryIn general, glycerophospholipids use a "sn" notation, which stands for stereospecific numbering. When the letters "sn" appear in the nomenclature, by convention the hydroxyl group of the second carbon of glycerol (sn-2) is on the left on a Fischer projection. The numbering follows the one of Fischer's projections, being sn-1 the carbon at the top and sn-3 the one at the bottom. The advantage of this particular notation is that the spatial conformation (R 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 and sn-glycero-1-phosphoric acid are enantiomers.
Examples of glycerophospholipids;Plasmalogens Plasmalogens 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. Phosphatidylethanoamines, phosphatidylcholines, and other phospholipids are examples of phosphatidates. ;Phosphatidylcholines Phosphatidylcholines are lecithins. 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 lecthins- which are commercially important as an emulsifying agent in products such as mayonnaise. Lecithins are also present in brain and nerve tissue. ;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.
Functions and use in membranesOne of the main functions of glycerophospholipid is to serve as a 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 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 erythrocytes the cytosolic side (the side facing the cytosol) of the plasma membrane 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 sphingolipid.
- the hydrophobic 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