thumb|right|upright=1.5|Membrane structures. Top, an archaeal phospholipid: 1, isoprene chains; 2, ether linkages; 3, [[Levorotation and dextrorotation|L-glycerol moiety; 4, phosphate group. Middle, a bacterial or eukaryotic phospholipid: 5, fatty acid chains; 6, ester linkages; 7, D-glycerol moiety; 8, phosphate group. Bottom: 9, lipid bilayer of bacteria and eukaryotes; 10, lipid monolayer of some archaea.]]
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes in eukaryotic cells. They are a type of lipid, of which its composition affects membrane structure and properties. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.
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, 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. 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 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 (<small>D</small> or <small>L</small>) 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.
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.
;Sphingomyelin
Sphingomyelin is a type of sphingolipid, which contains a backbone of sphingoid bases. It can be found in the myelin sheath 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 and is enriched at the plasma membrane 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 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.
Each glycerophospholipid molecule consists of a small polar head group and two long hydrophobic chains. In the cell membrane, the two layers of phospholipids are arranged as follows:
- 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
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 leukotrienes. 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.
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).