Phosphatidylethanolamines are found in all living cells, composing 25% of all phospholipids. In human physiology, they are found particularly in nervous tissue such as thewhite matter ofbrain, nerves, neural tissue, and inspinal cord, where they make up 45% of all phospholipids.[2]
As a polar head group, phosphatidylethanolamine creates a more viscous lipid membrane compared tophosphatidylcholine. For example, the melting temperature of di-oleoyl-phosphatidylethanolamine is -16 °C while the melting temperature of di-oleoyl-phosphatidylcholine is -20 °C. If the lipids had two palmitoyl chains, phosphatidylethanolamine would melt at 63 °C while phosphatidylcholine would melt already at 41 °C.[4] Lower melting temperatures correspond, in a simplistic view, to more fluid membranes.
In humans, metabolism of phosphatidylethanolamine is thought to be important in the heart. When blood flow to the heart is restricted, the asymmetrical distribution of phosphatidylethanolamine between membrane leaflets is disrupted, and as a result the membrane is disrupted. Additionally, phosphatidylethanolamine plays a role in the secretion oflipoproteins in the liver. This is because vesicles for secretion ofvery low-density lipoproteins coming off of theGolgi apparatus have a significantly higher phosphatidylethanolamine concentration when compared to other vesicles containing very low-density lipoproteins.[5] Phosphatidylethanolamine has also shown to be able to propagate infectiousprions without the assistance of anyproteins ornucleic acids, which is a unique characteristic of it.[6] Phosphatidylethanolamine is also thought to play a role in blood clotting, as it works withphosphatidylserine to increase the rate ofthrombin formation by promoting binding tofactor V andfactor X, two proteins which catalyze the formation of thrombin fromprothrombin.[7] The synthesis of endocannabinoidanandamide is performed from the phosphatidylethanolamine by the successive action of two enzymes,N-acetyltransferase andphospholipase-D.[8]
Where phosphatidylcholine is the principalphospholipid in animals, phosphatidylethanolamine is the principal one inbacteria. One of the primary roles for phosphatidylethanolamine in bacterial membranes is to spread out the negative charge caused byanionic membranephospholipids. In the bacteriumE. coli, phosphatidylethanolamine play a role in supportinglactose permeases active transport of lactose into the cell, and may play a role in other transport systems as well. Phosphatidylethanolamine plays a role in the assembly of lactose permease and other membrane proteins. It acts as a 'chaperone' to help the membrane proteins correctly fold theirtertiary structures so that they can function properly. When phosphatidylethanolamine is not present, the transport proteins have incorrect tertiary structures and do not function correctly.[9]
Phosphatidylethanolamine also enables bacterial multidrug transporters to function properly and allows the formation of intermediates that are needed for the transporters to properly open and close.[10]
As alecithin, phosphatidylethanolamine consists of a combination ofglycerol esterified with twofatty acids andphosphoric acid. Whereas the phosphate group is combined withcholine in phosphatidylcholine, it is combined withethanolamine in phosphatidylethanolamine. The two fatty acids may be identical or different, and are usually found in positions 1,2 (less commonly in positions 1,3).
Thephosphatidylserinedecarboxylation pathway and thecytidine diphosphate-ethanolamine pathways are used to synthesize phosphatidylethanolamine.Phosphatidylserine decarboxylase is the enzyme that is used to decarboxylate phosphatidylserine in the first pathway. The phosphatidylserine decarboxylation pathway is the main source of synthesis for phosphatidylethanolamine in the membranes of themitochondria. Phosphatidylethanolamine produced in the mitochondrial membrane is also transported throughout the cell to other membranes for use. In a process that mirrorsphosphatidylcholine synthesis, phosphatidylethanolamine is also made via the cytidine diphosphate-ethanolamine pathway, usingethanolamine as the substrate. Through several steps taking place in both thecytosol andendoplasmic reticulum, the synthesis pathway yields the end product of phosphatidylethanolamine.[11] Phosphatidylethanolamine is also found abundantly in soy or egg lecithin and is produced commercially using chromatographic separation.
Synthesis of phosphatidylethanolamine through thephosphatidylserinedecarboxylation pathway occurs rapidly in theinner mitochondrial membrane. However, phosphatidylserine is made in theendoplasmic reticulum. Because of this, the transport of phosphatidylserine from the endoplasmic reticulum to the mitochondrial membrane and then to the inner mitochondrial membrane limits the rate of synthesis via this pathway. The mechanism for this transport is currently unknown but may play a role in the regulation of the rate of synthesis in this pathway.[12]
Phosphatidylethanolamines in food break down to form phosphatidylethanolamine-linkedAmadori products as a part of theMaillard reaction.[13] These products acceleratemembranelipidperoxidation, causingoxidative stress to cells that come in contact with them.[14] Oxidative stress is known to cause food deterioration and several diseases. Significant levels of Amadori-phosphatidylethanolamine products have been found in a wide variety of foods such aschocolate,soybean milk,infant formula, and otherprocessed foods. The levels of Amadori-phosphatidylethanolamine products are higher in foods with high lipid and sugar concentrations that have high temperatures in processing.[13] Additional studies have found that Amadori-phosphatidylethanolamine may play a role invascular disease,[15] act as the mechanism by whichdiabetes can increase the incidence ofcancer,[16] and potentially play a role in other diseases as well. Amadori-phosphatidylethanolamine has a higherplasmaconcentration in diabetes patients than healthy people, indicating it may play a role in the development of the disease or be a product of the disease.[17]
^Wellner, Niels; Diep, Thi Ai; Janfelt, Christian; Hansen, Harald Severin (2012). "N-acylation of phosphatidylethanolamine and its biological functions in mammals".Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids.1831 (3):652–62.doi:10.1016/j.bbalip.2012.08.019.PMID23000428.
^abVance, Jean E.; Tasseva, Guergana (2012). "Formation and function of phosphatidylserine and phosphatidylethanolamine in mammalian cells".Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids.1831 (3):543–54.doi:10.1016/j.bbalip.2012.08.016.PMID22960354.
