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Phosphatidylethanolamine and Related Lipids



1.  Phosphatidylethanolamine – Structure and Occurrence

Phosphatidylethanolamine (once given the trivial name 'cephalin') is usually the second most abundant phospholipid in animal and plant lipids, after phosphatidylcholine, and it is frequently the main lipid component of microbial membranes. It can amount to 20% of liver phospholipids and as much as 45% of those of brain; higher proportions are found in mitochondria than in other organelles. As such, it is obviously a key building block of membrane bilayers. It is a neutral or zwitterionic phospholipid (at least in the pH range 2 to 7) and is concentrated in the inner leaflet of the plasma membrane, with the structure shown (with one specific molecular species illustrated as an example).

Structural formula of phosphatidylethanolamine

In animal tissues, phosphatidylethanolamine tends to exist in diacyl, alkylacyl and alkenylacyl forms, and data for the compositions of these various forms from bovine heart muscle are listed in our web pages on ether lipids. As much as 70% of the phosphatidylethanolamine in some cell types (especially inflammatory cells, neurons and tumor cells) can have an ether linkage. If biosynthesis of the plasmalogen form is inhibited by physiological conditions, it is replaced by the diacyl form so that the overall content of the phospholipid remains constant.

In general, animal phosphatidylethanolamine tends to contain higher proportions of arachidonic and docosahexaenoic acids than the other zwitterionic phospholipid, phosphatidylcholine. These polyunsaturated components are concentrated in position sn-2 with saturated fatty acids most abundant in position sn-1, as illustrated for rat liver and chicken egg in Table 1. In most other species, it would be expected that the structure of the phosphatidylethanolamine in the same metabolically active tissues would exhibit similar features.

Table 1. Positional distribution of fatty acids in phosphatidylethanolamine in animal tissues.
Position Fatty acid
14:0 16:0 18:0 18:1 18:2 20:4 22:6
 
Rat liver [1]
sn-1 25 65 8
sn-2 2 11 8 8 10 46 13
 
Chicken egg [2]
sn-1 32 59 7 1
sn-2 1 1 25 22 29 12
1, Wood, R. and Harlow, R.D., Arch. Biochem. Biophys., 131, 495-501 (1969).
2, Holub, B.J. and Kuksis, A. Lipids, 4, 466-472 (1969).

The positional distributions of fatty acids in phosphatidylethanolamine from the leaves of the model plant Arabidopsis thaliana are listed in Table 2. Here also saturated fatty acids are concentrated in position sn-1, and there is a preponderance of di- and triunsaturated in position sn-2. The pattern is somewhat different for the yeast Lipomyces lipoferus, where the compositions of the two positions are relatively similar.

Table 2. Composition of fatty acids (mol %) in positions sn-1 and sn-2 in the phosphatidylethanolamine from leaves of Arabidopsis thaliana [1] and from Lipoferus lipoferus [2].
Position Fatty acid
16:0 16:1 18:0 18:1 18:2 18:3
 
A. thaliana
sn-1 58 trace 4 5 15 18
sn-2 trace trace trace 2 60 38
 
L. lipoferus
sn-1 29 18 4 28 13 6
sn-2 23 15 3 34 17 6
1, Browse, J., Warwick, N., Somerville, C.R. and Slack, C.R. Biochem. J., 235, 25-31 (1986).
2, Haley, J.E. and Jack, R.C. Lipids, 9, 679-681 (1974).


2.  Phosphatidylethanolamine – Biosynthesis

The two main pathways employed by mammalian cells for the biosynthesis of phosphatidylethanolamine are the CDP-ethanolamine pathway, i.e. one of the general routes to phospholipid biosynthesis in plants and animals, and the phosphatidylserine decarboxylase pathway, which occur in two spatially separated organelles - the endoplasmic reticulum and mitochondria, respectively. Ethanolamine is obtained by decarboxylation of serine in plants, and in animals most must come from dietary sources and requires facilitated transport into cells (a small amount of ethanolamine phosphate comes from sphingolipid catabolism via sphingosine-1-phosphate). The first steps in phosphatidylethanolamine biosynthesis occur in the cytosol and are first the phosphorylation of ethanolamine by two specific ethanolamine kinases, and this is followed by the rate-limiting step, i.e. reaction of the product with cytidine triphosphate (CTP) to form cytidine diphosphoethanolamine catalysed by CTP:phosphoethanolamine cytidylyltransferase. In the final step, a membrane-bound enzyme in the endoplasmic reticulum (CDP-ethanolamine:diacylglycerol ethanolaminephosphotransferase) catalyses the reaction of the last compound with diacylglycerol to form phosphatidylethanolamine. The diacylglycerol precursor is formed from phosphatidic acid via the action of the enzyme phosphatidic acid phosphohydrolase (see our web pages on triacylglycerols and phosphatidylcholine).

Main pathway of phosphatidylethanolamine biosynthesis

The second major pathway is the conversion of phosphatidylserine to phosphatidylethanolamine (as discussed also in our web pages on phosphatidylserine). In prokaryotic cells, such as E. coli, in which phosphatidylethanolamine is the most abundant membrane phospholipid, all of it is derived from phosphatidylserine decarboxylation. However, this is also a major pathway in mammalian cells and yeasts, where phosphatidylserine decarboxylase is located on the external aspect of the mitochondrial inner membrane. The reaction is regulated by the transport of newly synthesised phosphatidylserine to the mitochondria from the endoplasmic reticulum or a specific domain of this termed the mitochondria-associated membrane (MAM). While various isoforms of the phosphatidylserine decarboxylation exist in prokaryotes, yeasts and mammals, the main forms designated 'PSD1' are found only in mitochondria and are related structurally. An isoform designated 'PSD2' is located in the endosomal membranes of yeasts, and the phosphatidylethanolamine formed is the preferred substrate for phosphatidylcholine biosynthesis. It is evident that cellular concentrations of phosphatidylethanolamine and phosphatidylserine are closely related and tightly regulated.

Phosphatidylethanolamine synthesis from phosphatidylserine

The relative importance of these two main pathways for phosphatidylethanolamine synthesis in mammalian cells appears to depend on the cell type. The spatially distinct pools within the cell are functionally distinct. Both are essential to life and for example, disruption of the phosphatidylserine decarboxylase gene causes misshapen mitochondria and has lethal consequences in embryonic mice, although phosphatidylethanolamine synthesis continues for a time in other cellular regions. Similarly, elimination of the endoplasmic reticulum route is embryonically lethal.

Three additional minor biosynthetic pathways are known. Phosphatidylethanolamine can be formed by the enzymatic exchange reaction of ethanolamine with phosphatidylserine, or by re-acylation of lysophosphatidylethanolamine. The second of these is associated with the mitochondria-associated membrane where the phosphatidylserine synthase II is located. Finally, the bacterial plant pathogen Xanthomonas campestris is able to synthesise phosphatidylethanolamine by condensation of cytidine diphosphate diacylglycerol with ethanolamine. It should be noted that all of these pathways for the biosynthesis of diacyl-phosphatidylethanolamine are very different and are separated spatially from that producing alkyl,acyl- and alkenyl,acyl-phosphatidylethanolamine), suggesting that there may be functional differences. In the protozoon Trypanosoma brucei, for example, it has been demonstrated that the diacyl and ether pools of phosphatidylethanolamine have separate functions and cannot substitute for each other.

The various mechanisms produce different pools of phosphatidylethanolamine species, which are often in different cellular compartments and have distinctive compositions. Studies with mammalian cell types in vitro suggest that the CDP-ethanolamine pathway produces molecular species with mono- or di-unsaturated fatty acids on the sn-2 position preferentially, while the phosphatidylserine decarboxylation reaction generates species with polyunsaturated fatty acids on the sn-2 position mainly. However, as with other phospholipids, the final fatty acid composition in animal tissues is attained by a process of remodelling known as the Lands’ cycle (see the web page on phosphatidylcholine, for example). The first step is hydrolysis by a phospholipase A2 to lysophosphatidylethanolamine, followed by reacylation by means of various acyl-CoA:lysophospholipid acyltransferases. The enzymes LPCAT1, 2 and 3, which are involved in phosphatidylcholine biosynthesis, are also active with phosphatidylethanolamine, while LPEAT1 utilizes lysophosphatidylethanolamine mainly and is specific for oleoyl-CoA. Some of these isoforms appear to be confined to particular tissues.


3.  Phosphatidylethanolamine – Biological Function

Phosphatidylethanolamine is a precursor for the synthesis N-acyl-phosphatidylethanolamine (see below) and thence of anandamide (N‑arachidonoylethanolamine), and it is the donor of ethanolamine phosphate during the synthesis of the glycosylphosphatidylinositol anchors that attach many signalling proteins to the surface of the plasma membrane. In bacteria, it functions similarly in the biosynthesis of lipid A and other lipopolysaccharides. It is also the substrate for the hepatic enzyme phosphatidylethanolamine N-methyltransferase, which provides about a third of the phosphatidylcholine in liver.

Although phosphatidylethanolamine has sometimes been equated with phosphatidylcholine in biological systems, there are significant differences in the chemistry and physical properties of these lipids, and they have different functions in biochemical processes. Both are key components of membrane bilayers and the ratio of the two may be important to many cellular functions. However, phosphatidylethanolamine has a smaller head group, which gives the lipid a cone shape. On its own, it does not form bilayers but inverted hexagonal phases. With other lipids in a bilayer, it is believed to exert a lateral pressure that modulates membrane curvature and stabilizes membrane proteins in their optimum conformations. It can hydrogen bond intermolecularly to adjacent lipids and to proteins through its polar head group. For example, the phosphate can be stabilized at the binding site by interactions with lysine and arginine side chains, or with hydroxyls from tyrosine side chains. There can also be strong interactions with acyl chains of the phospholipid.

In contrast to phosphatidylcholine, it is concentrated with phosphatidylserine in the inner leaflet of the plasma membrane. On the other hand, it is present in both the inner and outer membranes of mitochondria, but especially the former, where it facilitates membrane fusion and protein movement across membranes in addition to many other essential mitochondrial functions. It appears that a primary role for phosphatidylethanolamine in bacterial membranes at least is simply to dilute the high negative charge density of the anionic phospholipids. In eukaryotic cells, especially those of insects, studies suggest that it has a similar function to cholesterol in membranes in that it increases the rigidity of the bilayer to maintain membrane fluidity.

Membrane proteins amount to 30% of the genome, and they carry out innumerable biochemical functions, including transport, energy production, biosynthesis, signalling and communication. Within a membrane, most integral proteins consist of hydrophobic α-helical trans-membrane domains that zigzag across it and are connected by hydrophilic loops. Of those parts of the proteins outwith the bilayer, positively charged residues are much more abundant on the cytoplasmic side of membrane proteins as compared to the trans side (the positive-inside rule). Phosphatidylethanolamine is believed to have a key function in that it inhibits location of negative amino acids on the cytoplasmic side, supporting the positive-inside rule, and it has an appropriate charge density to balance that of the membrane surface and the protein. However, it can also permit the presence of negatively charged residues on the cytosolic surface in some circumstances in support of protein function.

Scottish thistleMuch of the evidence for the unique properties of phosphatidylethanolamine comes from studies of the biochemistry of E. coli, where this lipid is a major component of the membranes. In particular, phosphatidylethanolamine has a specific involvement in supporting active transport by the lactose permease, and other transport systems may require or be stimulated by it. There is evidence that phosphatidylethanolamine acts as a 'chaperone' during the assembly of this and other membrane proteins to guide the folding path for the proteins and to aid in the transition from the cytoplasmic to the membrane environment. In the absence of this lipid, the transport membranes may not have the correct tertiary structure and so will not function correctly. Whether the lipid is required once the protein is correctly assembled is not fully understood in all cases, but it may be needed to orient enzymes correctly in the inner membrane. It is certainly required both for proper functioning and to ensure the correct folding of the enzyme lactose permease (from E. coli) in membranes, although in contrast it inhibits folding of some multi-helical proteins. It appears that life in this organism can be maintained without phosphatidylethanolamine, but that life processes may be inhibited.

Although the mechanism has yet to fully elucidated, effects on protein conformation are believed to be behind a finding that phosphatidylethanolamine is the primary factor in brain required for the propagation and infectivity of mammalian prions.

Phosphatidylethanolamine binds non-covalently to a superfamily of cytosolic proteins with multiple functions termed 'phosphatidylethanolamine-binding proteins'. While four members have been identified in mammals (PEBP1-4), more than 400 members of the evolutionally conserved family are known from bacteria to higher eukaryotes. In animal tissues, phosphatidylethanolamine is especially important in the sarcolemmal membranes of the heart during ischemia, it is involved in secretion of the nascent very-low-density lipoproteins from liver and it has functions in membrane fusion and fission. It has a functional role in the Ca2+-ATPase in that one molecule of phosphatidylethanolamine is bound in a cavity between two transmembrane helices, acting as a wedge to keep them apart. This is displaced when Ca2+ is bound to the enzyme. Many other important proteins bind non-covalently to phosphatidylethanolamine in a similar way, including cytochrome c oxidase, rhodopsin and aquaporins.

The content of phosphatidylethanolamine in newly secreted VLDL particles and in apoB-containing particles isolated from the lumen of the Golgi is much higher than that in circulating VLDLs, suggesting that this lipid is involved in VLDL assembly and/or secretion. However, it is rapidly and efficiently removed from the VLDL in the circulation. With lipid droplets in cells, phosphatidylethanolamine is believed to promote coalescence of smaller droplets into larger ones.

A covalent conjugate of phosphatidylethanolamine with a protein designated 'Atg8' is formed by the action of cysteine protease ATG4 (belonging to the caspase family) and various other proteins, and is involved in the process of autophagy (controlled degradation of cellular components) in yeast by promoting the formation of membrane vesicles containing the components to be degraded. Similarly, phosphatidylethanolamine is the precursor of an ethanolamine phosphoglycerol moiety bound to two conserved glutamate residues in eukaryotic elongation factor 1A, which is an essential component in protein synthesis. This unique modification appears to be of great importance for the resistance of plants to attack by pathogens.

Host defence peptides are antimicrobial agents produced by both prokaryotic and eukaryotic organisms, and many of these have a high affinity for phosphatidylethanolamine as a lipid receptor to modulate their activities. For example, the peptide antibiotics cinnamycin and duramycins from Streptomyces have a hydrophobic pocket that fits around phosphatidylethanolamine such that the binding is stabilized by ionic interaction between the ethanolamine group of the lipid and the carboxylate moiety of the peptide; this complex exhibits activity against other Gram-positive organisms, such as Bacillus species.

In the seeds of higher plants, a deficiency of phosphorylethanolamine cytidylyltransferase, a rate-limiting enzyme in the biosynthesis of phosphatidylethanolamine, has profound effects upon the viability and maturation of embryos.


4.  Lysophosphatidylethanolamine

Formula of lysophosphatidylethanolamineLysophosphatidylethanolamine, with one mole of fatty acid per mole of lipid, is found in small amounts only in tissues. It is formed by hydrolysis of phosphatidylethanolamine by the enzyme phospholipase A2, as part of a de-acylation/re-acylation cycle that controls its overall molecular species composition as discussed above. A membrane-bound O-acyltransferase (MBOAT2) specific for lysophosphatidylethanolamine (and lysophosphatidic acid) has been characterized with a preference for oleoyl-CoA as substrate.

In plants, lysophosphatidylethanolamine is a specific inhibitor of phospholipase D, a key enzyme in the degradation of membrane phospholipids during the early stages of plant senescence. By this action, it retards the senescence of leaves, flowers, and post-harvest fruits. Indeed, it has a number of horticultural applications when applied externally, e.g. to stimulate ripening and extend the shelf-life of fruit, delay senescence and increase the vase life of cut flowers. In bacteria, lysophosphatidylethanolamine displays chaperone-like properties, promoting the functional folding of citrate synthase and other enzymes. Some biological properties have been reported in animal tissues in vitro, but a specific receptor has yet to be identified.


5.  N-Acyl Phosphatidylethanolamine

In N-Acyl phosphatidylethanolamine, the free amino group of phosphatidylethanolamine is acylated by a further fatty acid. This lipid has been detected in rather small amounts in several animal tissues (~0.01%), but especially brain, nervous tissues and the epidermis, when the N-acyl chain is often palmitic or stearic acid (human plasma: N16:0-PE (40%), N18:1-PE (23.3%), N18:0-PE (19%), N18:2-PE (16.6%) and N20:4-PE (1.4%)). Under conditions of degenerative stress, it can accumulate in significant amounts, for example as the result of ischemic injury, infarction or cancer. It is present in plasma after feeding a high fat diet to rats, and that it can cross into the brain where it accumulates in the hypothalamus.

Formula of N-acyl phosphatidylethanolamineIn animals, N-Acyl phosphatidylethanolamine is the precursor of anandamide and other biologically important amides in brain and other tissues (see our web pages on endocannabinoids for a more detailed discussion of N-acyl phosphatidylethanolamine synthesis and metabolism). In brief, it is formed biosynthetically by the action of a transferase exchanging a fatty acid from the sn-1 position of a phospholipid (probably phosphatidylcholine) to the primary amine group of phosphatidylethanolamine (without a hydrolytic step). In addition, some transfer can also occur from phosphatidylethanolamine per se by an intramolecular reaction. However, it should be noted that N-acyl phosphatidylethanolamine can also be formed artefactually as a result of faulty extraction procedures during analysis.

In plants, N-acyl phosphatidylethanolamine is a common constituent of cereal grains (e.g. wheat, barley and oats) and of some other seeds (1.9% of the phospholipids of cotton seeds but 10-12% of oats). In other plant tissues, it is detected most often under conditions of physiological stress. It has also been found in a number of microbial species. Its synthesis involves direct acylation of phosphatidylethanolamine by CoA esters, catalysed by a membrane-bound transferase. Activation of N-acyl phosphatidylethanolamine metabolism in plants with release of N-acylethanolamines seems to be associated with cellular stresses, but research is at an early stage.

N-Acetyl phosphatidylethanolamine was found in a filamentous fungus, Absidia corymbifera, where it comprised 6% of the total membrane lipids. It was accompanied by an even more unusual lipid 1,2-diacyl-sn-glycero-3-phospho(N-ethoxycarbonyl)-ethanolamine.


6.  Mono- and Dimethylphosphatidylethanolamines

Mono- and dimethylphosphatidylethanolamines are formed by sequential methylation of phosphatidylethanolamine by the enzyme phosphatidylethanolamine N-methyltransferase as intermediates in one of the mechanisms for the biosynthesis of phosphatidylcholine. This is a minor pathway in general in animals, although it is significant in liver, especially when choline is deficient in the diet. However, it is the major route in yeasts and bacteria. although these intermediate lipids do not seem to be essential components of yeast membranes.

Formulae of mono- and dimethylphosphatidylethanolamines

They are never found at greater than trace levels in animal or plant tissues, and it is not known whether they have any more specific functions. On the other hand as might be expected, they are more abundant in some species of bacteria, especially those that interact with plants.


7.  Non-Enzymatic Modification of Phosphatidylethanolamine by Carbonyl-Amine Reactions

Phosphatidylethanolamine can react non-enzymatically to form imine and/or Michael adducts with the hydroxy-alkenals and related compounds that are products of hydroperoxidation of unsaturated fatty acids, including malondialdehyde, acrolein, epoxyalkenals, hydroxyalkenals (4-hydroxy-trans-2-nonenal especially), oxoalkenals, and γ‑ketoaldehydes. The chemistry and biochemistry of bioactive aldehydes is discussed on a separate web page. As an example the reaction with glycoxal is illustrated.

Michael reaction of phosphatidylethanolamine

Such products accelerate the peroxidation of membrane lipids and are believed to be important for generating oxidative stress both in foods and in tissues. They are considered to be inflammatory mediators in vivo and have been implicated in a number of disease states, such as atherogenesis and diabetes, and during aging. However, it appears that little is known of how individual aldehyde-phosphatidylethanolamines differ in their biological effects. Alkyl modified phosphatidylethanolamines induce a negative membrane curvature in lipid vesicles in vitro.

Similarly, levuglandins and isolevuglandins are reactive cyclo-oxygenase metabolites of arachidonic and docosahexaenoic acids, which react rapidly with the free amine group of phosphatidylethanolamine (and with proteins) in vivo to form cytotoxic hydroxy-lactam derivatives.

In recent years, the concept of the Maillard reaction has been expanded to include glycation of amino-phospholipids. For example, phosphatidylethanolamine reacts with glucose and other sugars to form first unstable Schiff bases, which rearrange to produce Amadori products of phosphatidylethanolamine, as illustrated for glucose below, especially under hyperglycemic conditions. Indeed, there are suggestions that Amadori-phosphatidylethanolamine may be a useful predictive marker for hyperglycemia in the early stages of diabetes.

Formation of Amadori adduct of phosphatidylethanolamine

Once Amadori-phosphatidylethanolamine is formed, it can undergo further reactions, for example with reactive oxygen species to form carboxymethyl- and carboxyethyl-adducts, which also have the potential to trigger pathological processes including the neuropathy and retinopathy associated with diabetic complications. Thus, it has been demonstrated that glycated phosphatidylethanolamine and its oxidation products induce the production of pro-inflammatory cytokines and that they have a role in apoptotic cell signaling. Unsurprisingly, such changes to the phospholipid head group change the biophysical properties of membrane bilayers appreciably and can lead ultimately to the disintegration of cell membranes. Phosphatidylserine might be expected to form similar materials, but these have proved harder to detect in tissues.

Phosphatidylethanolamine reacts with all-trans-retinal in the photoreceptor outer segment membrane of the eye to form first N-retinylidene-phosphatidylethanolamine as part of a transport mechanism, but then further reaction can generate a troublesome bis-retinoid condensation product.

bis-Retinoid phosphatidylethanolamine condensation product

This lipid conjugate together with hydrolysis products, formed by cleavage of the ethanolamine-phosphate bond, can accumulate in retinal pigment epithelial cells with age, and it be involved in the pathogenesis of some retinal disorders.


8.  Phosphatidylethanol

Formula of phosphatidylethanolPhosphatidylethanol has little in common with phosphatidylethanolamine other than the obvious structural similarity. It is formed slowly in cell membranes, especially erythrocytes, by a transphosphatidylation reaction from phosphatidylcholine in the presence of ethanol, and catalysed by the enzyme phospholipase D. As it has a long half-life in serum relative to alcohol, it is a useful biochemical marker for alcohol abuse; chronic alcoholics have very much higher levels in the blood than healthy subjects who consume alcohol in moderation. While it is usually considered to be physiologically inert, there are suggestions that it may have a beneficial role in ethanol tolerance but a harmful one for colon cancer.


9.  Analysis

Analysis of phosphatidylethanolamine and related lipids present no particular problems. They are readily isolated by thin-layer or high-performance liquid chromatography methods for further analysis. Modern mass spectrometric methods are being used increasingly for the purpose.


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Lipid listings Credits/disclaimer Updated: August 10th, 2017 Author: William W. Christie LipidWeb icon