The Lipid Library

PHOSPHATIDYLETHANOLAMINE AND RELATED LIPIDS

STRUCTURE, OCCURRENCE, BIOCHEMISTRY and ANALYSIS

1.  Phosphatidylethanolamine – Structure and Occurrence

Phosphatidylethanolamine (once given the trivial name 'cephalin') is usually the second most abundant phospholipid in animal and plant lipids and it is frequently the main lipid component of microbial membranes. 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) with the structure shown (with one specific molecular species illustrated as an example).

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. In general, animal phosphatidylethanolamine tend 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.

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 differences between the two positions are relatively minor.

2. Phosphatidylethanolamine – Biosynthesis and Biological Function

One major pathway for biosynthesis of phosphatidylethanolamine de novo in animals and plants follows a general route to phospholipid biosynthesis -

The first step is phosphorylation of ethanolamine by an ethanolamine kinase, followed by reaction of the product with cytidine triphosphate (CTP) to form cytidine diphosphoethanol-amine. An enzyme, phosphoethanolamine transferase, catalyses the reaction of the last compound with diacylglycerol to form phosphatidylethanolamine.

Three other minor pathways exist, of which the most important 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 can also be a major pathway in mammalian cells and yeasts, where phosphatidylserine decarboxylase is located on the external aspect of the mitochondrial inner membrane. The relative importance of these two main pathways for phosphatidylethanolamine synthesis in mammalian cells appears to depend on the cell type. On the other hand, disruption of the phosphatidylserine decarboxylase gene causes misshapen mitochondria and has lethal consequences in embryonic mice. Phosphatidylethanolamine synthesised in the mitochondria is retained there.

Studies with mammalian cell types in vitro suggest that the CDP-ethanolamine pathway preferentially produces molecular species with mono- or di-unsaturated fatty acids on the sn-2 position, while the phosphatidylserine decarboxylation reaction generates species with polyunsaturated fatty acids on the sn-2 position mainly.

Phosphatidylethanolamine can also be formed by the enzymatic reaction of ethanolamine with phosphatidylserine, or by re-acylation of lysophosphatidylethanolamine.

Although phosphatidylethanolamine is sometimes 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. However, phosphatidylethanolamine has a smaller head group, which gives the lipid a cone shape, and it can hydrogen bond to proteins through its ionizable amine group. 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 stabilizes membrane proteins in their optimum conformations.

Much of the evidence for the unique properties of phosphatidylethanolamine come from studies of the biochemistry of E. coli, where this lipid is a major component of the membranes. It has become apparent that 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 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. It appears that life can be maintained without phosphatidylethanolamine, but life processes may be inhibited.

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.

In yeasts, a covalent conjugate of phosphatidylethanolamine with a protein designated 'Atg8' is involved in the process of autophagy (controlled degradation of cellular components) by promoting the formation of membrane vesicles containing the components to be degraded.

Trace levels of glucosylated phosphatidylethanolamine and of acetone adducts have been detected in human tissues, especially those of diabetic patients (see below). In addition, phosphatidylethanolamine can react to form Michael adducts with the hydroxy-alkenals that are products of hydroperoxidation of unsaturated fatty acids, such as 4-hydroxy-2(E)-nonenal derived from n-6 fatty acids. Under conditions of oxidative stress in vivo, these compounds may influence the properties of membranes.

3.  Lysophosphatidylethanolamine

Lysophosphatidylethanolamine, 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.

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 is used commercially in a spray to stimulate ripening of fruit and delay senescence. In bacteria, lysophosphatidylethanolamine displays chaperone-like properties, promoting the functional folding of citrate synthase and other enzymes.

4.  N-Acyl Phosphatidylethanolamine

N-Acyl phosphatidylethanolamine in which the free amino group of phosphatidylethanolamine is acylated by a further fatty acid is a common constituent of cereal grains (e.g. wheat, barley and oats) and of some other seeds, but it may occur in other plant tissues, especially under conditions of physiological stress. It has also been found in a number of microbial species.

This phospholipid has been detected in rather small amounts in several animal tissues, but especially brain, nervous tissues and the epidermis, when the N-acyl chain is often palmitic or stearic acid. Under conditions of degenerative stress, it can accumulate in significant amounts, for example as the result of ischemic injury, infarction or cancer.

N-Acyl phosphatidylethanolamine 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. N-Acyl phosphatidylethanolamine is the precursor of anandamide (see our web pages on this lipid for a more detailed discussion of the reactions involved) and other biologically important amides in brain and other tissues via a reaction catalysed by a phosphodiesterase.

The activation of N-acylphosphatidylethanolamine metabolism in plants seems to be associated with cellular stresses, with release of N-acylethanolamines, which act as lipid mediators to modulate ion flux and activate defense gene expression.

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.

5.  Mono- and Dimethylphosphatidylethanolamines

Mono- and dimethylphosphatidylethanolamines are formed by sequential methylation of phosphatidylethanolamine as part of a mechanism for biosynthesis of phosphatidylcholine. This is a minor pathway in animals, but is the major route in yeasts and bacteria. However, they do not seem to be essential components of yeast membranes. 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 bacteria, especially those that interact with plants.

6.  Amadori Adduct of Phosphatidylethanolamine

In recent years, the concept of the Maillard reaction has been expanded to include glycation of aminophospholipids. For example, phosphatidylethanolamine reacts with glucose and other sugars to form first unstable Schiff bases and then an Amadori product of phosphatidylethanolamine, as illustrated for glucose below.

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 involved in food deterioration and have been implicated in a number of disease states such as atherogenesis, diabetes and aging. Phosphatidylserine might be expected to form similar materials, but these have proved harder to detect in tissues.

7.  Phosphatidylethanol

Phosphatidylethanol 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 such, it has been proposed as a biochemical marker for alcohol abuse, since chronic alcoholics have very much higher levels in the blood than healthy subjects who consume alcohol in moderation.

8.  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.

Suggested Reading

• Bogdanov, M. and Dowhan, W. Lipid-assisted protein folding. J. Biol. Chem., 274, 36827-36830 (1999).
• Chapman, K.D. Emerging physiological roles for N-acylphosphatidylethanolamine metabolism in plants: signal transduction and membrane protection. Chem. Phys. Lipids, 108, 221-229 (2000).
• Christie, W.W. Lipid Analysis - 3rd Edition (Oily Press, Bridgwater) (2003).
• Cronan, J.E. Bacterial membrane lipids: where do we stand? Annu. Rev. Microbiol., 57, 203-224 (2003).
• Dowhan, W. Molecular basis for membrane phospholipid diversity: why are there so many lipids? Annu. Rev. Biochem., 66, 199-232 (1997).
• Oak, J.H., Nakagawa, K. and Miyazawa, T. UV analysis of Amadori-glycated phosphatidylethanolamine in foods and biological samples. J. Lipid Res., 43, 523-529 (2002).
• Schmid, H.H.O., Schmid, P.C. and Natarajan, V. N-Acylated glycerophospholipids and their derivatives. Prog. Lipid Res., 29, 1-43 (1990).
• Vance, D.E. and Vance, J. (editors) Biochemistry of Lipids, Lipoproteins and Membranes (4th Edition). (Elsevier, Amsterdam) (2002) - several chapters.

Updated: 17/3/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland