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Phosphatidylserine and Related Lipids

Phosphatidylserine is an important anionic phospholipid, which brings important physical properties to membranes in both eukaryotes and prokaryotes. Independently of this, it has many biological functions in cells, including effects on blood coagulation and apoptosis, and it is the precursor for phosphatidylethanolamine in prokaryotes and in eukaryote mitochondria. Its metabolite lysophosphatidylserine has signalling functions operating through specific receptors. Also, there is increasing interest in a structurally related lipid phosphatidylthreonine and other phospholipids linked to amino acids.

1.  Phosphatidylserine - Structure and Occurrence

Although phosphatidylserine or 1,2-diacyl-sn-glycero-3-phospho-L-serine is distributed widely among animals, plants and microorganisms, it is usually less than 10% of the total phospholipids, the greatest concentration being in myelin from brain tissue. For example, mouse brain and liver contain 14 and 3% phosphatidylserine, respectively. However, it may comprise 10 to 20 mol% of the total phospholipid in the plasma membrane, where it is concentrated in the inner leaflet, and in the endoplasmic reticulum of cells. In the yeast S. cerevisiae, it is a minor component of most cellular organelles other than the plasma membrane, where surprisingly it can amount to more than 30% of the total lipids. In most bacteria, it is a minor membrane constituent, although it is important as an intermediate in phosphatidylethanolamine biosynthesis. The 1‑octadecanoyl-2-docosahexaenoyl molecular species, which is especially important in brain tissue, is illustrated here.

Formula of phosphatidylserine

Formula of calcium-chelated phosphatidylserinePhosphatidylserine is an acidic (anionic) phospholipid with three ionizable groups, i.e. the phosphate moiety, the amino group and the carboxyl function. As with other acidic lipids, it exists in nature in salt form, but it has a high propensity to chelate to calcium via the charged oxygen atoms of both the carboxyl and phosphate moieties, modifying the conformation of the polar head group. This interaction may be of considerable relevance to the biological function of phosphatidylserine, especially during bone formation for example.

In animal cells, the fatty acid composition of phosphatidylserine varies from tissue to tissue, but it does not appear to resemble the precursor phospholipids, either because of selective utilization of specific molecular species for biosynthesis or because of re-modelling of the lipid via deacylation-reacylation reactions with lysophosphatidylserine as an intermediate (see below). In human plasma, 1-stearoyl-2-oleoyl and 1-stearoyl-2-arachidonoyl species predominate, but in brain (especially grey matter), retina and many other tissues 1-stearoyl-2-docosahexaenoyl species are very abundant and appear to be essential for normal functioning of the nervous system. Indeed, the ratio of n-3 to n-6 fatty acids in brain phosphatidylserine is much higher than in most other lipids. The positional distribution of fatty acids in phosphatidylserine from rat liver and bovine brain are listed in Table 1. As with most phospholipids, saturated fatty acids are concentrated in position sn-1 and polyunsaturated in position sn-2.

Table 1. Positional distribution of fatty acids in phosphatidylserine from rat liver and bovine brain
Position Fatty acid
16:0 18:0 18:1 18:2 20:4 22:6
  Rat liver [1]
sn-1 5 93 1
sn-2 6 29 8 4 32 19
  Bovine brain [2]
sn-1 3 81 13
sn-2 2 1 25 trace 1 60
1. Wood, R. and Harlow, R.D. Arch. Biochem. Biophys., 135, 272-281 (1969).
2. Yabuuchi, H. and O'Brien, J.S. J. Lipid Res., 9, 65-67 (1968).

In leaves of Arabidopsis thaliana, used as a 'model' plant in many studies, the fatty acid composition of phosphatidylserine resembles that of phosphatidylethanolamine. There is an intriguing report that the chain-lengths of the acyl groups increase with age and stress in phosphatidylserine quite specifically.

In marked contrast to phosphatidylethanolamine, phosphatidylserines with ether-linked moieties (alkyl and alkenyl) are not common in animal tissues, although they are reported to be relatively abundant in human retina and macrophages (they were first found in rat lung).

As a generality, the concentration of phosphatidylserine is highest in plasma membranes and endosomes, but is very low in mitochondria. As it is located entirely on the inner monolayer surface of the plasma membrane (and of other cellular membranes) and it is the most abundant anionic phospholipid, it may make the largest contribution to interfacial effects in membranes involving non-specific electrostatic interactions. This normal distribution is disturbed during platelet activation and cellular apoptosis.

N-Acylphosphatidylserine has been reported as a minor component of the lipids of sheep erythrocytes, bovine brain and the central nervous system of freshwater fish, amongst others. The N-arachidonoyl form may be the precursor of the endocannabinoid N-arachidonoylserine.

2.  Biosynthesis of Phosphatidylserine

L-Serine is a non-essential amino acid that is actively synthesised by most organisms. In animals, it is produced in nearly all cell types, although in brain it is synthesised by astrocytes but not by neurons, which must be supplied with this amino acid for the biosynthesis of phosphatidylserine (and of sphingoid bases).

In bacteria and other prokaryotic organisms, phosphatidylserine is synthesised by a mechanism comparable to that of most other phospholipids, i.e. by reaction of L-serine with CDP-diacylglycerol (see our web pages on phosphatidylglycerol, for example), and depends on Mg2+ or Mn2+.

Biosynthesis of phosphatidylserine in prokaryotes

Much of the phosphatidylserine thus formed is decarboxylated to phosphatidylethanolamine, and this may be the major route to the latter in bacteria. As phosphatidylcholine in yeast is produced via methylation of phosphatidylethanolamine, phosphatidylserine is the primary precursor phospholipid in these organisms.

In contrast in animal tissues, phosphatidylserine is synthesised solely by calcium-dependent base-exchange reactions in which the polar head-group of an existing phospholipid is exchanged for L-serine. There are two routes involving distinct enzymes (PS synthase I and II) with 30% homology and several membrane-spanning domains but utilizing different substrates. Phosphatidylserine is synthesised by both enzymes in the endoplasmic reticulum of the cell, or in a specific domain of this termed the mitochondria-associated membrane ('MAM'), because it is tethered transiently to the mitochondrial outer membrane (in yeast, a complex of integrated proteins ('ERMES') has been characterized with this function). The reaction involves exchange of L-serine with phosphatidylcholine, catalysed by PS synthase I, or with phosphatidylethanolamine (PE), catalysed by PS synthase II. It is strictly dependent on calcium ions. The new lipid is then transported to the mitochondria, probably by transient membrane contact, where it is decarboxylated to PE by a specific decarboxylase in the inner mitochondrial membrane; in yeast, this process also occurs at the Golgi/endosome membranes. The phosphatidylethanolamine formed in this way can return to the endoplasmic reticulum where it may be converted back to phosphatidylserine by the action of PS synthase II.

Biosynthesis of phosphatidylserine in animal tissues

Biosynthesis of phosphatidylserine - mitochondriaPhosphatidylserine synthase-I is expressed in all mouse tissues, but especially the kidney, liver and brain, while phosphatidylserine synthase-II is most active in the brain and testis and much less so in other tissues. The latter enzyme has a high specificity for docosahexaenoic acid. It is not known why such a complex series of coupled reactions is necessary, or why there should be two enzymes. One virtue of the second enzyme is that the free ethanolamine and choline formed are rapidly re-utilized for phospholipid synthesis. Thus, both phosphatidylserine and PE are produced without a reduction in the amount of phosphatidylcholine. Elimination of both enzymes is embryonically lethal in knock-out mice, but each of them can be knocked out separately and the mice survive, even though they have substantially reduced levels of phosphatidylserine and PE. In addition, mitochondrial production of PE from phosphatidylserine is not fully complemented by the CDP-ethanolamine pathway, as mice lacking the enzyme do not survive for long. It is evident that cellular concentrations of these two lipids are intimately related and tightly regulated.

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 lysophosphatidylserine, followed by reacylation by various acyl-CoA:lysophospholipid acyltransferases. One membrane-bound O-acyltransferase (LPCAT4 or MBOAT2) with a preference for oleoyl-CoA has been characterized, while a second (LPCAT3 or MBOAT5) incorporates linoleoyl and arachidonoyl chains (and also utilizes lysophosphatidylcholine).

In plants, much of the phosphatidylserine appears to be produced by a calcium-dependent base-exchange reaction in which the head-group of an existing phospholipid is exchanged for L-serine (i.e. mechanistically similar to PS synthase I), but a CDP-diacylglycerol pathway exists in some species, e.g. wheat.

Phosphatidylserine modulates membrane charge locally, enabling the recruitment of soluble cations and proteins, and so it contributes to the organization of processes within cell membranes. Its distribution within membranes is tightly controlled as it facilitates signalling within the various cellular compartments. Thus, it undergoes a transition from the lumenal leaflet of the endoplasmic reticulum to the cytosolic leaflet in the trans Golgi network, probably by the activity of flippases in the Golgi. Although it does not take part in membrane raft formation, it is present in caveolae, where it is believed to interact with caveolin-1.

3.  Phosphatidylserine – Biological Function

The asymmetric structure of the plasma membrane with high concentrations of anionic lipids such as phosphatidylserine in the cytosolic leaflet with zwitterionic lipids in the extracellular leaflet generates two surfaces with greatly different electrostatic potentials that influence the association of proteins with the membrane surface and the activities of integral membrane proteins. This distribution is maintained and can be altered, after specific activation, by various flippases, floppases and scramblases, including ATP-dependent translocases selective for phosphatidylserine that function at the plasma membrane.

Enzyme activation: In addition to its function as a component of cellular membranes and as a precursor for other phospholipids, phosphatidylserine is an essential cofactor that binds to and activates a large number of proteins, especially those with signalling activities. The negative charge on the lipid facilitates the binding to proteins through electrostatic interactions or Ca2+ bridges. For example, the presence of appreciable amounts of phosphatidylserine on the cytosolic leaflet of endosomes and lysosomes enables these compartments to dock with proteins with specific phosphatidylserine-binding domains, including several important signalling and fusogenic effectors. The cytoskeletal protein spectrin binds to phosphatidylserine in this way, and it is also required by enzymes such as Na+/K+ ATPase, where the 18:0/18:1 species is especially important, and the neutral sphingomyelinase. In addition, the high concentration of this anionic lipid results in an accumulation of negative surface charge to which poly-cationic proteins can bind. The effect is believed to be that certain proteins are re-directed from one target membrane to another. Similarly, phosphatidylserine participates directly in key signalling pathways in brain by binding to the cytosolic proteins involved in neuronal signalling and thereby activating them. At least three major pathways are affected, including those involving phosphatidylinositol 3-kinase and protein kinase C. For example, most enzymes of the protein kinase C family contain a 'C2' calcium-dependent cysteine-rich region that recognizes phosphatidylserine, and in coordination with the 'C1' domain that binds to diacylglycerols, is essential for activating and locating them to the plasma membrane of appropriately stimulated cells. Phosphatidylserine is not involved in cell signalling through the formation of metabolites, as is the case with phosphatidylinositol.

Blood coagulation: Phosphatidylserine is an important element of the blood coagulation process in platelets, where it is transported from the inner to the outer surface of the plasma membrane in platelets activated by exposure to fibrin-binding receptors, for example. Here, the exposed phosphatidylserine enhances the activation of prothrombin to thrombin (the key molecule in the blood clotting cascade) by triggering a cascade of reactions and providing the negatively charged platform that enables calcium ions to form bridges with γ-carboxyglutamic acid-containing domains on the coagulation factors. Membrane vesicles with phosphatidylserine exposed on the surface can also be released from platelets that promote the coagulation process. Apolipoprotein A-1 in high-density lipoproteins has a controlling function in that it neutralizes these procoagulant properties by arranging the phospholipid in surface areas that are too small to accommodate the prothrombinase complex. Blood coagulation is beneficial when it prevents the loss of blood from the circulatory system, but it is detrimental when it causes thrombosis, and the action of phosphatidylserine is essential to the regulation of the process.

Scottish thistleApoptosis: In addition in response to particular calcium-dependent stimuli, phosphatidylserine is known to have an important role in the regulation of apoptosis or programmed cell death, the natural process by which aged or damaged cells are removed from tissues before they can exert harmful effects. When cells are damaged, a mechanism is initiated in which the normal distribution of this lipid on the inner leaflet of the membrane bilayer is disrupted by stimulation of scramblases, which are ATP-independent and can move the lipid across the membrane, together with inhibition of aminophospholipid translocases, which return the lipid to the inner side of the membrane. In erythrocytes, phosphatidylserine is located in the inner leaflet of the membrane bilayer under low Ca2+ conditions when a phospholipid scramblase is suppressed by membrane cholesterol, but it is exposed to the outer leaflet under elevated Ca2+ concentrations which activate the scramblase. After the collapse of this asymmetry and transfer of phosphatidylserine to the outer leaflet of the cell, it is believed that it is recognized by a receptor on the surface of macrophages and related scavenger cells and these facilitate the removal of the apoptotic cells and their potentially toxic or immunogenic contents in a non-inflammatory manner. It is noteworthy that the transition from a pro-inflammatory to an anti-inflammatory state is defined by phagocytosis of neutrophils by macrophages via this phosphatidylserine-dependent process.

During apoptosis, generation of reactive oxygen species occurs, mainly hydrogen peroxide, which together with the enzyme cytochrome c bring about rapid oxidation of the fatty acids in phosphatidylserine before this lipid is externalized. Indeed, it is now apparent that molecular species of phosphatidylserine with an oxidatively truncated sn-2 acyl group that incorporates terminal γ-hydroxy(or oxo)-α,β-unsaturated acyl moieties are especially potent signals for scavenger receptors in macrophages as a prerequisite for engulfment of apoptotic cells. Such oxidized lipids are discussed in our web page dealing with biologically active aldehydes.

This has been described as "a dominant and evolutionarily conserved immunosuppressive signal that promotes tolerance and prevents local and systemic immune activation" or more succinctly as an "eat-me signal". Binding of phosphatidylserine to specific proteins, such as apolipoprotein H (β2-glycoprotein 1), enhances the recognition and clearance. This process is essential for the development of lung and brain, and it is also relevant to clinical situations where apoptosis plays an important part, such as cancer, chronic autoimmunity, and infections. In relation to atherosclerosis, phosphatidylserine is believed to have anti-inflammatory and protective effects as a component of the high-density lipoproteins, probably mediated by the apoptosis mechanism. In contrast, as this mechanism is important for the turnover of erythrocytes, it is relevant to thrombus formation and the stabilization of blood clots.

A similar apoptopic mechanism operates in retinal pigment epithelial cells to remove the large amounts of photoreceptor cell debris that are generated continuously. In addition, appreciable amounts of phosphatidylserine are translocated by an analogous mechanism to the surface of T lymphocytes that express low levels of the trans-membrane enzyme tyrosine phosphatase. This change in distribution acts then as a signalling mechanism to modulate the activities of several membrane proteins. The protein annexin V binds with high specificity to phosphatidylserine and is used as a probe to detect apoptotic cells.

Unfortunately, it appears that the protozoon Leishmania and other parasites and viruses can hijack this machinery by incorporating this phospholipid into their viral envelopes so conning cells into engulfing them; the viral glycoprotein/cellular receptor complex may then facilitate the entry of foreign organisms into other cells. As parasites ingested in this manner do not elicit production of proinflammatory cytokines, this mechanism, which has been termed 'apoptotic mimicry', is critical for survival of parasites within the macrophage.

Other activities: A further unusual function of phosphatidylserine is that it is a key component of the lipid-calcium-phosphate complexes that act as nucleation centers for hydroxyapatite formation and initiate mineral deposition during the formation of bone. It has been established that phosphatidylserine and inorganic phosphate must be present, before calcium ions are introduced, when the high affinity of phosphatidylserine for calcium ions becomes important. Nucleation is facilitated by the protein annexin V. In addition, this activity is relevant to cardiovascular disease and in particular to the phenomenon of "hardening of the arteries," where atherosclerotic plaques can undergo mineralization with the deposition of hydroxyapatite.

Among many other functions of phosphatidylserine, it is believed to be essential for the fusion of cells as part of the formation of fibres in muscle cells, and it provides stable membrane domains in spermatozoa that are essential for fertilization.

The high concentrations of docosahexaenoic acid (DHA) in brain and retinal phosphatidylserine are certainly important for the development and function of these tissues. Accumulation of phosphatidylserine in neuronal membranes is promoted by DHA, and this is important for the maintenance of neuronal survival. Phosphatidylserine may also be a reservoir of DHA for protectin formation in neuronal tissue. On the other hand, the Food and Drug Administration in the USA considers that there is little scientific evidence to support claims that dietary supplements of phosphatidylserine reduce the risk of dementia or cognitive dysfunction in the elderly. Other nutritional claims also appear doubtful. Antibodies to phosphatidylserine are formed in some disease states, including thrombosis and recurrent spontaneous pregnancy loss.

4.  Lysophosphatidylserine

Structural formula of lysophosphatidylserineLysophosphatidylserine, with a fatty acid in position sn-1 only, is known to be a mediator of a number of biological processes, especially in the context of the immune system. It is presumably formed primarily by deacylation by phospholipases; a secreted isoform that is phosphatidylserine-specific (PS-PLA1), which hydrolyses the sn-1 acyl group of phosphatidylserine to generate sn-2-lysophosphatidylserine, is upregulated greatly by various inflammatory stimuli. Lysophosphatidylserine has been detected after injury to animal tissues (tumor growth, graft rejection, burns), and it may have a similar function to lysophosphatidic acid in cell signalling, for example in regulating calcium flux and stimulating immune cells through specific receptors of which several have been detected in mice and humans. The lipid has been linked to certain cancers and to night blindness.

When cells are damaged, lysophosphatidylserine can be generated by a reaction dependent on activation of the NADPH oxidase. It can diffuse and transmit the information to other cells, especially mast cells, and it is produced to enhance clearance of activated and dying neutrophils. It thus has a role in the resolution of inflammation. In Schistosome infections, lysophosphatidylserine from the parasite is believed to be a key activator molecule in the host. Quite specifically, sn-2-lysophosphatidylserine stimulates degranulation of mast cells - most other lysophospholipids have no such activity.

Negatively charged lysophosphatidylserine forms tend to organize in non-bilayer structures and are believed to facilitate folding of certain membrane proteins in situ better than bilayer-forming lipids.

5.  Phosphatidylthreonine and Other Amino Acid-Containing Phospholipids

Formulae of phosphatidyl-L-threoninePhosphatidyl-L-threonine, which is closely related structurally and metabolically to phosphatidylserine, was first detected in animal brain and tuna muscle, before it was characterized definitively as a minor component of polyoma virus-transformed embryo fibroblasts in hamsters, cultured hippocampal neurons and macrophages. It has also been detected in some bacterial species. Biosynthetic studies with microsomes from rat brain suggest that it is synthesised by the same base-exchange enzyme involved in phosphatidylserine synthesis but with much lower activity. In laboratory animals, it is barely detectable in normal tissues such as brain, and it is decarboxylated in mitochondria in vitro to phosphatidylisopropanolamine. Phosphatidyl-L-aspartate and phosphatidyl-L-glutamate with carboxylate-phosphate anhydride bonds have recently been detected in rat brain.

Phosphatidylthreonine is now known to be of special importance as a major phospholipid, with 20:1 and 20:4 as the fatty acid constituents, of the protozoan parasite Toxoplasma gondii, which can infect animals and humans. It is produced by a novel phosphatidylthreonine synthase, which has evolved from the well-known phosphatidylserine synthase, and it is required for asexual reproduction and virulence of the parasite in vivo. A mutant strain of the organism lacking phosphatidylthreonine was able to protect vaccinated mice against acute and currently incurable chronic infection with obvious pharmacological implications.

Lysophosphatidylthreonine, with a fatty acid in position sn-1 only, displays many of the biological activities reported for lysophosphatidylserine in vitro, although it is not known whether it is also active in vivo.

Other amino acid-linked phospholipids include phosphatidyl-O-[N-(2-hydroxyethyl)glycine], which was isolated from brown algae of the family Phaeophyceae such as Fucus serratus, where it can amount to as much as 25% of the total lipids. The fatty acid composition is distinctive in that arachidonic acid comprises about 80% of the total. A minor phospholipid component from the bacterium Escherichia coli contains a dipeptide unit, i.e. phosphatidylserylglutamate.

Structural formula of phosphatidylserylglutamate and phosphatidyl-O-[N-(2-hydroxyethyl) glycine]

Phosphatidylethanolamineglutamate has been detected in the bacterium Peredibacter starrii. Some other amino acid-containing phospholipids (the complex lipoamino acids) are more closely related to phosphatidylglycerol in structure and biosynthesis.

6.  Analysis

As with other acidic lipids, the metal ions associated with phosphatidylserine hamper analysis, although the problem can be solved by an acid wash. It is easily separated from other phospholipids by two-dimensional thin-layer chromatography, but poorly shaped peaks are often seen with high-performance liquid chromatography. Mass spectrometry is being used increasingly for molecular species analysis and quantification.

Recommended Reading

Lipid listings Credits/disclaimer Updated: May 30th, 2017 Author: William W. Christie LipidWeb icon