Phosphatidylserine and Related Lipids
Phosphatidylserine or 1,2-diacyl-sn-glycero-3-phospho-L-serine is an important anionic phospholipid, which brings essential 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 and operates 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 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 phospholipids in the plasma membrane, where under normal conditions it is concentrated in the inner leaflet, and in the endoplasmic reticulum of cells. In the yeast Saccharomyces 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.
Phosphatidylserine 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
|Rat liver |
|Bovine brain |
|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, and 22:0 and 24:0 fatty acids have been reported to occur in this lipid in the plasma membrane of some plant species.
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 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 that can utilize different substrates. Phosphatidylserine is synthesised by both enzymes on the cytosolic face of the endoplasmic reticulum (ER) of the cell, but mainly in a specific domain of this termed the mitochondria-associated membrane ('MAM'), because it is tethered transiently to the mitochondrial outer membrane, presumably by a protein bridge (in yeast, a complex of integrated proteins ('ERMES') has been characterized with a similar function). The reaction involves exchange of L-serine with either phosphatidylcholine or phosphatidylethanolamine, catalysed by PS synthase I (although it was long thought that only phosphatidylcholine was a substrate for this enzyme), while PS synthase II catalyses a similar exchange with diacyl-phosphatidylethanolamine and its the plasmalogen form. Both enzymes are subject to feedback regulation by their product phosphatidylserine, thereby maintaining the correct amounts of this lipid.
Phosphatidylserine 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 molecular species containing docosahexaenoic acid. It is not known why such a complex series of coupled reactions is necessary, or why there should be two enzymes, but one virtue is that the free ethanolamine and choline formed are rapidly re-utilized for phospholipid synthesis. Thus, both phosphatidylserine and phosphatidylethanolamine 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 phosphatidylethanolamine.
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).
Following synthesis, phosphatidylserine molecules can diffuse laterally in a concentration-dependent manner to different regions of the membrane to fulfill their physiological functions. In humans, cytosolic transport proteins transfer phosphatidylserine and other acidic phospholipids between membranes, and this can also occur by a vesicular transport mechanism.
Some of the newly synthesised phosphatidylserine is transferred to the plasma membrane, while a proportion is transported to the mitochondria, probably again via transient membrane contact (MAM), where it is decarboxylated to produce phosphatidylethanolamine by a specific decarboxylase in the inner mitochondrial membrane; in yeast, this process also occurs at the Golgi/endosome membranes. All the phosphatidylethanolamine in mitochondria is formed in this way, but some can return to the endoplasmic reticulum where it may be converted back to phosphatidylserine by the action of the PS synthases. Mitochondrial production of phosphatidylethanolamine from phosphatidylserine is not fully complemented by the CDP-ethanolamine pathway, as mice lacking the enzyme do not survive for long. Evidently, cellular concentrations of these two lipids are intimately related and tightly regulated.
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 for this phospholipid in these organisms.
Bacteria and plants: In bacteria and other prokaryotic organisms and in yeast, 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+. Phosphatidylserine synthases belong to two different families: type I (non-integral membrane form) in the phospholipase D-like family as in E. coli, and type II (integral membrane form) in the CDP-alcohol phosphotransferase family as in Bacillus sp. and the yeast S. cerevisiae, although the latter shows no homology with the bacterial enzymes.
In many plants, including in the model plant Arabidopsis, much of the phosphatidylserine is produced by a calcium-dependent base-exchange reaction in which the head-group of an existing phospholipid is exchanged for L-serine in the luminal leaflet of the endoplasmic reticulum (i.e. mechanistically similar to PS synthase I). It is transferred to the cytoplasmic membrane leaflet by flippases and thence to the post-Golgi compartments before eventually accumulating at the plasma membrane. However, some vesicular transport may occur or there may be direct transfer at membrane contact sites. A CDP-diacylglycerol (prokaryotic-like) biosynthetic pathway exists in some species, e.g. wheat.
3. Phosphatidylserine – Biological Function
Membrane location: 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, and it is highly enriched on the inner, compared to outer, leaflet of the plasma membrane. Transport to the plasma membrane against a concentration gradient is aided in part by proteins designated 'ORP5' and 'ORP8' in humans (Osh 6 and Osh7 in yeast) with a PH binding domain for phosphatidylinositol 4,5-bisphosphate and an 'ORD' domain for phosphatidylserine. At a membrane contact site between the endoplasmic reticulum and plasma membrane, phosphatidylserine is exchanged for phosphatidylinositol 4-phosphate. Although it does not take part in membrane raft formation, phosphatidylserine is present in caveolae, where it is believed to interact with caveolin-1. It is also present in appreciable amounts in the endosomal compartment.
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 (transfer back into the cytoplasmic leaflet), floppases (transfer out of the cytoplasmic leaflet) and scramblases (bidirectional transfer), including ATP-dependent translocases selective for phosphatidylserine. Vesicular transport may occur also.
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 the neutral sphingomyelinase and the Na+/K+ ATPase, where the 18:0/18:1 molecular species is especially important. It is believed that the fatty acyl components of this species in the inner leaflet of the plasma membrane (and potentially other intracellular membranes) may interact (interdigitation or "hand-shake") with the very-long chains of sphingolipids in the outer leaflet in raft microdomains, resulting in a high local concentration of the anionic phospholipid and an accumulation of negative surface charge to which specific poly-cationic proteins in the membranes can bind. This may then enable transfer of signals across the membrane to the cytosol.
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.
Apoptosis: 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 plasma membrane bilayer is disrupted by stimulation of scramblases, which are ATP-independent and can move the lipid across the membrane to the outer leaflet, 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 an effete cell, it is believed that it is recognized by a cohort of receptors, either directly or indirectly, through bridging ligands on the surface of macrophages and related scavenger cells. These activate a family of cysteine-dependent aspartate-specific proteases, the caspases, and other enzymes to facilitate the engulfment 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 oxidized phospholipids.
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. For example, phosphatidylserine is a necessary component of the TAM family of receptor tyrosine kinases and the receptor-ligand complex of particular importance in cancer cells, where phosphatidylserine-TAM signalling regulates many aspects of inflammation and immune resolution and is seen as a target for therapeutic intervention. Exposure of phosphatidylserine is increased substantially on the surface of tumor cells or tumor cell-derived microvesicles, which have innate immunosuppressive properties and facilitate tumor growth and metastasis. 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. The innate immunosuppressive effect of externalized phosphatidylserine has been hijacked by numerous viruses and bacteria to facilitate infection.
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 anti-coagulant protein annexin V binds with high specificity to phosphatidylserine and is used as a probe to detect apoptotic cells. It is noteworthy that phosphatidylserine is a major component of the membranes of microvesicles in animal cells, and translocation to the outer leaflet upon cellular activation is essential for their biogenesis. In addition, exposure of phosphatidylserine on the cell surface is reported to be a factor in non-apoptotic forms of regulated inflammatory cell death, such as necroptosis.
Role in infections: Unfortunately, viruses such as Ebola and HIV viruses can hijack this apoptosis machinery by incorporating phosphatidylserine 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. Similarly, parasites ingested in this manner, including Leishmania and Trypanosoma species, utilize host phosphatidylserine to establish infections and facilitate disease progression as they do not then elicit production of proinflammatory cytokines. This mechanism has been termed 'apoptotic mimicry' and is critical for survival of parasites within the macrophage.
Other activities: Phosphatidylserine is required for the transmembrane movement of excess cholesterol, derived initially from the lysosomal degradation of low-density lipoproteins, from the plasma membrane to the endoplasmic reticulum thereby maintaining membrane integrity and ensuring cell survival. It is therefore an important element in cholesterol homeostasis. The mechanism is believed to involve proteins known as GRAMD1s embedded in the endoplasmic reticulum membrane at sites in contact with the plasma membrane. These have two functional domains: the StART-like domain that binds cholesterol and the GRAM domain that binds anionic lipids, such as phosphatidylserine, and so are able to form a link between the two membranes that enables the transfer of cholesterol.
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. Similarly, during bone repair and maintenance, the fusion of osteoclasts requires the non-apoptotic exposure of phosphatidylserine at the surface of fusion-committed cells with the aid of a transmembrane protein (DC-STAMP) expressed in dendrocytes. 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. Over-production of phosphatidylserine is a key factor in the congenital disease Lenz-Majewski syndrome.
Among many other functions of phosphatidylserine, it is believed to be an essential surface membrane component for the fusion of cell types other than osteoclasts, including during the formation of fibres in muscle cells, and fusion of macrophages into inflammatory giant cells and myoblasts into myotubes. Such cell fusions require the nonapoptotic exposure of phosphatidylserine at the surface of fusing cells, where it interacts with phosphatidylserine-recognizing proteins to regulate the time and place of cell-fusion. Phosphatidylserine provides stable membrane domains in spermatozoa that are essential for fertilization, and it also an essential component of the plasma membrane microdomains known as caveolae, where it is required both for their formation and stability possibly through specific binding to the cavin proteins.
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, and other nutritional claims appear to be dubious also. Antibodies to phosphatidylserine are formed in some disease states, including thrombosis and recurrent spontaneous pregnancy loss. The rare genetic disease Lenz-Majewski syndrome is caused by a mutation in the phosphatidylserine synthase I gene that greatly increases the activity of the enzyme while preventing feedback inhibition.
In yeasts such as Candida albicans, phosphatidylserine and the enzyme phosphatidylserine decarboxylase, which generates phosphatidylethanolamine, are both essential for the virulence of the organism towards a host species.
Lysophosphatidylserine, i.e. with a fatty acid in one position only, is known to be a mediator of a number of biological processes, especially in the context of the immune system in animal tissues. It has been found in the thymus, peripheral lymphoid tissues, central nervous system and colon, but is barely detectable in plasma. Deacylation of the diacyl lipid by phospholipases is the primary source. For example, a secreted isoform that is phosphatidylserine-specific (PS‑PLA1) removes the sn-1 acyl group of phosphatidylserine to generate sn‑2‑lysophosphatidylserine containing unsaturated fatty acids, and this is upregulated greatly by various inflammatory stimuli. This extracellular enzyme utilizes phosphatidylserine exposed on the cell membrane as a substrate, although other phospholipases may operate intracellularly and produce sn‑1‑lysophosphatidylserine. In addition, platelets in some species (not significantly in humans) secrete a phospholipase A2 group IIA (ABHD16A), which generates saturated sn‑1‑lysophosphatidylserine (and other lysophospholipids).
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 G protein-coupled receptors of which three (GPR34, P2Y10 and GPR174, LPS1-3) have been detected in mice and humans. 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. One specific molecular species, i.e. 1‑(11Z‑eicosenoyl)-glycero-3-phosphoserine, is reported to be a true agonist of the Toll-like receptor 2/6 heterodimer of importance to the immune response to pathogens; both its polar head group and the length of the acyl chain are required for this activity. On the other hand, sn-2-lysophosphatidylserine has proinflammatory reactions in that it augments mast cell degranulation and mast cell-dependent anaphylactic shock; most other lysophospholipids have no such activity. Deregulated lysophosphatidylserine metabolism has been linked to certain cancers, night blindness and the human genetic neurological disorder PHARC. In Schistosome infections, lysophosphatidylserine from the parasite is believed to be a key activator molecule in the host.
Negatively charged lysophosphatidylserine species 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
Phosphatidyl-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 such as Bdellovibrio bacteriovorus. 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.
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. Targeted inhibition of this enzyme leads to dysregulation of calcium metabolism with effects on many essential functions as well as virulence. 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: In addition to phosphatidylthreonine, phosphatidyl-L-aspartate and phosphatidyl-L-glutamate with unique carboxylate-phosphate anhydride bonds have been detected in rat brain. Other phospholipids related to phosphatidylthreonine 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 E. coli contains a dipeptide unit, i.e. phosphatidylserylglutamate. Porphyromas gingivalis, a Gram-negative anaerobic periodontal pathogen, contains phosphatidylserylglycine together with a form in which the dipeptide component is further acylated (and could be classified as a lipopeptide). The acylated form is a ligand for Toll-like receptor 2 (TLR2) and is believed to be relevant to chronic periodontitis in humans.
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.
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.
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