Phosphatidylglycerol and Related Lipids
Phosphatidylglycerol is a lipid present in the three kingdoms of life that can be a major component of some bacterial membranes, and it is found also in membranes of plants and animals often of low abundance but with many essential functions. The charge on the phosphate group means that it is an anionic lipid at neutral pH with a larger head group than might be expected because of hydration and with a cylindrical shape overall. As an example, the dihexadecanoyl species, which is an important constituent of lung surfactant, is illustrated.
Aside from its importance as a membrane constituent, phosphatidylglycerol is a key intermediate in the biosynthesis of a number of other lipids but especially of cardiolipin, which is located in the inner mitochondrial membrane and is required for proper functioning of the enzymes involved in oxidative phosphorylation.
1. Phosphatidylglycerol in Bacteria and Plants
Anaerobic organisms: Phosphatidylglycerol is found in almost all bacterial types, representing 20-25% of the phospholipids in most Gram-negative bacteria, i.e. with a double (outer and inner) phospholipid envelope, where it is present only in the inner membrane (there is a schematic diagram in our web page on Lipid A). In Gram-positive bacteria, which have a single phospholipid bilayer coated with peptidoglycans, it can be as high as 60% of the phospholipids (together with glycosyl diacylglycerols). For example, Escherichia coli, a widely studied 'model' organism, has up to 20% of phosphatidylglycerol in its membranes (phosphatidylethanolamine makes up much of the rest with a little cardiolipin). In many bacteria, the diacyl form of the lipid predominates, but in others the alkyl,acyl and alkenyl,acyl forms are more abundant.
There is conflicting evidence on as to whether E. coli has an absolute requirement for phosphatidylglycerol in its membranes. For example, studies with mutants deficient in phosphatidylglycerol have suggested that its absence results in defective DNA replication and a lack of a necessary modification to the main cellular lipoprotein or proteolipid, leading to membrane welding and eventually cell death. However, others have concluded from similar experiments that phosphatidylglycerol and cardiolipin are dispensable and can be substituted by phosphatidylethanolamine and anionic phospholipids such as phosphatidic acid. Phosphatidylglycerol is the biosynthetic precursor of many other phospholipids (see below). There seems little doubt that phosphatidylglycerol is important for optimal functioning of the bacterial machinery under normal conditions. It has a role in protein folding and binding, while activating a glycerol phosphate acyl transferase to suggest that it may be involved in a positive feedback loop that produces phosphatidic acid and thus all other membrane lipids.
There is evidence that in some bacterial membranes, phosphatidylglycerol may be segregated into distinct domains, which differ in lipid and protein composition and degree of order from other regions. For example, the conjugative E. coli F pilus, a channel important for the reproduction of the organism, is assembled from protein-phospholipid units lined with stoichiometrically arranged phosphatidylglycerol molecules in which their head groups are directed to the interior of the pilus and the acyl chains buried entirely between subunits.
Two unusual phosphatidylglycerol derivatives based on an archaeol backbone, i.e. phosphatidylglycerol sulfate and phosphatidylglycerol phosphate methyl ester, are unique constituents of the primitive organisms, the Haloarchaea. In these organisms, they are important constituents of bacteriorhodopsin, a retinal-containing integral membrane protein of the cytoplasmic membrane, which forms two-dimensional crystalline patches known as the purple membrane. Further phosphatidylglycerol analogues, the complex lipoamino acids and acylphosphatidylglycerols, are discussed below.
Photosynthetic organisms: In the photosynthetic membranes of leaf tissue of higher plants, including the model plant Arabidopsis thaliana, phosphatidylglycerol is in essence the only phospholipid and is also unique in that in contains a high proportion of trans‑3‑hexadecenoic acid, which is located exclusively in position sn-2 (Table 1). This fatty acid is not found in other lipids of the thylakoid membrane, and it must be significant that the rate of its synthesis in leaves deprived of light is greatly reduced (with accumulation of the precursor palmitic acid); it increases in concentration by a factor of 20 between the youngest (basal) and oldest (distal) leaf sections. In studies with Arabidopsis mutants, it has been demonstrated that phosphatidylglycerol biosynthesis is essential for the development of embryos and the normal membrane structures of chloroplasts and mitochondria.
Table 1. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of phosphatidylglycerol from leaves of Arabidopsis thaliana.
|Data from: Browse, J. et al. Biochem. J., 235, 25-31 (1986); DOI.|
It is interesting to note that saturated and monoenoic fatty acids are concentrated in position sn-2 and polyunsaturated in position sn-1, the opposite of that found for most animal phospholipids. This is because phosphatidylglycerol is synthesised in chloroplasts via the so-called "prokaryotic" pathway (now known to be a misnomer - see our web-pages on mono- and digalactosyldiacylglycerols for further discussion of this phenomenon). In some plant species, position sn-2 of the thylakoid phosphatidylglycerol is occupied exclusively with C16 fatty acids giving a rather distinctive molecular species distribution.
In cyanobacteria and plants that are able to carry out aerobic photosynthesis, phosphatidylglycerol is found in all cellular membranes - 2 to 5% of the plasma membrane, for example, but it appears to be especially important in the thylakoid membrane, which surrounds the chloroplast. There, it is the only phospholipid, comprising up to 10% of the total lipids with a high proportion (up to 70%) in the outer monolayer (much of the remaining lipid is digalactosyldiacylglycerols). Phosphatidylglycerol is essential for the oligomerization of photosystems I and II in cyanobacteria. Analysis of the crystal structure of the photosystem I of cyanobacteria (Thermosynechococcus elongatus) has shown that it contains three molecules of phosphatidylglycerol and one of monogalactosyldiacylglycerol as integral components, while in the PSII of T. vulcanus , all five phosphatidylglycerol molecules are deeply buried near the reaction center. It binds to a specific polypeptide component of the photosystem II complex and appears to be involved in electron transport. This phospholipid also appears to be required for crystallization and polymerization of the light-harvesting complex II in pea chloroplasts, where it may be the 'glue' that binds the individual protein components. A report that trans-3-hexadecenoic acid in phosphatidylglycerol is essential for the latter process has been questioned, but it appears to be true for Chlamydomonas at least. When phosphate is limiting in plants, the glycolipid sulfoquinovosyldiacylglycerol accumulates to compensate for decreased phosphatidylglycerol levels, but this compensation does not apply to photosynthetic electron transport. Indeed, Arabidopsis mutants lacking phosphatidylglycerol synthases are unable to produce thylakoid membranes (and viable seeds).
Disaturated molecular species of phosphatidylglycerol in plants are believed to be an important factor in sensitivity to chilling, and experiments with genetic modifications to increase the degree of unsaturation of this lipid have produced plants with a greater resistance to cold. However, there are discrepancies between the results of different experimental approaches, and other factors are certainly involved.
In cyanobacteria such as Synechocystis sp., in addition to its role in photosynthesis, phosphatidylglycerol is intimately involved in the regulation of enzymes involved in respiration, metabolism, transport, transcription, and translation (ca. 80 proteins). Here, its propensity to form non-bilayer structures in the presence of calcium ions may be important, aided by its ability to bind to specific proteins. There may be parallels in plastids of higher plants.
2. Phosphatidylglycerol in Animal Tissues
Phosphatidylglycerol is present at a level of 1-2% in most animal tissues, but it can be the second most abundant phospholipid in lung surfactant at up to 11% of the total, especially within the alveolar hypophase (in a few species, including the rhesus monkey, it is replaced by another acidic lipid, phosphatidylinositol). It is well established that the concentration of phosphatidylglycerol increases during foetal development, coincident with the formation of stable lamellar phases. The fatty acid composition of lung tissue from several species is listed in Table 2.
Table 2. Fatty acid composition (weight % of the total) in lung phosphatidylglycerol from various species.
|Data from: Okano, G. and Akino, T. Lipids, 14, 541-546 (1979); DOI.|
With each species, the content of saturated fatty acids is high while that of the polyunsaturated components is relatively low in comparison to phospholipids in other tissues. It has also been shown that lung phosphatidylglycerol in many animals contains a high proportion of disaturated molecular species, although this does not appear to be true of human lung surfactant, where palmitoyl-oleoyl phosphatidylglycerol is the main molecular species. It can be used as a clinical treatment for respiratory distress syndrome, because it prevents alveolar epithelial apoptosis and blocks the effects of the inflammatory agents that cause acute lung injury. It seems that the acidic head-group is more important to the physical properties relevant to the surfactant function of phosphatidylglycerol than the precise molecular species composition.
However, the complex nature of how phosphatidylglycerol and a single molecular species function in humans is only slowly being revealed. For example, it may aid the spreading of dipalmitoyl-phosphatidylcholine, which is presumed to be the main functional component of lung surfactant. Together with phosphatidylinositol, it has a role in the regulation of the innate immune response in the lungs in part by attenuating inflammation induced by bacterial lipopolysaccharides and in part by blocking certain viral infections. With the latter, it antagonizes the cognate ligand activation of the toll-like receptors (TLRs) 2 and 4, and it disrupts the binding of virus particles to the plasma membrane receptors required for viral uptake in host cells. In addition, it suppresses pathogen-induced eicosanoid production resulting from Mycoplasma pneumoniae infection in macrophages, as the alveolar surfactant protein A binds strongly to the phosphatidylglycerol in the surface membranes of this organism to attenuate its activity.
As an example of another tissue, the positional distribution of fatty acids in rat liver phosphatidylglycerol is listed in Table 3. Like cardiolipin, there is a very high proportion of linoleate, much of which is concentrated in position sn-1, with relatively little polyunsaturates.
Table 3. Positional distribution of fatty acids in phosphatidylglycerol from rat liver.
|Data from: Wood, R. and Harlow, R.D. Arch. Biochem. Biophys., 135, 272-281 (1969); DOI.|
In mammalian cells, phosphatidylglycerol activates a nuclear protein kinase C while inhibiting some of the activities of platelet activating factor. The enzyme cytochrome c oxidase, the last enzyme in the respiratory electron transport chain of mitochondria, contains four molecules of phosphatidylglycerol in its crystal structure. The lipid is also known to bind non-covalently to various membrane transporter and channel proteins.
Phosphatidylglycerol from bacteria such as Mycobacterium tuberculosis and Listeria monocytogenes interacts with the CD1 family of antigen-presenting molecules in Type II Natural Killer T (NKT) cells in mammalian hosts, suggesting a role in protective immunity towards these pathogens. Presumably, the mammalian cells are able to recognize the changes in structure and physical properties arising from the very different fatty acid compositions (saturated and branched-chain) of the bacterial lipids.
3. Biosynthesis of Phosphatidylglycerol
In animal, plant and microbial tissues, phosphatidylglycerol is formed from phosphatidic acid by a sequence of enzymatic reactions that proceeds via the intermediate, cytidine diphosphate diacylglycerol (CDP-diacylglycerol), which is rarely detected as a normal component of tissues amounting to only 0.05% or so of the total phospholipids. Biosynthesis starts with the condensation of phosphatidic acid and cytidine triphosphate with elimination of pyrophosphate via the action of phosphatidate cytidyltransferases (or CDP-synthases - mainly the CDS1 and TAMM41/TAM41 forms), believed to be rate-limiting - step 1 in the figure. The same liponucleotide is the key intermediate in the biosynthesis of phosphatidylinositol, but different routes are taken to phosphatidylcholine and phosphatidylethanolamine.
In Step 2, the first committed step in phosphatidylglycerol synthesis, CDP-diacylglycerol reacts with glycerol-3-phosphate via phosphatidylglycerophosphate synthase (PGPS) to form 3-sn-phosphatidyl-1'-sn-glycerol 3'-phosphoric acid, with the release of cytidine monophosphate (CMP). One enzyme (PgsA) is found in E. coli in the mitochondria, but two such enzymes are present in the chloroplasts of eukaryotes such as A. thaliana. With the latter PGPS1 is targeted to both mitochondria and chloroplasts, whereas PGPS2 is present only in the endoplasmic reticulum and makes a relatively minor contribution to phosphatidylglycerol synthesis. In animal tissues, biosynthesis occurs in the endoplasmic reticulum and in mitochondria. Finally, phosphatidylglycerol is formed by the action of one of two phosphatases in animals - step 3, but the Arabidopsis PGPP1 protein is located in the chloroplast and is crucial for plastidial phosphatidylglycerol synthesis. As biosynthesis is via glycerol-3-phosphate, the second glycerol moiety is attached to the phosphate group via position sn-1. However, there are other minor biosynthetic routes to phosphatidylglycerol, e.g. by phospholipase D-catalysed catabolism of diphosphatidylglycerol (cardiolipin) or by glycerolysis of other phospholipids (also catalysed by phospholipase D), which can change the stereochemistry of the second glycerol moiety in part (in effect, racemization). For example, in E. coli in addition to the main route, a cardiolipin synthase acts in a similar manner to phospholipase D to produce phosphatidylglycerol from phosphatidylethanolamine and glycerol.
The eventual fatty acid composition of phosphatidylglycerol in animal tissues is attained by the 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 lysophosphatidylglycerol, followed by reacylation by means of an acyl-CoA:lysophosphatidylglycerol acyltransferase. The human form of the latter, designated LPGAT1, has been characterized and was found to have a preference for 16:0-, 18:0-, and 18:1-CoA esters as donors.
In the plastids of higher plants, the selectivity of the acyltransferases is such that the initial molecular species formed contains oleic acid in position sn-1 and palmitic acid in position sn-2. Some of the palmitate in position sn-2 is desaturated to the trans-3 isomer by FAD4, while the oleate in position sn-1 is desaturated to 18:2 and 18:3 fatty acids (see our web page on polyunsaturated fatty acids). In the endoplasmic reticulum in contrast, the initial molecular species contain palmitic and oleic acids in position sn-1 and oleic acid in position sn-2. The oleate in both positions, but not the palmitate, is further desaturated by acyl-lipid desaturases until the final fatty acid compositions are attained. These details of the biosynthetic processes that occur in mitochondria have still to be determined. In cyanobacteria, a disaturated molecular species of phosphatidylglycerol is synthesised first, and the fatty acid in position sn-1 is subsequently desaturated by specific acyl-lipid desaturases; that in position sn-2 is not affected. In some bacteria, including E. coli, the cardiolipin synthase ClsB catalyses headgroup exchange between phosphatidylethanolamine and glycerol to form small amounts of phosphatidylglycerol and free ethanolamine.
As discussed in other web pages of this site, much of the phosphatidylglycerol formed is utilized for the biosynthesis of cardiolipin, but it is also the precursor for bis(monoacylglycero)phosphate and many glycophospholipids, as well as bacterial proteolipids, lipoteichoic acids and the complex lipoamino acids (the last are discussed below).
Lysophosphatidylglycerol, with a fatty acid in position sn-1 only, has been reported to have some biological properties in animal tissues in vitro, but it is not known whether these are relevant in vivo. Elevated concentrations have been detected in acute coronary syndrome and these may be linked to the pathogenesis of cardiovascular diseases. It is the conserved terminal element of bacterial capsular lipopolysaccharides, which are important virulence factors for many pathogens.
Acylphosphatidylglycerol or (1,2-diacyl-sn-glycero-3-phospho-(3'-acyl)-1'-sn-glycerol) was first isolated as a minor component of the phospholipids of the bacterium Salmonella typhimurium, and it has since been found in a number of prokaryotic species, including E. coli. In particular, it is a characteristic lipid in the membranes of Corynebacteria and is especially abundant in those species that lack mycolic acids. C. amycolatum, for example, contains 20-29% of this lipid, with mainly C14 to C18 saturated and monoenoic fatty acid components; the fatty acid on the head group glycerol is mainly oleate. It has also been found in parasitic protozoa, such as Trichomonas vaginalis and T. foetus. The only report of its occurrence in plants is from oats (Avena sativa), which are also known to contain N‑acylphosphatidylethanolamine in appreciable amounts. However, acylphosphatidylglycerol with one of the fatty acids an oxylipin has been detected in stressed A. thaliana.
Acylphosphatidylglycerol is formed in vitro in experiments designed to study the biosynthesis of lysobisphosphatidic acid in animal cells, and in this instance the fatty acid on the glycerol head group is presumed to be in the sn-2' position. It is not clear whether it occurs naturally in animal tissues.
Bis-phosphatidic acid or phosphatidyldiacylglycerol (fully acylated phosphatidylglycerol) is occasional reported from bacteria and it can be produced as a minor component of animal cells by trans-phosphatidylation of phosphatidylcholine with diacylglycerol, catalysed by the enzyme phospholipase D, a possible mechanism for removing excess messenger diacylglycerol. In this instance, the stereochemistry of the second glycerol is presumably different from that in normal phosphatidylglycerol, i.e. the phosphate will be attached to the sn-3/sn-3' positions. The bis-phosphatidic acid found in lysosomes is related to bis(monoacylglycero)phosphate.
5. Complex Lipoamino Acids
In some species of Gram-positive bacteria or rarely in certain Gram-negative bacteria, the 3'-hydroxyl of the phosphatidylglycerol moiety may be esterified to an amino acid (lysine, ornithine or alanine, or less commonly arginine or glycine) to form an O-aminoacylphosphatidylglycerol. Such lipids been termed lipoamino acids, though it might be better to call them "complex lipoamino acids" to distinguish them from those consisting simply of a fatty acids linked to an amino acid, such as the ornithine lipids, which are presumed to have similar functions. There are related complex lipoamino acids derived from cardiolipin in some bacterial species.
For example, lysyl-phosphatidylglycerol (lysyl-PG) is a major membrane lipid (20 to 40%) in Staphylococcus aureus, while ornithyl-PG is found in Mycobacterium tuberculosis and alanyl-PG in Clostridium perfringens. N-succinylated L-lysylphosphatidylglycerol has been detected in B. subtilis. Enterococcus faecalis has been reported to contain alanyl-PG, 2'-lysyl-PG, 3'-lysyl-PG, 2',3'-dilysyl-PG and arginyl-PG, not to forget a diglucosyl derivative of PG. Alkyl- and alkenyl-forms have been detected also. It should be noted that 2'-lysyl-PG can undergo acyl migration to yield 3'-lysyl-PG. Related lipids containing glycine and ornithine have been found in other bacterial species.
An enzyme MprF ("multiple peptide resistance factor") is a highly conserved protein family of aminoacyl phosphatidylglycerol synthases in bacteria, which is able to transfer lysine or alanine from the appropriate aminoacyl-tRNAs (the same substrates as for protein biosynthesis) to the 3'-hydroxyl group of phosphatidylglycerol (or cardiolipin) to form lysyl- or alanyl-phosphatidylglycerol, respectively. In Staphylococcus aureus, the C-terminal domain of MprF is sufficient for the full production of lysylphosphatidylglycerol at the inner leaflet of the cytoplasmic membrane, whereas the N-terminal MprF domain translocates the newly formed lipid from the inner to the outer leaflet and therefore acts as a flippase. Related enzymes are present in many other bacterial species, some of which can only produce a single lipoamino acid while others can produce several products. In comparison to the precursor phosphatidylglycerol and cardiolipin, which have negatively charged head groups, the aminoacylated products are cationic or zwitterionic. It is now evident that an important function of these complex lipoamino acids in the membranes of bacteria is to lower the net negative charge of their cellular envelope in order to protect them from antimicrobial cationic polypeptides produced by other bacteria (bacteriocins), plants and animals, and perhaps to facilitate the interaction of pathogenic bacteria with their host. They also protect against environmental stresses such as those encountered during extreme osmotic or acidic conditions. Membranes containing these lipids are much less permeable than those containing phosphatidylglycerol per se. The hydrophobic flippase domain of MprF is seen as a target for the development of new antibiotics.
Strictly speaking, the betaine lipids, phosphatidylserine, phosphatidylthreonine, lysyl-diacylglycerol and related lipids discussed elsewhere on this site for various practical reasons could also be termed complex lipoamino acids (not related to phosphatidylglycerol).
Phosphatidylglycerol is not the easiest phospholipid to analyse, as it tends to elute close to phosphatidic acid in many chromatographic systems, although it can usually be resolved by two-dimensional thin-layer chromatography. Electrospray mass spectrometry under negative ionization conditions appears to be well suited to determination of molecular species compositions and to the analysis of the complex lipoamino acids.
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|Credits/disclaimer||Updated: June 29th, 2021||Author: William W. Christie|