Phosphatidylglycerol & Related lipids
1. Phosphatidylglycerol in Bacteria and Plants
Phosphatidylglycerol is a ubiquitous lipid that can be the main component of some bacterial membranes, and it is found also in membranes of plants and animals where it appears to perform specific functions. The charge on the phosphate group means that it is an anionic lipid at neutral pH. The dihexadecanoyl species is illustrated as an example.
Phosphatidylglycerol is found in almost all bacterial types. For example, Escherichia coli, a widely studied 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 alkylacyl- and alkenylacyl forms are more abundant. Phosphatidylglycerol is synthesised only in mitochondria of non-photosynthetic eukaryotes, and it is used as the precursor for cardiolipin, which is located in the inner mitochondrial membrane and is required for proper functioning of the enzymes involved in oxidative phosphorylation.
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 entirely dispensable and can be substituted by other anionic phospholipids such as phosphatidic acid. 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.
In cyanobacteria and plants, which are able to carry out aerobic photosynthesis, phosphatidylglycerol is found in all cellular membranes, but it appears to be especially important in the thylakoid membrane, which surrounds the chloroplast and where 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 glycosyldiacylglycerols). While sulfoquinovosyldiacylglycerol can substitute for phosphatidylglycerol to a certain extent, especially under conditions of phosphate deficiency and presumably to maintain a required level of anionic lipids, a minimum level of phosphatidylglycerol appears to be essential for photosynthesis and growth.
In the photosynthetic membranes of leaf tissue of higher plants, the phosphatidylglycerol is 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. The rate of its synthesis in leaves deprived of light is greatly reduced (with accumulation of the precursor palmitic acid).
|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., Warwick, N., Somerville, C.R. and Slack, C.R. Biochem. J., 235, 25-31 (1986).|
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 other than phosphatidylglycerol. This is because phosphatidylglycerol is synthesised in chloroplasts via the so-called "prokaryotic" pathway only (see below and in 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.
The role of phosphatidylglycerol in the photosynthetic apparatus of bacteria and higher plants is gradually being clarified, and it is known that in cyanobacteria, it is essential for the oligomerization of photosystems I and II. Analysis of the crystal structure of the photosystem I of cyanobacteria has shown that it contains three molecules of phosphatidylglycerol and one of monogalactosyldiacylglycerol as integral components, while phosphatidylglycerol is one of up to 25 lipid molecules bound to the photosystem II complex. 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.
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 (~ 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 with the division of plastids in higher plants.
A fully acylated phosphatidylglycerol, termed bis-phosphatidic acid or phosphatidyldiacylglycerol, and plasmalogen analogues have been found in bacteria. While this structure has on occasion been ascribed in error to other lipids in developing seeds or brain, it can indeed be formed in animal tissues (see below). This lipid with one of the fatty acids an oxylipin has been detected in stressed Arabidopsis thaliana. Two other 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. 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. The complex lipoamino acids and acylphosphatidylglycerols are discussed below.
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 (in a few species, 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, but its precise function is a matter of conjecture. For example, it may aid the spreading of dipalmitoyl-phosphatidylcholine, which is presumed to be the main functional component of lung surfactant. However, there are recent suggestions that it may have a role in the regulation of the innate immune response and the effects of viral infection. 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).|
In each, 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 seems that the acidic head-group is more important to surfactant function than the precise molecular species composition.
The lung aside, phosphatidylglycerol may be present in animal tissues merely as a precursor for diphosphatidylglycerol (cardiolipin). 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.
|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).|
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 proceeds by condensation of phosphatidic acid and cytidine triphosphate with elimination of pyrophosphate via the action of an enzyme phosphatidate cytidyltransferase (or CDP-synthase). The same liponucleotide is an important intermediate in the biosynthesis of phosphatidylinositol, but rather different routes are taken to phosphatidylcholine and phosphatidylethanolamine.
CDP-diacylglycerol reacts with glycerol-3-phosphate via phosphatidylglycerophosphate synthase to form 3-sn-phosphatidyl-1'-sn-glycerol 3'-phosphoric acid, with release of cytidine monophosphate (CMP). Finally, phosphatidylglycerol is formed by the action of one of two phosphatases. As biosynthesis is via glycerol-3-phosphate, the second glycerol moiety is attached at position sn-1 to the phosphate group. 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 in part (in effect, racemization).
In animal tissues, the eventual fatty acid composition is attained by the process of remodelling known as the Lands' cycle (see the webpage 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 found to have a preference for 16:0-, 18:0-, and 18:1-CoA esters as donors.
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 higher plants, phosphatidylglycerol is synthesised in three cellular compartments, plastids, endoplasmic reticulum and mitochondria. In the plastids, 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, while the oleate in position sn-1 is desaturated to 18:2 and 18:3 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.
Phosphatidylglycerol is the biosynthetic precursor of cardiolipin, lysobisphosphatidic acid 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.
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 component of the membranes of Corynebacteria and is especially abundant in those species that lack mycolic acids. C. amycolatum, for example, contained 20-29% of this lipid, with mainly C14 to C18 saturated and monoenoic fatty acid components; the fatty acid on the head group glycerol was 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 .
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) 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 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 lysobisphosphatidic acid.
In some species of Gram-positive 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 (or it can be linked to another fatty acid or to glucose). 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 acid linked to an amino acid (see our web page on simple amides). There are related complex lipoamino acids derived from cardiolipin in some bacterial species.
For example, lysyl-phosphatidylglycerol (lysyl-PG) is a major membrane lipid in Staphylococcus aureus, while ornithyl-PG is found in Mycobacterium tuberculosis and alanyl-PG in Clostridium perfringens. 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. 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") has been characterized from a number of bacterial species, 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 to form lysyl- or alanyl-phosphatidylglycerol, respectively. The reaction is catalysed by membrane-bound aminoacyl-phosphatidylglycerol synthases. There is evidence that the function of these complex lipoamino acids in the membranes of pathogenic bacteria is to lower the net negative charge of their cellular envelope in order to protect them from antimicrobial cationic polypeptides produced by plants and animals. 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.
Phosphatidylglycerol is not the easiest phospholipid to analyse. It tends to elute close to phosphatidic acid in many chromatographic systems, but 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 composition. Similarly, modern mass spectrometric methods seem to be suited to the analysis of the complex lipoamino acids.
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Updated: June 12, 2014