Monogalactosyldiacylglycerols and digalactosyldiacylglycerols (together with the plant sulfolipid – see below) are the main lipid components of the various membranes of chloroplasts and related organelles, and indeed they are the most abundant lipids in all photosynthetic tissues, including those of higher plants, algae and certain bacteria. For example, mono- and di-galactosyldiacylglycerol amount to 50% and 25% of chloroplast glycerolipids, respectively. In photosynthetic tissues, monogalactosyldiacylglycerols are located exclusively in plastid membranes, but digalactosyldiacylglycerols can also be found in extra-plastidic membranes under some conditions. In non-photosynthetic tissues of plants, the proportion of these glycosyldiacylglycerols is much lower under normal growth conditions, although flowers contain appreciable amounts. The predominant structures are 1,2-di-O-acyl-3-O-β-D-galactopyranosyl-sn-glycerol and 1,2-di-O-acyl-3-O-(α-D-galactopyranosyl-(1→6)-O-β-D-galactopyranosyl)-sn-glycerol.
In higher plants, the galactolipids of photosynthetic tissues contain a high proportion of polyunsaturated fatty acids, up to 95% of which can be linolenic acid (18:3(n-3)). In this instance, the most abundant molecular species of mono- and digalactosyldiacylglycerols must have 18:3 at both sn-l and sn-2 positions of the glycerol backbone. Plants such as the pea, which have 18:3 as almost the only fatty acid in the monogalactosyldiacylglycerols, have been termed "18:3 plants". Other species, and the 'model' plant Arabidopsis thaliana is an example, contain appreciable amounts of hexadecatrienoic acid (16:3(n-3)) in the monogalactosyldiacylglycerols, and they are termed "16:3 plants". A further distinctive feature is that this acid is located entirely at the sn-2 position of the glycerol backbone (see Table 1). Palmitic acid tends to be found only in digalactosyldiacylglycerols, usually in small amounts, when the positional distribution appears to depend on species. In non-photosynthetic tissues, such as tubers, roots or seeds, the Cl8 fatty acids are usually more saturated (c.f. the data for wheat flour lipids).
|Table 1. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of mono-and digalactosyldiacylglycerols from leaves of Arabidopsis thaliana and from wheat flour.|
|Arabidopsis thaliana |
|Wheat flour |
| Browse, J., Warwick, N., Somerville, C.R. and Slack, C.R. Biochem. J., 235, 25-31 (1986).
 Arunga, R.O. and Morrison, W.R. Lipids, 6, 768-776 (1971).
On the basis of these structures, galactolipids are classified into two groups. The first has mainly C18 fatty acids at the sn-1 position of the glycerol backbone, and only C16 fatty acids at the sn-2 position, and it is termed a "prokaryotic" structure (as it is characteristic of cyanobacteria - see Table 2 below also). The second class has C16 or C18 fatty acids at the sn-l position but only C18 fatty acids in the sn-2 position, and this is termed a "eukaryotic" structure, as it is present in most glycerolipids, such as the phospholipids, of all eukaryotic cells. The exception is phosphatidylglycerol, which is synthesised in chloroplasts via the prokaryotic pathway only. Some plants contain both eukaryotic and prokaryotic structures in the monogalactosyldiacylglycerols.
The structural differences in the diacylglycerol moiety of galactolipids from algae and higher plants are believed to originate in compartmentalization of the biosynthetic pathways or precursors in eukaryotic cells, each compartment having its own distinctive enzymes.
The main galactosyldiacylglycerols consist of 1,2-di-O-acyl-3-O-β-D-galactopyranosyl-sn-glycerol and its digalactosyl homologue formed biosynthetically by addition of an α-D-galactopyranosyl residue to C6 of the first galactose unit. However, other homologues occur that are formed by a continuation of this process, i.e. tri- and tetragalactosyldiacylglycerols. For example, trigalactosyldiacylglycerols have been found in pumpkins and potatoes, and tri- and tetragalactosyldiacylglycerols in oats and rice bran. Formation of such compounds in membranes is now known to be a normal process that contributes to freezing tolerance.
In addition, a second series has been identified consisting of all-beta-linked homologues, i.e. linear (1→6)-linked all-β-galactolipids with two to four galactose units. These have been found in a variety of plant families, algae and some bacteria. A third series has been described from one plant species that is derived from the normal (α→β)-linked digalactosyldiacylglycerol by sequential addition of 6-O-β-D-galactopyranosyl residues, resulting in alternative types of tri- and tetragalactosyldiacylglycerols. An interesting polyglycosyl glycerolipid has been found in mung beans with a terminal rhamnose unit and unusually an alkyl group in position sn-2 (in animal glycerolipids, the ether moiety is invariably in position sn-1 - see our web pages on Ether lipids).
Oat seeds contain an interesting form of digalactosyldiacylglycerol with an estolide linkage, i.e. 15-hydroxylinoleic acid is esterified to position sn-2 of the glycerol moiety, and the hydroxyl group of the fatty acid is esterified with linoleic acid. Further tri- and tetragalactosyldiacylglycerols with up to three estolide-linked fatty acids have now been identified.
1,2-Di-O-acyl-3-O-β-D-glucopyranosyl-sn-glycerol has been found in rice bran, where it occurs with the corresponding galactolipids in an approximate ratio of 1:2. Interestingly, the two forms differ appreciably in their fatty acid compositions. Triglycosyldiacylglycerols containing a high proportion of glucose have also been found in rice, but the structures have not been confirmed definitively. Although glucosyldiacylglycerols have been found in some other plants, they are always rather minor components.
The phytoplackton Chrysochromulina polylepis contains monogalactosyldiacylglycerol linked via the sugar moiety and an ester bond to a chlorophyll pigment. A marine algal species contains sn-1,2-di-palmityl-3-(N-palmityl-6'-desoxy-6'-amino-α-D-glucosyl)-glycerol and a homologue of this.
Under stress by mechanical wounding, bacterial infection and freezing/thawing, plants synthesise monogalactosyldiacylglycerols in which the galactose unit is acylated, often but not always by an oxylipin (see the web page on these compounds), depending on species.
Biosynthesis of plant galactosyldiacylglycerols is discussed in much greater detail in the Plant Biochemistry section of this site. The basic biochemical mechanisms of galactolipid synthesis require the synthesis of 1,2-diacyl-sn-glycerols either by dephosphorylation of phosphatidic acid (prokaryotic diacylglycerols) or from phosphatidylcholine (eukaryotic diacylglycerols) in the endoplasmic reticulum, the latter probably via the action of a phospholipase D. A monogalactosyldiacylglycerol synthase, located in the inner envelope membrane of the chloroplast, then effects the reaction of the diacylglycerols with uridine 5-diphosphate(UDP)-galactose to form monogalactosyldiacylglycerols. The enzyme must first be activated by phosphatidic acid, a key signalling molecule in plants, although phosphatidylglycerol may also have a role.
A further enzyme system catalyses the addition of another galactose unit from UDP-galactose to form digalactosyldiacylglycerols. In the prokaryotic pathway, which is located in the chloroplast envelope, 16:1-ACP and 18:1-ACP newly synthesised in the plastid are utilized for production of phosphatidic acid, which contains only oleic acid in position sn-1 and palmitic acid in position sn-2. The eukaryotic pathway utilizes phosphatidylcholine in the endoplasmic reticulum and yields diacylglycerols with C18 fatty acids in position sn-2 and a C18 or a C16 fatty acid in position sn-1. There is extensive trafficking of diacylglycerols between the various cellular compartments, and the acyl moieties of these are actively desaturated in situ to produce the eventual fatty acid and molecular species compositions. Thus, the final galactolipid structures are governed by the relative activities of the various enzyme systems in different cellular organelles and the rates of exchange between each. The mechanism of these transfers is now becoming clearer; various lipid transporters have been characterized, and some vesicular transport may be involved. All of these steps are important for the biogenesis of chloroplasts.
There are now known to be three different sets of lipid galactosyltransferases or monogalactosyldiacylglycerol synthases that catalyse the final step in the process in the plastid envelope of Arabidopsis thaliana. The first, designated MGD1, is an inner envelope membrane-associated protein of chloroplasts, and this is responsible for most galactolipid biosynthesis in green tissues. It is indispensable for the biogenesis of thylakoid membranes and for embryogenesis. Under conditions of phosphate limitation and in non-photosynthetic tissues, a second set consisting of two isoforms, designated MGD2 and MGD3, and located in the outer envelope of plastids is more active; it has no function in chloroplast biogenesis or plant development when there is sufficient nutrient. Similarly, there are two digalactosyldiacylglycerol synthases, DGD1 and DGD2; the first is responsible for the most digalactosyldiacylglycerol synthesis, while DGD2 is most active during phosphate deficiency. As DGDG1 is located on the chloroplast outer membrane, the precursor monogalactosyldiacylglycerol must be transported across the membrane by some means.
A third pathway for the biosynthesis of di- and oligogalactosyldiacylglycerols in the outer chloroplast membrane does not use UDP-galactose as the donor, but involves a processive enzyme, which transfers galactose from one galactolipid to another with concomitant formation of diacylglycerols, i.e. it is a galactolipid: galactolipid galactosyltransferase. This process appears to be involved in freezing tolerance.
It is now evident that galactolipids, and digalactosyldiacylglycerols especially, are important for the lipid composition of the extra-plastid membranes in plants under phosphate-limiting conditions, assisting to conserve this important nutrient by acting as a replacement for phospholipids to maintain membrane homeostasis. Indeed, it is now apparent that phospholipids undergo a remodelling process in which they are first hydrolysed to diacylglycerols with release of phosphate for other purposes prior to glycolipid formation. In addition, galactolipid synthesis is regulated by light, plant hormones, redox state, phosphatidic acid levels, and many other stress conditions, including drought.
Because of its small head group, monogalactosyldiacylglycerol has a cone-like geometry with galactose at the point and the two fatty acyl chains oriented towards the base. Therefore, in aqueous systems, it tends to form a hexagonal-II phase, with the polar head group facing towards the centre of micellar structures rather than forming a bilayer. In contrast, digalactosyldiacylglycerols with two galactose moieties in the head group have a more cylindrical shape, so they form lamellar phases and thence bilayers. The ratio of these two lipids must be under tight control for proper membrane function.
It is clear that the galactosyldiacylglycerols have important functions in photosynthesis, and the nature of these functions is slowly being clarified. However, the photosystem I complex of cyanobacteria has been crystallized and found to contain three molecules of monogalactosyldiacylglycerol and one of phosphatidylglycerol, while the photosystem II complex has up to 11 moles of monogalactosyldiacylglycerols and four moles of digalactosyldiacylglycerols. These lipids are required for crystallization of the light-harvesting complex II in pea chloroplasts (again together with phosphatidylglycerol). There is evidence that digalactosyldiacylglycerols bind together the various extrinsic proteins in the photosystem II complex to enable it to function in an efficient manner.
As with other biomembranes, the thylakoid membrane (which encloses the chloroplasts and the photosynthetic apparatus in plants) has an asymmetric distribution of glycolipids between the two leaflets, with much of the digalactosyldiacylglycerol on the luminal leaflet. For example, there is a suggestion that the polar head group of the lipid assists the movement of protons along the luminal membrane surface to the ATPase. It is also evident that individual glycolipids are associated in a highly specific way with various membrane proteins, where the ability of monogalactosyldiacylglycerols to form inverted micelles may be important. The presence of this lipid may be required to assist the transport of proteins and other nutrients across membranes. As they are concentrated in the peribacteroid membrane surrounding nitrogen-fixing rhizobia in the nodules of legumes, they may be needed for the exchange of ammonium and nutrients, in this instance between the bacteria and the host cell. The axial hydroxyl group at C4 of galactose appears to be essential for certain of these interactions and may explain why galactolipids are favoured over those containing glucose.
Mono- and digalactosylmonoacylglycerols (lyso derivatives) are found from time to time in small amounts in plant tissues. Usually the sn-1 isomer is identified, but acyl migration could occur quickly to give this, the more thermodynamically stable isomer. It is not clear whether these lyso-compounds play a part in galactolipid turnover and fatty acid re-modelling.
Sulfoquinovosyldiacylglycerol or 1,2-di-O-acyl-3-O-(6'-deoxy-6'-sulfo-α-D-glucopyranosyl)-sn-glycerol (quinovose = 6-deoxyglucose), the plant sulfolipid, is the single glycolipid most characteristic of photosynthetic organisms, including both higher plants and cyanobacteria.
In many species of higher plants, the sn-1 position is enriched in 16:0 and the sn-2 position in 18:3 and 18:2. Biosynthesis of the sulfoquinovose head-group involves a unique set of enzymes that serve no other function. Much remains to be learned regarding the details of the biosynthetic pathway, but it is believed that it involves synthesis of UDP-sulfoquinovose from UDP-glucose and sulfite, followed by the transfer of sulfoquinovose to position sn-3 of 1,2-diacyl-sn-glycerols. The process occurs entirely in the plastids, although diacylglycerols transferred from the endoplasmic reticulum can be used as substrates.
Other than in active photosynthetic organisms (cyanobacteria - see below), this lipid has only been found in a few bacterial species, mainly of the genus Rhizobium, which have a symbiotic relationship with plants in root nodules and may have obtained the required genes by horizontal gene transfer. However, it was also found to comprise half the lipids of the halophilic eubacteria Planococcus sp. and Haloferax volcanii.
Photosystem II contains up to 25 lipid molecules of which three are sulfolipid. As with the neutral galactosyldiacylglycerols, it seems clear that the negatively charged sulfoquinovosyldiacylglycerol is important for photosynthesis and for the function of the thylakoid membrane in plants, where it is located mainly on the inner leaflet, possibly by assisting in the process of protein insertion and passage through the membranes. This membrane has a very large surface area, as is necessary for photosynthesis, and the sulfolipid appears to provide the required negative charge with a minimum demand for phosphate (the only phospholipid present is a small amount of phosphatidylglycerol, which appears to have a similar function). This is particularly important for plants when phosphate concentrations are limiting, and it seems that sulfolipid may even be dispensable otherwise.
An acylated derivative of this sulfolipid, 2'-O-acyl-sulfoquinovosyldiacylglycerol has been found in the unicellular alga Chlamydomonas reinhardtii, i.e. with an additional acyl group attached to the 2'-hydroxyl of the sulfoquinovosyl head group. While the fatty acids of sulfoquinovosyldiacylglycerol were mostly saturated, the 2’-acylated analogue contained mainly unsaturated fatty acids with an 18-carbon fatty acid with four double bonds linked to the head group.
5. Glycosyldiacylglycerols of Bacteria
Cyanobacteria are oxygenic photosynthetic bacteria (Gram negative) that are distinct from most other bacteria in their lipid compositions, as they contain appreciable amounts of mono- and digalactosyldiacylglycerols together with sulfoquinovosyldiacylglycerol in which the configuration of the anomeric head groups is identical to that of the corresponding plant lipids. Indeed, the membrane architecture of cyanobacteria and chloroplasts in higher plants is very similar. This may be explained by the theory that an ancestral cyanobacterial cell, which was photosynthetically active, was engulfed by a eukaryotic organism to become the precursor of the first plant cell, the composition of which has been largely conserved throughout evolution. The role of digalactosyldiacylglycerols in the photosynthetic apparatus in these organisms is discussed above.
As can be seen from the data in Table 2, the overall fatty acid compositions of the lipids of the cyanobacterium Synechocystis PCC6803 resemble that of photosynthetic tissues in higher plants although the polyunsaturated fatty acids (C18) are concentrated in position sn-1 in this instance with saturated fatty acids (C16) in position sn-2. Phosphatidylglycerol is often the only phospholipid present in appreciable amounts.
|Table 2. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of mono- and digalactosyl- and sulphoquinovosyldiacylglycerols from Synechocystis PCC6803*.|
|* Grown at 22°C; ** mainly α-18:3; tr = trace.
Data from Wada, H. and Murata, N. Plant Physiology, 92, 1062-1069 (1990).
On average, cyanobacteria contain ~52% MGDG, ~15% DGDG and ~9% SQDG, together with ~22% phosphatidylglycerol and ~1% minor components (Petroutsos, D. et al. (2014).
Although the nature of the lipids is highly conserved in plants and photosynthetic bacteria, the biosynthetic mechanisms are somewhat different. Cyanobacteria contain trace amounts of a monoglucosyldiacylglycerol in which the glucosyl group is in the β conformation, i.e. 1,2-diacyl-3-O-(β-D-glucopyranosyl)-sn-glycerol. This is also found in Bacillus subtillis where it amounts to 10% of the total lipids. It is now known that the production of monoglucosyldiacylglycerol in cyanobacteria is the first step in biosynthesis of galactosyldiacylglycerols by means of conversion by an epimerization reaction to the galactosyl form. The second galactose unit is added to the mongalactosyl product by a digalactosyldiacylglycerol synthase with UDP-galactose as the carbohydrate donor.
Many species of anoxic photosynthetic bacteria contain monogalactosyldiacylglycerols, but digalactosyldiacylglycerols are rarely found in other bacteria. However, the latter are major membrane components of free-living and bacteroid forms of Bradyrhizobium japonicum, which normally live symbiotically with plants in root nodules. The green photosynthetic bacterium Chlorobium tepidum contains rhamnosylgalactosyldiacylglycerols.
A wide variety of glycosyldiacylglycerols are found in non-photosynthetic bacteria; those with one to three glycosyl units linked to sn-1,2-diacylglycerol are most common, although others with up to five glycosyl units are found. These are very different from the plant glycosyl diacylglycerols, in that glucose is much more common than galactose, while the fatty acid components are mainly saturated, monoenoic and branched-chain or cyclopropanoid. The nature of the glucose linkages is also variable. For example, some Streptococcus species contain mono- and diglucosyldiacylglycerols, with the diglucoside unit having an α-(1→2) linkage as in kojibiose, and so can be termed ‘kojibiosyldiacylglycerols’. Related lipids together with diglucosyl-1-monoacyl-sn-glycerol and glycerophosphoryldiglucosyldiacylglycerol are present in S. mutans. S. pneumoniae contains glucopyranosyl- and galactoglucopyranosyldiacylglycerol, while this and other species contain similar lipids with a fatty acyl group attached to a carbohydrate moiety (usually in position 3 or 6).
Some microorganisms accumulate galactofuranosyl-diacylglycerols rather than the galactopyranosyl form, and a variety of unusual glycosyldiacylglycerols with differing carbohydrate moieties, or with differences in the glycosidic bonds from those in higher plants, have been found. For example, Micrococcus luteus synthesises mono- and dimannosyldiacylglycerols. Other bacteria have glycosyldiacylglycerols with a glycerophosphate group linked to a carbohydrate moiety (‘phosphoglycolipids’). Bacillus megaterium contains N-acetylgalactosamine linked to a diacylglycerol. As might be expected, even greater complexity exists in the triglycosyldiacylglycerols. In mechanistic terms, the biosynthesis of these lipids is analogous to that in higher plants described above.
In gram positive bacteria such as Staphylococcus aureus, lipoteichoic acid is anchored in the membrane by a diglucosyldiacylglycerol moiety. The membranes of this organism also contain 8 mol% of the free glycolipid, and the ratio of mono- to diglucosyldiacylglycerol may play an important role in determining bilayer stability; only the latter will form a bilayer. Similarly, the human pathogen Enterococcus faecalis produces diglucosyldiacylglycerol as a membrane component and as a lipoteichoic acid precursor in a secreted biofilm, which is involved in adherence to host cells and virulence in vivo. There is increasing interest in such lipids as it has been demonstrated that galactosyldiacylglycerols from Borrelia burgdorferi, the causative agent of Lyme disease, are involved in the antigen response via specific receptors.
Certain bacteria, fungi and algae contain the ionic 1,2-diacyl-3-O-α-D-glucuronyl-sn-glycerol among their membrane lipids, and a conjugate of this with taurine is known (see our webpage on sulfonolipids). Of course, the algal lipid illustrated has a very different fatty acid composition from those of bacteria. In addition, glucosylglucuronyl- and galacturonyldiacylglycerols have been detected in bacteria.
The complex diether isoprenoid glycerolipids (discussed elsewhere) from the extreme halophilic bacteria of the Archaea family exist in the form of glycosyldiacylglycerols, both as neutral lipids and in sulfated form, with two to four glycosyl units attached to glycerol.
The main neutral galactolipids in plants present no particular difficulties for analysis. They are easily separated from phospholipids by adsorption chromatography, usually by making use of the fact that they, unlike phospholipids, are soluble in acetone. Because of its highly polar acidic nature, sulfoquinovosyldiacylglycerol presents more analytical problems, but methods have been devised for its analysis that make use of adsorption or ion-exchange chromatography. Electrospray-ionization tandem mass spectrometry now appears to hold particular promise for structural analyses. The review by Heinz cited below is essential reading for anyone who wishes to study these lipids.
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Updated May 22, 2014