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Glycosyldiacylglycerols: 1. Plants and Microorganisms



The glycosyldiacylglycerols of higher plants, yeasts and many lower organisms are important membrane constituents with innumerable vital functions. In many ways they are equivalent to and may substitute for phosphoglycerolipids, especially when the supply of phosphorus is restricted. They share a common 1,2-diacyl-sn-glycerol backbone, but polar carbohydrate rather than phosphate moieties occupy position sn-3. In particular, mono- and digalactosyldiacylglycerols together with sulfoquinovosyldiacylglycerols are key components of the thylakoid membranes in chloroplasts, and are intimately involved in the process of photosynthesis; they are therefore essential for life on Earth. Related lipids are important cell wall constituents of bacteria, and they are the anchor element of the lipoteichoic acids. Glycosyldiacylglycerols in animals are very different in structure and function, and they are discussed separately on this website here...


1.  Mono- and Digalactosyldiacylglycerols from Plants

Monogalactosyldiacylglycerols and digalactosyldiacylglycerols (together with the plant sulfolipid – see below and phosphatidylglycerol) are the main lipid components of the various membranes of chloroplasts and related organelles. 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, and they are conserved from cyanobacteria through green algae to vascular plants.

Structural formulae for mono- and digalactosyldiacylglycerols

These are the most abundant lipids in all photosynthetic tissues, including those of higher plants, algae and certain bacteria, and for example, mono- and digalactosyldiacylglycerol amount to 27% and 31%, respectively, of spinach chloroplast glycerolipids, and they are accompanied by 6% sulfoquinovosyldiacylglycerol and 9% phosphatidylglycerol. 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 relative proportions of the two galactolipids and the ratio of galactolipids to phospholipids are stable when plants are grown under favourable conditions, but they can change markedly when these are subjected to stress.

In higher plants, the galactolipids of photosynthetic tissues contain a high proportion of polyunsaturated fatty acids, up to 95% of which can be α‑linolenic acid (18:3(n-3)), and the most abundant molecular species of mono- and digalactosyldiacylglycerols have 18:3 at both sn-1 and sn-2 positions of the glycerol backbone. In microalgae, especially those of marine origin, these lipids can also contain more highly unsaturated fatty acids, including eicosapentaenoic (20:5(n-3)) and docosahexaenoic (22:6(n-3)) acids.

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 7,10,13-hexadecatrienoic acid (16:3(n-3)) in the monogalactosyldiacylglycerols, and they are termed "16:3 plants". A further distinctive feature is that 16:3 is located entirely at the sn-2 position of the glycerol backbone (see Table 1). Palmitic acid tends to be found mainly in digalactosyldiacylglycerols, usually in small amounts and largely in position sn-1, although the positional distribution appears vary somewhat with species. In non-photosynthetic tissues, such as tubers, roots or seeds, the Cl8 fatty acids are usually more saturated in that they tend to contain more linoleate (18:2(n-6)) (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 and of sulfoquinovosyl-diacylglycerols from leaves of A. thaliana and from wheat flour.
Position Fatty acids
16:0 16:3(n‑3) 18:0 18:1 18:2 18:3(n‑3)
 
Arabidopsis thaliana [1]
  MGDG
sn-1 2 1 trace trace 4 93
sn-2 trace 70 trace trace 1 28
  DGDG
sn-1 15 2 trace 2 3 76
sn-2 9 3 trace trace 4 83
  SQDG
sn-1 23 - 2 2 6 68
sn-2 63 - trace 2 3 32
 
Wheat flour [2]
  MGDG
sn-1 11 - 1 5 81 1
sn-2 trace - trace 9 83 7
  DGDG
sn-1 26 - 2 4 63 4
sn-2 2 - trace 7 83 7
[1] Browse, J. et al. Biochem. J., 235, 25-31 (1986).
[2] Arunga, R.O. and Morrison, W.R. Lipids, 6, 768-776 (1971).

On the basis of these structures and of what is known of the biosynthetic mechanisms, 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, such as 16:3(n-3), at the sn-2 position, and it is termed a "prokaryotic" structure (as it is characteristic of cyanobacteria - see Table 3 below). The second class has C16 or C18 fatty acids at the sn-1 position but only C18 fatty acids, especially 18:3(n‑3), 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, and in fact, Arabidopsis has roughly equal amounts synthesised by each pathway.

The structural differences in the diacylglycerol moiety of galactolipids from algae and higher plants originate in compartmentalization of the biosynthetic pathways or precursors in cells, especially between the chloroplasts and endoplasmic reticulum, each compartment having its own distinctive enzymes (as discussed below). Digalactosyldiacylglycerols are the only galactolipid in the plasma membrane, where they are located on the inner leaflet. Although they do not occur in plant mitochondrial membranes under normal growth conditions, digalactosyldiacylglycerols can accumulate until they amount to 18% of the total during phosphate deprivation.


2.  Biosynthesis of Glycosyldiacylglycerols in Plants

The basic biochemical mechanisms of galactolipid synthesis require the synthesis of 1,2-diacyl-sn-glycerols either by dephosphorylation of phosphatidic acid in the chloroplasts (prokaryotic diacylglycerols) or of phosphatidylcholine (eukaryotic diacylglycerols) in the endoplasmic reticulum (or the inner envelope membrane), 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 (produced in the cytoplasm) 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. It is noteworthy that the MGDG synthase changes the configuration of the α-galactose in UDP-galactose to the β-form, but the DGDG synthesis preserves the α-configuration

Biosynthesis of monogalactosyldiacylglycerols

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 A. 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 such as roots and pollen, two further isoforms designated MGD2 and MGD3 and located in the outer envelope of plastids are more active; these have 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 most digalactosyldiacylglycerol synthesis, while DGD2 is most active during phosphate deficiency. As DGD1 is located on the chloroplast outer membrane, the precursor monogalactosyldiacylglycerol must be transported across the membrane by some means. Transfer of digalactosyldiacylglycerols into mitochondria during phosphate deprivation is believed to involve a contact site in the endoplasmic reticulum, possibly after remodelling of glycerolipids in the tonoplast membranes, which contain an active phospholipase D.

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. This specificity is achieved by the stepwise action of two acyltransferases, ATS1 in the stroma and ATS2 in the inner envelope membrane of the chloroplast. The palmitic acid in position sn-2 then serves as a substrate for desaturases to produce 16:3. The eukaryotic pathway utilizes phosphatidylcholine in the endoplasmic reticulum as the precursor to yield diacylglycerols with C18 fatty acids in position sn-2 and a C18 or a C16 fatty acid in position sn-1. There is little or no phosphatidylcholine in the chloroplast membranes. As the digalactosyldiacylglycerols contain very little 16:3, the DGDG synthase must utilize specific molecular species of monogalactosyldiacylglycerols as substrates.

There is extensive trafficking both of diacylglycerols and fatty acids 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.

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 transferase that 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. The enzyme also has the capacity to produce tri-and tetragalactosyldiacylglycerols in vitro at least.

Digalactosyldiacylglycerol biosynthesis by a galactolipid:galactolipid galactosyltransferase

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.

Catabolism: Acylhydrolases are present in plants that rapidly remove fatty acids from both positions of galactolipids, and α- and β-galactosidases complete the breakdown. Most of the acylhydrolases ('patatin'-like) are also capable of hydrolysing phospholipids, although at least one is specific for galactolipids.


3.  Other Non-acidic Glycosyldiacylglycerols from Plants

The main galactosyldiacylglycerols in plants are those described above, but other homologues occur. 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). Similarly, an unusual galactoglycerolipid with phytol ether-linked to position 1 of glycerol has been partially characterized from algae and cyanobacteria, and it may occur at trace levels in some higher plants.

Scottish thistle1,2-Di-O-acyl-3-O-β-D-glucopyranosyl-sn-glycerol has been found in rice bran, but not in the chloroplasts, 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.

Seaweeds (multicellular algae) contain the conventional range of galactolipids, including sulfoquinovosyldiacylglycerol discussed below, though often with distinctive fatty acid compositions as might be expected of marine organisms, and some species of red algae, such as Gracilaria species, contain glycosyldiacylglycerols that are highly enriched in arachidonic acid, for example 57% of the 20:4/20:4 molecular species and 18% of the 16:0/20:4 species in the monogalactosyldiacylglycerols.. In addition, some species contain monoglucopyranosyldiacylglycerols. The phytoplankton Chrysochromulina polylepis contains monogalactosyldiacylglycerol linked via the sugar moiety and an ester bond to a chlorophyll pigment, while a marine algal species contains sn‑1,2‑dipalmitoyl-3-(N-palmitoyl-6'-desoxy-6'-amino-α-D-glucosyl)-glycerol and a homologue of this.

Oat seeds contain a novel 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.

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 such as 12-oxo-10,15c-phytodienoic acids (12-oxo-PDA or 'OPDA'). It is now apparent that head-group acylation of mono- and digalactosyldiacylglycerols is a common stress response in plants, depending on species, and a phylogenetically conserved enzyme has been identified as responsible for the accumulation of acyl-monogalactosyldiacylglycerols in A. thaliana; the activity of this is strongly enhanced by stress conditions. Environmental stresses can also induce the accumulation of digalactosyldiacylglycerols in which both the galactose units have the β-configuration. In addition, oxylipins such as OPDA are found esterified to positions sn-1 and 2 of the glycerol moiety of mono- and digalactosyldiacylglycerols and sometimes to galactose also in Arabidopsis; these lipids have been termed 'arabidopsides', and they are discussed further in our web page on plant oxylipins.


4.  Sulfoquinovosyldiacylglycerol and Related Acidic Glycolipids

Sulfoquinovosyldiacylglycerol (SQDG) 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 higher plants, algae, chloromonads and cyanobacteria, and it is most abundant in the photosynthetic tissues. In contrast to the neutral galactosyldiacylglycerols, it is an anionic lipid with a negative charge on the head group.

Formula of sulfoquinovosyldiacylglycerol

In higher plants, the concentration of SQDG is very variable (2% to 11% of the total glycerolipids), but the proportion in the thylakoid membranes is much higher. In many species, including A. thaliana (Table 1), the sn-1 position is enriched in 18:3 and the sn-2 position in 16:0, a very different pattern from the mono- and digalactosyldiacylglycerols (or from the phospholipids); it is noteworthy that there is no 16:3 in this instance. The compositions in cyanobacteria are very different (see Table 3 below).

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 by UDP-SQ synthase (SQD1), a soluble enzyme in the chloroplast stroma, followed by the transfer of sulfoquinovose to position sn-3 of 1,2-diacyl-sn-glycerols by SQDG synthase (SQD2). The process occurs entirely in the plastids, although diacylglycerols transferred from the endoplasmic reticulum can be used as substrates. Together with phosphatidylglycerol, it has an indispensable function in the thylakoid membrane during photosynthesis as discussed below.

Other than in active photosynthetic organisms (cyanobacteria - see below), sulfoquinovosyldiacylglycerol 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. Surprisingly, it has been detected in a sea urchin (Scaphechinus mirabilis), where the fatty acid components are mainly saturated and monoenoic (C14 to C24).

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. In the diatom P. tricornutum, the 2' fatty acid is 20:5 in this lipid.

Catabolism:  SQDG is an important reservoir of organic sulfur in the biosphere that must be conserved. Catabolism involved first de-lipidation by lipases, followed by one of two sulfoglycolytic pathways to produce C3-sulfonates; these undergo biomineralization to form inorganic sulfur species to complete the sulfur cycle.

Formula of glucuronosyldiacylglycerolGlucuronosyldiacylglycerol:  When A. thaliana is stressed by being deprived of phosphate, a second anionic glycosyldiacylglycerol is produced, i.e. 1,2-diacyl-3-O-α-glucuronosyl-sn-glycerol, which appears to be just as important as sulfoquinovosyldiacylglycerol in protecting plants from the effects of phosphate deprivation, presumably by maintaining the negative charge on the membranes (see next section). While the lipid is present at low levels in plant species grown under normal conditions, its concentration is greatly elevated when phosphorus is limiting. There is now evidence that this is a wide-spread phenomenon in higher plants. Biosynthesis requires the sulfoquinovosyldiacylglycerol synthase SQD2, located in the chloroplast envelope, which transfers glucuronic acid from its UDP conjugate to diacylglycerols. The molecular species compositions of the two lipids are almost identical. Although this lipid had been reported earlier from bacteria, fungi and algae, little is know of its function or metabolism in these organisms (see below).


5.  Function of Glycosyldiacylglycerols in Plants

Chloroplasts are double-membrane organelles specific to plants and algae that perform oxygenic photosynthesis. In addition to inner and outer envelope membranes, chloroplasts have an extensive internal membrane system, the thylakoid membrane, where the photochemical and electron transport reactions of photosynthesis take place. The galactosyldiacylglycerols and sulfoquinovosyldiacylglycerol especially are key lipid components of the chloroplast membranes in plants and are essential for their function (Table 2). Under phosphate-limiting conditions, these lipids assist in conserving this important nutrient by acting as a replacement for phospholipids to maintain membrane homeostasis. Thus, phospholipids undergo a remodeling process in which they are first hydrolysed to diacylglycerols with release of phosphate for other purposes immediately prior to glycolipid formation. There is evidence that the biosynthesis of galactolipids is coordinated with the synthesis of chlorophyll and the proteins involved in photosynthesis. In addition, galactolipid synthesis is regulated by light, plant hormones, redox state, phosphatidic acid levels, and many other stress conditions, including drought. In pollen, galactolipids are concentrated in the plasma membrane.

Table 2. Lipid class composition (mol %) of membranes in plants and cyanobacteria.
MGDG DGDG SQDG PG PI PC Others
 
Chloroplast thylakoid membrane (a) 53 27 7 7 2 - 4
Chloroplast inner envelope (a) 49 30 5 8 1 6 1
Chloroplast outer envelope (a) 17 29 6 10 5 32 1
 
Non‑green plastid (b) 32 27 6 9 4 20 2
 
Synechocystis  sp. 54 81 15 13 - - -
(a) Spinach;   (b) cauliflower buds
Table adapted from Kobayashi, K. J. Plant Res., 129, 565-580 (2016).

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. As with other biomembranes, the thylakoid membrane has an asymmetric distribution of glycolipids between the two leaflets, often with much of the digalactosyldiacylglycerols on the luminal leaflet, where hydrogen bonding effects of its polar head group are essential to balance the repulsive electrostatic contributions of the charged lipids phosphatidylglycerol and sulfoquinovosyldiacylglycerol. In addition, there is a suggestion that the polar head group of this lipid assists the movement of protons along the luminal membrane surface to the ATPase.

Scottish thistleIt is clear that the galactosyldiacylglycerols have important functions in photosynthesis, and the nature of these functions is slowly being clarified. For example, 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 contains up to 25 lipid molecules, including eleven moles of monogalactosyldiacylglycerols, four of digalactosyldiacylglycerols and three of sulfolipid. 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.

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 symbiotic bacteria and the host cell. Similarly, the digalactosyl moiety of digalactosyldiacylglycerols plays a special role in the plant immune response towards bacterial infection (systemic acquired resistance). 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.

As with the neutral galactosyldiacylglycerols, the negatively charged sulfoquinovosyldiacylglycerol is essential 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. 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 has a similar function. This is particularly important for plants when phosphate concentrations are limiting, when glucuronosyldiacylglycerol assumes greater importance also. It is evident that there is a reciprocal relationship between the concentrations of the anionic lipids and of phosphatidylglycerol in these circumstances in all photosynthetic organisms. The function of digalactosyldiacylglycerols in mitochondrial membranes during phosphate deprivation is not known.


5.  Glycosyldiacylglycerols of Bacteria

Photosynthetic 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 (see Table 2). Indeed, the membrane architecture of cyanobacteria and chloroplasts in higher plants is very similar, and for example, cyanobacteria possess thylakoid membranes with comparable lipid compositions and functional properties. 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. As in higher plants, galactolipids are produced in greater relative amounts during phosphate deprivation.

As can be seen from the data in Table 3, 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 3. Composition (mol %) of fatty acids in positions sn-1 and sn-2 of mono- and digalactosyl- and sulphoquinovosyldiacylglycerols from Synechocystis PCC6803*.
Position Fatty acids
16:0 16:1 18:0 18:1 18:2 18:3** 18:4
  MGDG
sn-1 14 4 tr - 8 54 20
sn-2 94 2 tr 2 tr tr tr
  DGDG
sn-1 16 4 2 2 8 50 18
sn-2 94 2 2 tr - - -
  SQDG
sn-1 34 8 2 10 16 28 tr
sn-2 92 tr 4 tr tr tr -
Grown at 22°C; ** mainly 18:3(n-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 subtilis 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 monogalactosyl product by a digalactosyldiacylglycerol synthase with UDP-galactose as the carbohydrate donor. Monoglucosyldiacylglycerols of undefined stereochemistry have been detected in Synechococcus sp. PCC 7002.

Biosynthesis of galactolipids in cyanobacteria

Mutants of Synechocystis sp. in which the epimerase has been 'knocked out' accumulate monoglucosyldiacylglycerols only in the thylakoid membranes, but the organisms continue to function in photosynthesis, if less efficiently. While the role of digalactosyldiacylglycerols (and of sulfoquinovosyl-diacylglycerol) in the photosynthetic apparatus in these organisms is discussed briefly above, it should be noted that digalactosyldiacylglycerol is essential in Synechococcus elongatus PCC 7942. Its loss cannot be compensated by other lipids, including glucosylgalactosyldiacylglycerol, so the second galactose molecule may be the key to its function.

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 as well as monogalactosyldiacylglycerols. In the latter species, biosynthesis is by a different mechanism involving a unique UDP-galactose diacylglyceroltransferase.

Other bacteria: 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. For example, αGal1→2αGlc- and αGal1→6αGal1→2αGlc-diacylglycerols are often detected in Lactobacillus species, while mono- and diglucosyldiacylglycerols are present in the opportunistic pathogen Enterococcus faecalis. These lipids often differ from the plant glycosyl diacylglycerols in that glucose is much more common than galactose; a UDP-glucose:1,2-diacylglycerol-3-β-D-glucosyl transferase that is capable of transferring one or more sugars to create mono-, di-, or polyglycosylated diacylglycerols has been characterized from Bacillus subtilis. There is a related enzyme in Mesorhizobium loti that is capable of utilizing both UDP-galactose and UDP-glucose. In this instance, processive glycosyltransferases are responsible for the transfer of each sugar moiety, and they can catalyse three transferase steps, each using the glycosyldiacylglycerol produced by the earlier step as a substrate. The digalactosyldiacylglycerols in this organism differ from the normal plant lipid in that both galactose units are of the β-conformation. As noted earlier, Rhizobium and related species produce sulfoquinovosyldiacylglycerols. The fatty acid components of these lipids are mainly saturated, monoenoic and branched-chain or cyclopropanoid. In addition, many species of anaerobic bacteria contain alk-1'-enyl moieties (plasmalogens) in position sn-1 of the glycosyldiradylglycerols.

Formula of a bacterial glycolipidThe 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 galactoglucopyranosyldiacylglycerols, while this and other species contain similar lipids with a fatty acyl group attached to a carbohydrate moiety (usually in position 3 or 6). Rhodobacter sphaeroides contains 1,2-di-O-acyl-3-O-[α-D-glucopyranosyl-(1→4)-O-β-D-galactopyranosyl]glycerol and three other glycosyldiacylglycerols with 11-18:1 as more than 80% of the fatty acid constituents. α-Glucosyl-(1→3)-α-mannosyl-diacylglycerol produced in sub-nanomolar concentrations by Rhizobium leguminosarum may be important for the induction of symbiosis-related processes.

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, Bifidobacterium longum subs. infantis from the intestinal tract of infants contains a galactofuranosyl-diacylglycerol with a novel acetal linkage to glycerol, and this was found to suppress the innate immune response in the host. Amongst other species, Micrococcus luteus synthesises mono- and dimannosyldiacylglycerols, while glycosyldiacylglycerols with a glycerophosphate group linked to a carbohydrate moiety (‘phosphoglycolipids’) are known from other bacteria. The lipids of Bacillus megaterium contain 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-glucuronosyl-sn-glycerol (glucuronosyldiacylglycerol) among their membrane lipids, and this may have a functional relationship to sulfoquinovosyldiacylglycerol as discussed above. A conjugate of this with taurine is also known (see our web page on other sulfolipids). Of course, the bacterial lipid has a very different fatty acid composition from that in algae or in higher plants. 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.

A number of novel and interesting glycosyldiacylglycerol derivatives have been isolated from primitive members of the animal kingdom, such as sponges and corals, but they are now known to be the product of symbiotic bacteria (some sponges can contain more bacterial than animal cells). For example, new glycosyldiacylglycerols in which the sugar moiety is replaced by an unusual five-membered cyclitol occur widely in sponges and were termed 'crasserides' from their initial discovery in the sponge Pseudoceratina crassa.

Formula of a crasseride

A branched-chain alkyl moiety is ether-linked to position 1 of the glycerol moiety, while position 2 can contain one of several fatty acids. They are believed to be natural deterrents against fish predation. The iso-crasserides are related lipids in which in which the acyl chain is linked to a cyclitol hydroxyl group rather than to the glycerol moiety.


6.  Analysis

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 theyare soluble in acetone in contrast to phospholipids. 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.


Recommended Reading



Lipid listings Credits/disclaimer Updated: May 29th, 2017 Author: William W. Christie LipidWeb icon