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 GLYCOLIPIDS BASED ON GLYCEROL

These lipids consist of a mono- or oligosaccharide moiety linked glycosidically to the hydroxyl group of glycerol which may be acylated (or alkylated) with one or two fatty acids. Furthermore, these glycolipids may be uncharged and, therefore often called neutral glycoglycerolipids, or may contain a sulfate or a phosphate group.
The last IUPAC recommendations for the nomenclature of glycolipids may be found on the IUPAC site.

According to their structure they may be classified into the following groups :

 



NEUTRAL GLYCOGLYCEROLIPIDS

These compounds contain most frequently one or two sugars linked glycosidically to glycerol or diacylglycerol. Glycolipids with three or four sugars are also known. They are especially important in higher plants, algae and bacteria where they are located in photosynthetic membranes, they are also found in animals but in lesser amounts. 
Photosynthetic membranes of all oxygenic photosynthetic organisms are constituted of about 85 % of neutral glycoglycerolipids (MGDG and DGDG) (Murata et al., Biochim Biophys Acta 1990, 1019, 261).
Therefore, based on the natural abundance of photosynthetic organisms, these glycolipids constitute the most profuse lipid class on Earth (Gounaris K et al., Trends Biochem Sci 1983,  8, 378). The discovery of a plastid in Plasmodium falciparum, the causative agent of malaria, without galactoglycerolipids raised the question of the loss of galactoglycerolipids during evolution (Botté CY et al., Trends Plant Sci 2014, 19, 71).
Galactosyl monoacylglycerol with various numbers of galactose moiety has also been described in several plants.

Neutral glycoglycerolipids may be separated into two families :

1 – compounds with non acylated glycoside moiety

2 – compounds with acylated glycoside moiety 

1 – Non acylated glycoside moiety

Monogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG).

Glycoglycerolipids were unknown until 1956, when Carter HE et al. reported the presence of these lipids in lipid extracts of wheat flour (J Amer Chem Soc 1956, 78, 3735). Their specific association with photosynthetic tissues (tylakoid membranes of chloroplasts) was known from 1958 (Benson AA et al., J Am Chem Soc 1958, 80, 4740) but their correct structure was elucidated in 1961 (Carter HE et al., J Lipid Res 1961, 2, 215 and 223). This tight assocIation is exemplified by the herbicide or algaecide activity of a monogalactosyldiacylglycerol synthase inhibitor (Galvestine) (Bonneau AL et al., Patent EU 07290684.5). 

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R1 and R2 are the two fatty chains

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Higher homologues of galactolipids (tri- and tetragalactosyl diacylglycerols) were later identified in all plant tissues. A new glyceroglycolipid whose structure was elucidated to be trigalactosyl monolinolenylglycerol has been isolated from a Chinese folk medicine Premna microphylla (Zhan ZJ et al., Lipids 2003, 38, 1299).

Both MGDG and DGDG contain large amounts of linolenic acid (18:3n-3) and a specific trienoic acid (16:3 n-3). In higher plants, linolenic acid is almost the only fatty acid in MGDG, this composition led to name these plants "18:3 plants". In angiosperms, linolenic acid is concentrated in both sn-1 and -2 positions of both MGDG and DGDG and 16:3 n-3 is absent. By contrast, lower plants (green algae, mosses, ferns, conifers) and some angiosperms (Solanaceae, Brassicaceae, Chenopodiaceae) have 16:3 n-3 concentrated in the sn-2 position of galactolipids, while the low proportion of 18:3 n-3 is acylated in both positions. Such plants are called "16:3 plants" and have a "prokaryotic" structure similar to that observed in cyanobacteria. In red algae and photosynthetic diatoms, galactolipids are characterized by a high proportion of 20:5 n-3. Several n-3 fatty acids have been determined in MGDG isolated from the brown alga Sargassum thunbergii (Kim YH et al., Lipids 2007, 42, 395). Combinations of 18:3/20:5, 18:3/18:4, 18:4/20:5 and 18:3/18:3 were described.
It has been shown that MDA oxidatively generated from fatty acids in Arabidopsis may be recycled back into chloroplast 18:3-16:3-MGDG that may be again fragmented into MDA (Schmid-Siegert E et al., J Biol Chem 2016, 291, 13005).

Galactolipids have been found in only low amounts in most fungi, the main glycolipids being glycosphingolipids.
Another unusual fatty acid (18:3 n-1) was described to be abundant (about 25 %) in MGDG from marine diatom Skeletonema costatum (d’Ippolito G et al., Biochim Biophys Acta 2004, 1686, 100). These two fatty acids were shown to be metabolized into short-chain aldehydes, octadienal (8:2 n-4) and octatrienal (8:3 n-1), which may have deleterious effects on zooplankton crustaceans. A third aldehyde, heptadienal (7:2 n-3), was also shown to be produced from MGDG eicosapentaenoic acid.
A monogalactosyl diacylglycerol containing two linolenic acid (18:3 n-3) acyl groups has been described in fruits of rose hips (Rosa canina) and was shown to be an anti-inflammatory agent (inhibition of cell migration). This may be directly related to the clinically observed anti-arthritis properties of rose hip herbal remedies (Larsen E et al., J Nat Prod 2003, 66, 994). 
Other studies reported that galactosyl diglycerides from various sources have antitumor-promoting (Shirahashi H et al., Chem Pharm Bull 1996, 44, 1404), oxygen scavenging (Nakata K , J Biochem 2000, 127, 731), and virus neutralizing (Nakata K et al., J Biochem 2000, 127, 191) activities. More recently, DGDG synthesized or isolated from Clinacanthus leaves from Thailand exhibited anti-herpes simplex virus activity (Janwitayanuchit W et al., Phytochemistry 2003, 64, 1253). MGDG extracted from the invasive brown alga Sargassum muticum was shown to exhibit anti-microfouling activity (inhibition of the growth of bacteria and fungi), thus protecting any surface immersed in sea water (Plouguerné E et al., Mar Biotechnol 2010, 12, 52).
There is evidence that specific galactolipids containing 16:3n-3 or 18:4n-3 in the sn-1 position and 20:5n-3 or 18:5n-3 in the sn-2 position may be directly involved in cytotoxic reactions induced by marine diatoms (Phaeodactylum tricornutum) (Andrianasolo EH et al., J Nat Prod 2008, 71, 1197). This is the first report of an apoptosis induction by galactolipids.

It has been discovered that the chloroplasts of Arabidopsis contained a particular MGDG named arabidopside characterized by the presence of 12-oxo-phytodienoic acid (Stelma