3.2. Glycoglycerolipids

added because of possible variations in the saccharide units. Often GSLs are classified based on their saccharide units, that can range from a single to 20 or more carbohydrate residues [27]. Most of GSLs have a neutral core structure which is used for their classification into different series (Table 1). Roman numerals are assigned, starting from the ceramide end while referring to a particular residue of the core and an Arabic numeral superscript is given to indicate the position at which a substituent is attached if any are present [28]. GSLs are further subclassified as neutral, sulfatides or gangliosides [29]. Gangliosides are sialylated GSLs. Gangliosides are written using Svennerholm abbreviations, where the first letter G stands for ganglioside, the number of sialic acid residues is denoted by a letter, defined as M-mono, D-di, T-tri and Q-tetra, and is followed by a number which represents the order of migration on thin

IV III II I

GSLs can participate in both donating and receiving hydrogen bonds through the hydroxyls of the sphingoid base, fatty acids, carbohydrates and the acylamide group. Because of this hydrogen bonding ability, GSLs can cluster together to form rigid highly organized domains on the surface of the biomembrane. These clusters of GSLs often have signal transducer proteins, growth factors or adhesion receptors organized in them and are involved in carbohydrate dependent intercellular adhesion, which triggers the signaling transducers leading to modification of the cellular phenotype. These GSL enriched domains that are involved in GSL-dependent cell adhesion and signaling are termed "glycosynapses" [30]. Glycosynapses differ from other membrane domains such as caveolae and lipid rafts in that neither of these microdomains are involved in carbohydrate dependent cell to cell

The major form of glycoconjugates found in animal brains are glycolipids which includes galactosylceramide (GalCer), its 3-O-sulfated form sulfatide and gangliosides. GalCer and sulfatide make up a significant portion of myelin lipid and gangliosides are found in neuronal plasma membrane [31]. Inherent defects in the biosynthesis and catabolism of gangliosides results in neurodegenerative diseases. So far very few incidences of diseases caused by mutations of genes responsible for synthesis of gangliosides have been reported [32, 33]. Inherited defects in catabolism of gangliosides are well documented. Defects in catabolism of gangliosides

layer chromatography.

Table 1. Series of glycosphingolipids.

218 Cell Culture

Series Symbol Core structure

globo Gb GalNAcβ3Galα4Galβ4GlcCer isoglobo iGb GalNAcβ3Galα3Galβ4GlcCer ganglio Gg Galβ3GalNAcβ4Galp4Gcer lacto Lc Galβ3GlcNAcβ3Galβ4GlcCer neolacto nLc Galβ4GlcNAcβ3Galβ4GlcCer mollu Mu GlcNAcβ2Manα3Manβ4GlcCer artho At GalNAcβ4GlcNAcβ3Manβ4GlcCer

adhesion.

Glycoglycerolipids constitute a major portion of the lipids found in chloroplasts of plants and in cyanobacteria with digalactosyldiacylglycerol (DGDG), monogalactosyldiacylglycerol (MGDG), and sulfoglycolipid sulfoquinovosyl diacylglycerol (SQDG) composing more than 80% of total lipid composition [36]. In general, organisms performing oxygenic photosynthesis tend to have higher percentage of galactolipids. Many different glycoglycerolipids besides galactolipids are found in bacteria where they contribute to membrane stability and survival of bacterial species in phosphate limited environments.

Glycoglycerolipids have been explored for their biological activities for the past few decades. Natural and synthetic analogs of MGDGs, DGDGs and SQDGs have been studied for their antitumor [37–39], antiviral [40–42], antifungal [43], anti-inflammatory [44, 45] and other biotechnological applications [46, 47].
