3. Biological significance of glycolipids

The notions of how biomolecules are arranged in membranes have been continuously evolving. If one is to attempt building a timeline depicting major events in the development of biological membrane models, one could divide it into three periods: pre Singer-Nicolson, Singer-Nicolson and post Singer-Nicolson [22]. Among the various models proposed, two that stand out are fluid-mosaic [23] and lipid raft [24]. The fluid-mosaic model proposed by Singer and Nicolson pictured the membrane as a lipid bilayer, predominately of phospholipids, embedded within which were transmembrane proteins. The texture of the matrix was hypothesized to be like a viscous fluid which would allow the translational diffusion of the embedded proteins. Although

Figure 3. Structures of (a) D-erythyro-sphingosine, (b) N-dodecanoyl-β-D-galactosylceramide, (c) ganglioside GM1, and (d) a glycoglycerolipid.

thoughtful considerations were taken to address thermodynamic limitations and attempts were made to correlate the experimental evidence with the proposed model, there were some anomalies hinting at the presence of regions in the membrane where the lipids would behave differently, i.e. were different in composition and/or phase. To explain these discrepancies a new hypothesis was proposed which suggested that certain lipids within the cell membrane have unique properties which would allow them to self-associate and form segregated regions which were named "lipid rafts". Originally it was proposed that these rafts were made of sphingolipids and cholesterol and functioned as platforms for trafficking proteins; however, it was later found that there was more to lipid rafts than trafficking of proteins [25].

Glycolipids are an important group of raft forming lipids. Their ability to aggregate together to form microdomains indicates their involvement in various cellular activities. Glycolipids can broadly be divided into two major categories-glycosphingolipids (GSLs) and glycoglycerolipids, the first one being widely present in animal cells and the latter in plant and microbial cells with an exception of sulfated glycoglycerolipids called seminolipids which are found in mammalian testis [26]. The major difference between these two classes of glycolipids is their lipid moiety. While GSLs have ceramide as their lipid component, which is made of an aminoalcohol base (sphingoid base) and fatty acid joined by an amide bond, glycoglycerolipids have diacylglycerol as their lipid component. The sugar units are attached to GSLs through glycosidic linkage to hydroxyl groups at the C-1 carbon of the ceramide. In glycoglycerolipids the glycosylation occurs at the C-3 hydroxyl group of glycerol, see Figure 3. From here on we attempt to understand what structural features of these glycosylated lipids gives them their unique property to cluster and form microdomains.

#### 3.1. Glycosphingolipids

components of the LB trough are a Teflon trough which holds the subphase, a barrier which helps compress the spread monolayer to a targeted area or surface pressure at specified compression rates, a surface pressure transducer for measurement of surface pressure and a dipper which helps in transferring the monolayer film onto a solid substrate. Some details on the mechanism of transfer will be discussed later in the chapter. The trough can be accessorized with temperature, pH and surface potential sensors. It can also be coupled with optical and spectroscopic instruments such as a fluorescence microscope, a Brewster angle microscope or an infrared spectrometer which help in visualization and characterization of the

Figure 2. Schematics of Langmuir-Blodgett trough along with some associated measurement techniques. (1) Trough with subphase and deposited monolayer, (2) side view of barrier, (3) surface pressure transducer with Wilhelmy plate, (4) dipping system with a solid support, (5) microscopic measurements (BAM, fluorescence), (6) spectroscopic measure-

The notions of how biomolecules are arranged in membranes have been continuously evolving. If one is to attempt building a timeline depicting major events in the development of biological membrane models, one could divide it into three periods: pre Singer-Nicolson, Singer-Nicolson and post Singer-Nicolson [22]. Among the various models proposed, two that stand out are fluid-mosaic [23] and lipid raft [24]. The fluid-mosaic model proposed by Singer and Nicolson pictured the membrane as a lipid bilayer, predominately of phospholipids, embedded within which were transmembrane proteins. The texture of the matrix was hypothesized to be like a viscous fluid which would allow the translational diffusion of the embedded proteins. Although

monolayers.

216 Cell Culture

3. Biological significance of glycolipids

ments, and (7) AFM image of transferred monolayer on solid support.

As mentioned above, GSLs have ceramide as their lipid moiety. Ceramides can have a variety of structures depending upon the sphingoid bases and the fatty acid combinations. This variability in the structure of ceramide adds diversity to GSLs and further diversity is


results in their accumulation inside lysosomes known as gangliosidoses. Gangliosidoses can occur in any age group, although most patients showing the symptoms are infants. Tay-Sachs disease is caused by deficiency of the enzyme β-hexosaminidase A which causes the lysosomal accumulation of GM2 gangliosides and is an example of gangliosidoses [34]. Gaucher and Fabry disease are other examples of GSL storage diseases in which lysosomal accumulation of

Monolayers of Carbohydrate-Containing Lipids at the Water-Air Interface

http://dx.doi.org/10.5772/intechopen.76440

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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

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

Glycosylphosphatidylinositols (GPI) are complex structures to which the C-terminus of proteins gets attached during their post-translational modification [48]. All GPI have a core glycan structure sandwiched between an ethanolamine phosphate linker, bridging the C-terminal of the protein with the highly conserved glycan core, and a phosphatidylinositol (PI) group. The fatty acids of the PI moiety attach the GPI to the cell membrane. So far more than 200 proteins have been found to be anchored by a GPI to the cell surface and more than 20 GPI structures have been elucidated [49]. Some of the proteins attached to GPI anchor are enzymes like

defense proteins like decay accelerating factor (DAF or CD55), CD59, and mammalian antigens like Thy-1, or protozoan antigens like variant surface glycoprotein (VSG) found on the

Unlike GPI-anchored proteins not much is known about biological functions of GPI anchors, apart from their role as a membrane anchor for proteins. Given the complexity and diversity of their structures, they are thought of as being involved in many different biological functions but there are not sufficient experimental evidences to draw definitive conclusions [50]. However, there are several studies implicating their involvement in sorting of proteins in the lipid raft and in signal transduction [51–53]. Other studies have shown that the structure and conformation of proteins change upon binding to GPI anchors [54, 55]. Besides this physiological role, deficiency of GPI anchors on red blood cells causes a chronic pathological disorder


neutral GSLs occurs [35].

3.2. Glycoglycerolipids

of bacterial species in phosphate limited environments.

alkaline phosphatase (APase), acetylcholinesterase (AChE) and 5<sup>0</sup>

biotechnological applications [46, 47].

3.3. Glycophosphatidylinositol

cell surface of Trypanosoma [48].

paroxysmal nocturnal hemoglobinuria (PNH) [56].

Table 1. Series of glycosphingolipids.

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 layer chromatography.

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 adhesion.

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 results in their accumulation inside lysosomes known as gangliosidoses. Gangliosidoses can occur in any age group, although most patients showing the symptoms are infants. Tay-Sachs disease is caused by deficiency of the enzyme β-hexosaminidase A which causes the lysosomal accumulation of GM2 gangliosides and is an example of gangliosidoses [34]. Gaucher and Fabry disease are other examples of GSL storage diseases in which lysosomal accumulation of neutral GSLs occurs [35].
