What are Glycolipids?

Glycolipids are lipids that are an important class of organic compounds in chemistry that have simple to complex applications. They contain carbohydrates, fatty acids, sphingolipids or a glycerol group. In other words, they are the modifications of lipids like acylglycerols, prenols and ceramides. They are all part of a wider group of compounds known as glycoconjugates.

Chemical Structure and Synthesis

Glycolipids are major components of the cellular membrane. They are located on the outer part of cellular membranes where they serve as receptors, protein anchors and signal transduction regulators providing intercellular communications. They also help in maintaining membrane stability. They are commonly distributed in all cells along with glycoproteins and glycosaminoglycans.

A glycolipid's basic structure is a mono or an oligosaccharide group bound to a sphingolipid or a glycerol group with one or more fatty acids. Fatty acids attached to this backbone give the lipid a polar head group and a non-polar tail group. In the cell membrane, it exists as a polar head pointing towards outer surfaces and the non-polar fatty acid tail facing the inner portion of the membrane. The polar saccharide unit is the ligand part of glycolipids and hence they are water-soluble. An anomeric glyosidic linkage is formed between lipid and saccharide and is covalent in nature. These saccharides have different structures according to the configuration of molecules they bind to.

Enzymes help in the synthesis of glycolipids by sequentially adding sugars to the lipid. Glycosphingolipids are made from lactosylceramide with D- erythro-sphinganine being acylated and desaturated first. Lactosylceramide is formed when ceramide is glucosylated and then galactosylated extracellularly. Glycosyltransferases and sulfotransferases may help in the further elongation.

Classification of Glycolipids

The following are the different types of glycolipids:

Glycoglycerolipids

Glycerolipids with mono, di or trisaccharides glycosidically bound to the hydroxyl band of diglycerides are referred to as glycoglycerolipids. The main glycolipid elements of the various walls of chloroplasts as well as the most abundant fats in all photosynthetic cells, such as those of higher plants plankton and algae are monogalactosyl-diacylglycerols and digalactosyldiacylglycerols.

There are mainly three types of glycoglycerolipids:

1. Neutral glycoglycerolipids: In these, glycerol or diacylglycerols is linked to two sugars. They are found in photosynthetic membranes and plays an important role in higher plants, algae and bacteria. Non acylated and acylated glycoside moieties may be isolated from neutral glycoglycerolipids.

2. Glycophospholipids: These compounds contain at least one phosphate group bound to the sugar or glycerol. The most basic of these compounds is glycosylated phosphatidic acid, which is present in red blood cells.

3. Sulfoglycoglycerolipids: these compounds bear sulfur atoms and are seen in acidic membranes. Significant quantities of these lipids are found in photosynthetic membranes of plants, algae, cyanobacteria and certain fungi.

Glycosphingolipids

Fats with at least one monosaccharide residue connected to the ceramide moiety are referred to as glycosphingolipids. Ceramides have long chain saturated or monounsaturated fatty acid in their acyl group. Glycosphingolipids are mainly present in the nerve tissue and helps in cell signaling. They are further classified into further categories:

1. Neutral glycosphingolipids: Ceramides with glycosyl moieties are the main component of these lipids. An example for monoglycosylceramides is cerebrosides which contain an O-ester attachment connecting the glucose or galactose sugar residue to the alcohol of ceramide. They are located in the brain as well as peripheral nervous tissue.

2. Oligoglycosylceramides: They are made up of glycosphingolipids that contain more than one glucose moiety. The cell membranes of many of the eukaryotic organisms and viruses contain this as a major constituent. The most significant and plentiful of the oligosylceramides is β-D-galactosyl-(1-4)-β-D-glucosyl-(1-1')-ceramide, additionally called lactosylceramide (LacCer).

3. Acidic glycosphingolipids: They exist as negatively charged species at physiological pH. They are located mostly in medullated nerve fibers and are connected to immune responses. They are subdivided into sulfoglycosphingolipids and gangliosides.

Functions

• They mainly help in cell-cell communications. It is done by binding saccharide of glycolipid with a complimentary carbohydrate of the nearest cell.
• They play a role in immune responses. During inflammation, the binding of carbohydrates of glycolipids with selectins (a class of lectins) of leukocytes and endothelial cells is an example of how these lipids work in the body. This binding is responsible for the migration of leukocytes towards the inflammation site.

There are many advanced articles and journals having detailed contents related to the functions and applications of glycolipids.

Role of Glycolipids in Calcium Homeostasis

Calcium particle interactions with glycolipids, especially gaglioside, were linked to neuronal capacity. Micelles made of ganglioside bind calcium particles with high partiality. Sphingosine and ceramide were found to play a role in the significant distribution of calcium particles from intracellular stores.

Calcium homeostasis and flagging are two functions of gangliosides. It was discovered that gangliosides induced changes in cell calcium, which were cultivated by the balance of calcium deluge channels, calcium trade proteins, and various calcium subordinate proteins, all of which were changed by the relationship with gangliosides. 

Glycolipids in Signal Transduction

Glycolipids were recognized to be modulators of signal transduction.  Glycosphingolipids and sphingomyelin in creature cells are grouped and coordinated as film microdomains firmly related with different sign transducer particles, for example, cSrc, Src family kinases, little G-proteins (for example RhoA, Ras), and central grip kinase.

Glycolipids as Receptors of Bacteria

Limiting of pathogenic microorganisms and bacterial harms to have cell surfaces may be an essential progress in setting up the disease in tissues and conveying hurtful affect. Glycolipids on cell surfaces are receptors for limiting to cells. Various pathogenic microscopic living beings tie to glycolipids of host cell surface for colonization and disease.

Glycolipids in Modulation of Cell Proliferation

Some perceptions underline the work of glycolipids within the rule of cell improvement by interfacing through development factor receptors. Auge At Al. saw the intracellularly conveyed ceramide. Lac Cer animated DNA union in endothelial smooth muscle cells. They moreover potentiated mitogenesis initiated by different development factors including platelet-determined development factors.

Glycolipids as biosurfactant

Biosurfactants are amphiphilic compounds that will reduce surface strain and interfacial strains between singular particles at the cell surface and interface individually. Glycolipids are the most well-known kinds of biosurfactants. The polar moiety is sugar and the non-polar moiety is a long carbon chain of unsaturated fat. Mannosylethritol (MELs) glycolipids delivered by Candida Antarctica T-34 showed magnificent surface and interfacial strain bringing down activities and shown antimicrobial movement especially against gram-positive microorganisms, consequently proposing a solid potential for their mechanical use. The brilliant surface pressure lessening and emulsifying qualities of MELs glycolipids and their dependability over a wide scope of temperature, pHs made MELs significant for business application.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry and Biochemistry.