Glycolipids

Fig 1. Glycolipid (Wiki)

Glycolipids (GLs) consist of glycosyl and lipid moieties and are normally found on the extracellular surface of eukaryotic cellular membranes. They function to maintain the stability of membranes and to facilitate cell-cell interactions. Moreover, glycolipids also serve as receptors for viruses or other pathogens to enter cells.

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Introduction

Glycolipids as glycoconjugates of lipids are broadly designated to any compound that contains one or more monosaccharide residues bound by a glycosidic linkage to a hydrophobic moiety. The definition encompasses a wide diversity of structurally heterogeneous biological compounds produced by animals, plants, and microbes. Now, this concept is even broader to include glycoside and non-glycoside glycolipids in which the glycosyl and lipid residues are linked together by glycosidic (e.g. N- or O-glycosidic bonds) and non-glycosidic linkages (e.g. ester or amide bonds), respectively.

Fig.2 Classification of glycolipids and main types of linkages between their glycosyl and lipid residues. (Abdel-Mawgoud, 2017)

The glycosyl can be mono-/di-/oligo-/polysaccharides (e.g. glucose, cellobiose or glycan), alcohol sugars (e.g. mannitol, erythritol, and arabinol), amino sugars (e.g. desosamine) or sugar acids (e.g. glucuronic acids). The lipid moiety of glycolipids ranges from fatty acids, fatty alcohols, fatty amino alcohols, sterols, polyketides, hopanoids, and carotenoids with different substitutions, chain lengths, saturation levels, branched chains, and di-/oligo-/polymerizations.

Types of Glycolipid

Glycolipids constitute ~3% of the outer layer of the plasma membrane, and their structures are composed of a carbohydrate head and a lipid tail. Based on different lipid components, glycolipids are classified into three main groups, including glycosphingolipids (GSLs), glycoglycerolipids, and glycosylphosphatidylinositols (GPIs). Gangliosides and cerebrosides are two classes of glycolipids that form GLSs (carbohydrate and sphingolipid). And GSLs are the most widely overexpressed on tumors.

Functions

• Cell Proliferation: Glycolipids have been observed to play roles in the regulation of cell growth through interactions with growth factor receptors. Intracellular ceramide can stimulate DNA synthesis in endothelial smooth muscle cells and also induce mitogenesis by platelet-derived growth factors.
• Cell-cell Interaction: Carbohydrates on glycolipids are the most exposed structures on the extracellular surface and show flexible with numerous binding sites which make them optimal for cell signaling. Since the lipid is usually buried within the membrane, carbohydrate interactions are the major interactions that may occur between glycolipids.
• Signal Transduction: GLs and sphingomyelin are clustered into microdomains where they’re associated with several different proteins such as cSrc, G-proteins, and focal adhesion kinase, to mediate cellular events. Besides, gangliosides are related to calcium ions which are thought to be implicated in neuronal function. Ganglioside micelles bind to calcium ions with high affinity and may involve in synaptic transmission.
• Immune Response: Selectins (lectins) on the surface of leukocytes and endothelial cells bind to the carbohydrates attached to glycolipids and can initiate an immune response. This binding lead leukocyte to leave circulation and gather near the site of inflammation. Glycolipids are also responsible for other activities, notably the recognition of host cells by viruses.

Reference:

1. Abdel-Mawgoud, A.M.; Stephanopoulos, G. Simple glycolipids of microbes: Chemistry, biological activity and metabolic engineering. Synth Syst Biotechnol. 2017, 3(1): 3-19.