Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine. Sphingolipids are often found in neural tissue, and play an important role in both signal transmission and cell recognition.
There are three main types of sphingolipids:
- ceramides. Ceramides are the simplest type of sphingolipid. They consist simply of a fatty acid chain attached through an amide linkage to sphingosine.
- sphingomyelins. Sphingomyelins have a phosphorylcholine or phosphoroethanolamine molecule esterified to the 1-hydroxy group of a ceramide.
- glycosphingolipids, which differ in the substituents on their head group (see image). Glycosphingolipids are ceramides with one or more sugar residues joined in a β-glycosidic linkage at the 1-hydroxyl position. Glycosphingolipids may be further subdivided into cerebrosides and gangliosides.
Function and transport
Sphingolipids are commonly believed to protect the cell surface against harmful environmental factors by forming a mechanically stable and chemically resistant outer leaflet of the plasma membrane lipid bilayer. Certain complex glycosphingolipids were found to be involved in specific functions, such as cell recognition and signaling. The first feature depends mainly on the physical properties of the sphingolipids, whereas signaling involves specific interactions of the glycan structures of glycosphingolipids with similar lipids present on neighboring cells or with proteins.
Recently, relatively simple sphingolipid metabolites, such as ceramide and sphingosine-1-phosphate, have been shown to be important mediators in the signaling cascades involved in apoptosis, proliferation, and stress responses. Ceramide-based lipids self-aggregate in cell membranes and form separate phases less fluid than the bulk phospholipids. These sphingolipid-based microdomains, or "lipid rafts" were originally proposed to sort membrane proteins along the cellular pathways of membrane transport. At present, most research focuses on the organizing function during signal transduction.
Sphingolipids are synthesized in the ER and Golgi apparatus, but are enriched in the plasma membrane and in endosomes, where they perform many of their functions, thus travelling and evolving between organelles. Transport occurs via vesicles and monomeric transport in the cytosol. Sphingolipids are virtually absent from mitochondria and the ER, but constitute a 20-35 molar fraction of plasma membrane lipids.
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