Cholesterol sphingolipids and

Lipid rafts are mlcrodomalns containing cholesterol, sphingolipids, and certain membrane proteins that form in the plane of the bilayer. These aggregates are sites for signaling across the plasma membrane. 

These compounds are members of a broader group of chemical substances called lipids, which has been classified by the National Research Council into (1) nonpolar lipids— including esters of fatty acids (triacylglycerols and cholesteryl esters) that are virtually insoluble in water but soluble in most organic solvents, and enter metabolic pathways only after hydrolysis and (2) polar or amphi-pathic lipids—including fatty acids, cholesterol, sphingolipids, and glycerophospholipids (mainly lecithins). The term phospholipids... 

A. The three major types of amphipathic lipids found in membranes are the gly-cerophospholipids (also called phosphoglycerides), the sphingolipids, and cholesterol. 

Sphingolipids and Cholesterol Cluster Together in Membrane Rafts... 

Lipids and proteins can diffuse laterally within the plane of the membrane, but this mobility is limited by interactions of membrane proteins with internal cytoskeletal structures and interactions of lipids with lipid rafts. One class of lipid rafts consists of sphingolipids and cholesterol with a subset of membrane proteins that are GPI-linked or attached to several long-chain fatty acyl moieties. 

On the other hand, as we have already seen, cholesterol tends to reduce the mobility of molecules in membranes and causes phospholipid molecules to occupy a smaller area than they would otherwise. Myelin is especially rich in long-chain sphingolipids and cholesterol, both of which tend to stabilize artificial bilayers. Within our bodies, the bilayers of myelin tend to be almost solid. Bilayers of some gram-positive bacteria growing at elevated temperatures are stiffened by biosynthesis of bifunctional fatty acids with covalently joined "tails" that link the opposite sides of a bilayer.149... 

Targeting of proteins to specialized domains of a membrane are less well understood. These include caveolae and lipid rafts, domains that are high in cholesterol and sphingolipids and which function in endocytosis and in cell signaling. A recent proposal is that proteins with hydrophobic surfaces needed in these domains become coated with a lipid "shell" before entering the membrane.6173... 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

Lipids are far more diverse chemically than other typical biomolecules such as amino acids, carbohydrates, and nucleotides. The definition of lipids includes simple fatty acids and their glycerol esters, sterols such as cholesterol, and phospholipids, sphingolipids, and cerebrosides. Lipids are generally defined by their common hydrophobic character, which makes them soluble in organic solvents such as chloroform. Virtually all lipids also have a hydrophilic group, which makes them surface active. 

Brown, D. A. and London, E. (2000). Structure and function of sphingolipid- and cholesterol-rich membrane rafts. ]. Biol. Chem. 275(23), 17221-17224. 

This chapter examines the biosynthesis of three important components of biological membranes—phospholipids, sphingolipids, and cholesterol (Chapter 12). Triacylglycerols also are considered here because the pathway for their synthesis overlaps that of phospholipids. Cholesterol is of interest both as a membrane component and as a precursor of many signal molecules, including the steroid hormones progesterone, testosterone, estrogen, and cortisol. The biosynthesis of cholesterol exemplifies a fundamental mechanism for the assembly of extended carbon skeletons from five-carbon units. 

Membranes contain lipids, particularly phosphoglycerides, sphingolipids, and cholesterol. 

The major membrane lipids are phosphoglycerides, but sphingolipids and cholesterol are also present. 

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