Structural Lipids in Membranes:- Sterols Have Four Fused Carbon Rings
Sterols are structural lipids present in the membranes of most eukaryotic cells. The characteristic structure of this fifth group of membrane lipids is the steroid nucleus, consisting of four fused rings, three with six car bons and one with five (Fig. 10–16). The steroid nucleus is almost planar and is relatively rigid; the fused rings do not allow rotation about C-C bonds. Cholesterol, the major sterol in animal tissues, is amphipathic, with a polar head group (the hydroxyl group at C-3) and a nonpolar hydrocarbon body (the steroid nucleus and the hydrocarbon side chain at C-17), about as long as a 16 carbon fatty acid in its extended form. Similar sterols are found in other eukaryotes: stigmasterol in plants and ergosterol in fungi, for example. Bacteria cannot syn thesize sterols; a few bacterial species, however, can in corporate exogenous sterols into their membranes. The sterols of all eukaryotes are synthesized from simple five carbon isoprene subunits, as are the fat-soluble vitamins, quinones, and dolichols described in Section 10.3. In addition to their roles as membrane constituents, the sterols serve as precursors for a variety of products with specific biological activities. Steroid hormones, for example, are potent biological signals that regulate gene expression. Bile acids are polar derivatives of cholesterol that act as detergents in the intestine, emulsifying dietary fats to make them more readily accessible to digestive lipases. We return to cholesterol and other sterols in later chapters, to consider the structural role of cholesterol in biological membranes, signaling by steroid hormones, the remarkable biosynthetic pathway to cholesterol, and the transport of cholesterol by lipoprotein carriers.
FIGURE 10–15 The specificities of phospholipases. Phospholipases A1 and A2 hydrolyze the ester bonds of intact glycerophospholipids at C-1 and C-2 of glycerol, respectively. Phospholipases C and D each split one of the phosphodiester bonds in the head group. Some phospholipases act on only one type of glycerophospholipid, such as phosphatidylinositol 4,5-bisphosphate (shown here) or phosphatidyl choline; others are less specific. When one of the fatty acids has been removed by a type A phospholipase, the second fatty acid is cleaved from the molecule by a lysophospholipase (not shown).
FIGURE 10–16 Cholesterol. The stick structure of cholesterol is visible through a transparent surface contour model of the molecule (from coordinates supplied by Dave Woodcock). In the chemical structure, the rings are labeled A through D to simplify reference to derivatives of the steroid nucleus, and the carbon atoms are numbered in blue. The C-3 hydroxyl group (pink in both representations) is the polar head group. For storage and transport of the sterol, this hydroxyl group condenses with a fatty acid to form a sterol ester.
