O

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Sphingolipid

▲ FIGURE 18-2 Chemical structures of fatty acids and some of their derivatives. Palmitate, a saturated fatty acid, contains 16 carbon atoms; arachidonate, a polyunsaturated fatty acid, contains 20 carbons atoms. Both saturated and unsaturated fatty acids are stored as triglycerides in which three fatty acyl chains (R = hydrocarbon portion of fatty acid) are esterified to a glycerol molecule. Fatty acids are also components of phospholipids (glycerol phospholipids, plasmalogens, and sphingolipids), which along with cholesterol are the major lipids present in membranes. The common phospholipids (e.g., phosphatidylcholine) have two acyl chains esterified to glycerol; in plasmalogens, one hydrocarbon chain is attached to glycerol by an ether linkage and the other by an ester linkage. Sphingolipids are built from sphingosine, an amino alcohol that contains a long, unsaturated hydrocarbon chain. Several types of polar X groups are found in all three of these classes of membrane lipids (see Figure 5-5).

Unesterified Fatty Acids Move Within Cells Bound to Small Cytosolic Proteins

Unesterified fatty acids within cells are commonly bound by fatty acid-binding proteins (FABPs), which belong to a group of small cytosolic proteins that facilitate the intracellular movement of many lipids. These proteins contain a hydrophobic pocket lined by p sheets (Figure 18-3). A long-chain fatty acid can fit into this pocket and interact noncovalently with the surrounding protein.

The expression of cellular FABPs is regulated coordi-nately with cellular requirements for the uptake and release of fatty acids. Thus FABP levels are high in active muscles

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