Harpers

H114 / CHAPTER 14OCOO –more unsaturated than storage lipids. Lipids in tissuesthat are subject to cooling, eg, in hibernators or in theextremities of animals, are more unsaturated.Figure 14–4. Leukotriene A 4 (LTA 4 ).contribution comes from the ingestion of ruminant fatthat contains trans fatty acids arising from the action ofmicroorganisms in the rumen.Physical and Physiologic Propertiesof Fatty Acids Reflect Chain Lengthand Degree of UnsaturationThe melting points of even-numbered-carbon fattyacids increase with chain length and decrease accordingto unsaturation. A triacylglycerol containing three saturatedfatty acids of 12 carbons or more is solid at bodytemperature, whereas if the fatty acid residues are 18:2,it is liquid to below 0 °C. In practice, natural acylglycerolscontain a mixture of fatty acids tailored to suittheir functional roles. The membrane lipids, whichmust be fluid at all environmental temperatures, areTRIACYLGLYCEROLS (TRIGLYCERIDES)*ARE THE MAIN STORAGE FORMS OFFATTY ACIDSThe triacylglycerols (Figure 14–6) are esters of the trihydricalcohol glycerol and fatty acids. Mono- and diacylglycerolswherein one or two fatty acids are esterifiedwith glycerol are also found in the tissues. Theseare of particular significance in the synthesis and hydrolysisof triacylglycerols.Carbons 1 & 3 of Glycerol AreNot IdenticalTo number the carbon atoms of glycerol unambiguously,the -sn- (stereochemical numbering) system isused. It is important to realize that carbons 1 and 3 ofglycerol are not identical when viewed in three dimensions(shown as a projection formula in Figure 14–7).Enzymes readily distinguish between them and arenearly always specific for one or the other carbon; eg,glycerol is always phosphorylated on sn-3 by glycerolkinase to give glycerol 3-phosphate and not glycerol1-phosphate.12018CH 3 CH 3Cis form(oleic acid)10CC9HHTrans form(elaidic acid)CCH110PHOSPHOLIPIDS ARE THE MAIN LIPIDCONSTITUENTS OF MEMBRANESPhospholipids may be regarded as derivatives of phosphatidicacid (Figure 14–8), in which the phosphate isesterified with the ⎯ OH of a suitable alcohol. Phosphatidicacid is important as an intermediate in the synthesisof triacylglycerols as well as phosphoglycerols butis not found in any great quantity in tissues.Phosphatidylcholines (Lecithins)Occur in Cell MembranesPhosphoacylglycerols containing choline (Figure 14–8)are the most abundant phospholipids of the cell mem-1COO – COO –Figure 14–5. Geometric isomerism of ∆ 9 , 18:1 fattyacids (oleic and elaidic acids).* According to the standardized terminology of the InternationalUnion of Pure and Applied Chemistry (IUPAC) and the InternationalUnion of Biochemistry (IUB), the monoglycerides, diglycerides,and triglycerides should be designated monoacylglycerols,diacylglycerols, and triacylglycerols, respectively. However, theolder terminology is still widely used, particularly in clinical medicine.