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Chapter 26

CHMC47 Chapter 26.doc

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University of Toronto Scarborough
Lana Mikhaylichenko

Chapter 26: Lipids Fatty Acids Are Long-Chain Carboxylic Acids • Lipids: organic compounds that are soluble in nonpolar solvents, due to their significant hydrocarbon component. • Fatty Acids: carboxylic acids with long hydrocarbon chains and the most naturally occurring fatty acids contain an even number of carbon atoms that are unbranched. o Polyunsaturated Fatty Acids: fatty acids with more than one double done. o The melting points of saturated fatty acids increase with increasing molecular weight because of increased van der Waals interactions between the molecules. o The melting points of unsaturated fatty acids (alkenes) also increase with increasing molecular weight, but are lower than those of saturated fatty acids (alkanes) with comparable molecular weights. o Double bonds are in cis configuration and are always separated by one CH 2roup. o The melting points of the unsaturated fatty acids decrease as the number of double bonds increases. Waxes Are High-Molecular Weight Esters • Waxes: esters formed from long-chain carboxylic acids and long-chain alcohols. • Waxes have a large even number of carbons on both sides of the ester. Fats and Oils • Triacylglycerols or Triglycerides: compounds in which each of the three OH groups of glycerol has formed an ester with a fatty acid • Simple Triacylglycerol: if the three fatty acid components of a triacylglycerol are the same. • Mixed Triacylglycerol: contains two or more different fatty acid components and are more common than simple triacylglycerols. • Fats: triacylglycerols that are solids or semisolids at room temperature. o They are either saturated or have only one double bond. o They are packed closely together, giving them high melting points so they are solids at room temperature. o Typically comes from animals. • Oils: liquid triacylglycerols. o Typically comes from plant products. o They are composed of unsaturated components and cannot be packed tightly. o They have low melting points, causing them to be liquid at room temperature. • The fat content of the average man is about 21% whereas the fat content of the average woman is about 25%. • Polyunsaturated fats and oils are easily oxidized by O 2y means of a radical chain reaction. o A radical removes a hydrogen from methylene group that is flanked by two double bonds and reacted with O , f2rming a peroxy radical with conjugated double bonds. o The peroxy radical removes a hydrogen from a methylene group of another amolecule of fatty acid, forming an alkyl hydroperoxide. Phospholipids and Sphingolipids Are Components of Membranes • Phospholipids: lipids that contain a phosphate group. o Phosphoacylglycerols: major component of cell membranes and belongs to the phospholipids group. o They are similar to triacylglycerols except that a terminal OH group of glycerol is esterified with phosphoric rather than with a fatty acid, forming a phosphatidic acid. • The most common phosphoacylglycerols are phosphodiesters. • Phosphoacylglycerols from membranes by arranging themselves in a lipid bilayer, where the polar heads are on both surfaces of the bilayer and the fatty acid chains form the interior of the bilayer. • Saturated fatty acids decrease membrane fluidity because their hydrocarbon chains pack closely together while unsaturated fatty acids increase fluidity because they pack less closely together. • Sphingolipids: major lipid components in the myelin sheaths of nerve fibers and contain an amino alcohol called sphingosine instead of glycerol. o The amino group of sphingosine is bonded to the acyl group of a fatty acid. o Both asymmetric centers in sphingosine have the S configuration. Prostaglandins Regulate Physiological Responses • Prostaglandins: found in all body tissues and are responsible for regulating a variety of physiological responses like inflammation, blood pressure, fever and pain. o All have five-membered rings with a seven-carbon carboxylic acid substituent and an eight-carbon hydrocarbon substituent. o The two substituents are trans to each other. • Prostaglandins are classified using the formula PGX, where X designates the functional groups of the compound’s five-membered ring. o PGAs, PGBs and PGCs all contain a carbonyl group and a double bond in the five-membered ring. o PGDs and PGEs are β-hydroxy ketones. o PGFs are 1,3-diols, where α indicates a cis diol and β indicates a trans diol. • Biosynthesis of Prostaglandins, Thromboxanes and Prostacyclins o In the first step, a hydrogen atom is removed from a carbon flanked by two double bonds. This hydrogen is removed easily because the resulting radical is stabilized by electron delocalization. o The radical reacts with oxygen to form a peroxy radical. o The peroxy radical rearranges and reacts with a second molecule of oxygen. o The enzyme then uses its hydroperoxidase activity to concert the OOH group into an OH group, forming PGH ,2which rearranges to form PGE , 2 prostaglandin. • PGH 2s a precursor for the synthesis of prostaglandins, thromboxanes and prostacyclins. • Aspirin inhibits the cyclooxygenase activity of prostaglandin endoperoxide synthase; therefore, it inhibits the synthesis of prostaglandins and decreases the inflammation produced by these compounds. Terpenes Contain Carbon Atoms in Multiples of Five • Terpenes: they can be hydrocarbons, contain oxygen or be alcohols, ketones or aldehydes. o Terpenoids: oxygen-containin
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