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BCH2011: Textbook summary - Lectures 25 - 27

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Monash University

LECTURES 25 – 27 Fatty Acids: The fatty acids are hydrocarbon derivatives, at the same low oxidation state as the hydrocarbons in fossil fuels. The cellular oxidation of fatty acids (to CO2 and H2O), like the controlled, rapid burning of fossil fuels in internal combustion engines is highly exergonic. They are carboxylic acids with hydrocarbon chains ranging from 4-36 carbons long. In some fatty acids, this chain is unbranched and fully saturated (contains no double bonds); in others the chain contains one or more double bonds. A few contain three-carbon rings, hydroxyl groups, or methyl-group branches. The physical properties of the fatty acids, and of compounds that contain them, are largely determined by the length and degree of unsaturation of the hydrocarbon chain. The nonpolar hydrocarbon chain accounts for the poor solubility of fatty acids in water. Melting points are strongly influenced by the length and degree of unsaturation of the hydrocarbon chain. The difference in melting points is due to different degrees of packing of the fatty acid molecules. In the fully saturated compounds, free rotation around each carbon-carbon bond gives the hydrocarbon chain great flexibility; the most stable conformation is the fully extended form, in which the steric hindrance of neighbouring atoms is minimized. These molecules can pack together tightly in nearly crystalline arrays, with atoms all along their lengths in van der Waals contact with the atoms of neighbouring molecules. In unsaturated fatty acids, a cis double bond forces a kink in the hydrocarbon chain. Fatty acids with one or several such kinks cannot pack together as tightly as fully saturated fatty acids, and their interactions with each other are therefore weaker. Because less thermal energy is needed to disorder these poorly ordered arrays of unsaturated fatty acids, they have markedly lower melting points than saturated fatty acids of the same chain length. Polyunsaturated fatty acids (PUFAs): The family of polyunsaturated fatty acids with a double bond between the third and fourth carbon from the methyl end of the chain are of special importance in human nutrition. Because the physiological role of PUFAs is related more to the position of the first double bond near the methyl end of the chain than to the carboxyl end, an alternative nomenclature is sometimes used for these fatty acids. Triacylglycerols: They are the simplest lipids constructed from fatty acids. Triacylglycerols are composed of three fatty acids each in ester linkage with a single glycerol. Those containing the same kind of fatty acid in all three positions are called simple triacylglycerols and are named after the fatty acid they contain. Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules, essentially insoluble in water. Lipids have lower specific gravities than water, which explains why mixtures of oil and water have two phases: oil, with the lower specific gravity, floats on the aqueous phase. There are two significant advantages to using triacylglycerols as stored fuels, rather than polysaccharides such as glycogen and starch. First, the carbon atoms of fatty acids are more reduced than those of sugars, and oxidation of triacylglycerols yields more than twice as much energy as the oxidation of carbohydrates. Second, becayse triacylglycerols are hydrophobic and therefore unhydrated, the organism that carries fat as fuel does not have to carry the extra weight of water of hydration that is associated with stored polysaccharides. Phospholipids: The hydrophobic moieties in these amphipathic compounds may be as simple as a single –OH group at one end of the sterol ring system, or they may be much more complex. In glycerophospholipids and some sphingolipids, a polar head group is joinsed to the hydrophobic moiety by a phosphodiester linkage; these are the phospholipids. Other sphingolipids lack phosphate but have a simple sugar or complex oligosaccharide at their polar ends; these are the glycolipids. Glycerophospholipids: They are membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol, and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon. Glycerol is prochiral; it has no asymmetric carbons, but attachment of phosphate at one end converts it into a chiral compound, which can be correctly named either L- glycerol 3-phosphate, D-glycerol 1-phosphate, or sn-glycerol 3-phosphate. Glycerophospholipids are named as derivatives of the parent compound, phosphatidic acid. Sphingolipids: They have a polar head group and two nonpolar tails, but unlike glycerophospholipids and galactolipids, they contain no glycerol. Sphingoipids are composed of one molecule of the long-chain amino alcohol sphingosine or one of its derivatives, one molecule of a long-chain fatty acid, and a polar head group that is joined by a glycosidic linkage in some cases and a phosphodiester in others. Carbons C-1, C-2 and C-3 of the sphingosine moelcules are structurally analogous to the three carbons of glycerol in glycerophospholipids. When a fatty acid is attached in amide linkage to the –NH2 on C-2, the resulting compound is ceramide, which is structurally similar to a diacylglycerol. Ceramide is the structural parent of all sphingolipids. Phosphatidylinositols: Phosphatidylinositols act at several levels to regulate cell structure and metabolism. Phosphatidylinositols 4,5-bisphosphate in the cytoplasmic (inner) face of plasma membranes serves as a reservoir of messenger molecules that are released inside the cell in response to extracellular signals interacting with specific surface receptors. Extracellular signals suc
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