Storage Lipids:- Fatty Acids Are Hydrocarbon Derivatives

Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C₄to C₃₆). 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 (Table 10–1). A few contain three-carbon rings, hydroxyl groups, or methyl group branches. A simplified nomenclature for these compounds specifies the chain length and number of double bonds, separated by a colon; for example, the 16-carbon saturated palmitic acid is abbreviated 16:0, and the 18-carbon oleic acid, with one double bond, is 18:1. The positions of any double bonds are specified by superscript numbers following Δ (delta); a 20-carbon fatty acid with one double bond between C-9 and C-10 (C-1 being the carboxyl carbon) and another between C-12 and C-13 is designated 20:2(Δ^9,12). The most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons (Table 10–1). As we shall see in Chapter 21, the even number of carbons results from the mode of synthesis of these compounds, which involves condensation of two-carbon (acetate) units. There is also a common pattern in the location of double bonds; in most monounsaturated fatty acids the double bond is between C-9 and C-10 (Δ⁹), the other double bonds of polyunsaturated fatty acids are generally Δ ¹² and Δ ¹⁵.

(Arachidonic acid is an exception to this generalization.) The double bonds of polyunsaturated fatty acids are almost never con jugated (alternating single and double bonds, as in -CH=CH-CH=CH-), but are separated by a methylene group: -CH=CH-CH₂-CH=CH-. In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis configuration. Trans fatty acids are produced by fermentation in the rumen of dairy animals and are obtained from dairy products and meat.

They are also produced during hydrogenation of fish or vegetable oils. Because diets high in trans fatty acids correlate with increased blood levels of LDL (bad cholesterol) and decreased HDL (good cholesterol), it is generally recommended that one avoid large amounts of these fatty acids. Unfortunately, French fries, dough nuts, and cookies tend to be high in trans fatty acids. 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 hydro carbon chain. The nonpolar hydrocarbon chain accounts for the poor solubility of fatty acids in water. Lauric acid (12:0, Mr200), for example, has a solubility in water of 0.063mg/g—much less than that of glucose (Mr180), which is 1,100 mg/g. The longer the fatty acyl chain and the fewer the double bonds, the lower is the solubility in water.

FIGURE 10–1 The packing of fatty acids into stable aggregates. The extent of packing depends on the degree of saturation. (a) Two representations of the fully saturated acid stearic acid (stearate at pH 7) in its usual extended conformation. Each line segment of the zigzag represents a single bond between adjacent carbons. (b) The cis double bond (shaded) in oleic acid (oleate) does not permit rotation and introduces a rigid bend in the hydrocarbon tail. All other bonds in the chain are free to rotate. (c) Fully saturated fatty acids in the extended form pack into nearly crystalline arrays, stabilized by many hydro phobic interactions. (d) The presence of one or more cis double bonds interferes with this tight packing and results in less stable aggregates.

The carboxylic acid group is polar (and ion ized at neutral pH) and accounts for the slight solubil ity of short-chain fatty acids in water. Melting points are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain. At room temperature (25C), the saturated fatty acids from 12:0 to 24:0 have a waxy consistency, whereas unsaturated fatty acids of these lengths are oily liquids. This difference in melting points is due to different de grees of packing of the fatty acid molecules (Fig. 10–1). 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 neigh boring 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 neighboring molecules. In unsaturated fatty acids, a cis double bond forces a kink in the hydrocar bon 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 it takes less thermal energy 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 (Table 10–1). In vertebrates, free fatty acids (unesterified fatty acids, with a free carboxylate group) circulate in the blood bound noncovalently to a protein carrier, serum albumin. However, fatty acids are present in blood plasma mostly as carboxylic acid derivatives such as esters or amides. Lacking the charged carboxylate group, these fatty acid derivatives are generally even less soluble in water than are the free fatty acids.

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مواضيع ذات صلة

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From Genes to Genomes:- Genome Sequences Provide the Ultimate Genetic Libraries

From Genes to Genomes:- The Polymerase Chain Reaction Amplifies Specific DNA Sequences

From Genes to Genomes:- DNA Libraries Provide Specialized Catalogs of Genetic Information

DNA Cloning: The Basics:- Alterations in Cloned Genes Produce Modified Proteins