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21: Lipids
Preview 21 - 2
21.1 Structures of Lipids 21 - 2
Fats, Oils, and Related Compounds (21.1A) 21 - 2 Fatty Acids. A Comparison of Fats and Oils Hydrogenation of Fats and Oils Soaps Detergents Waxes Glycerophospholipids The Biological Origin of Fatty Acids Prostaglandins (21.1B) 21 - 7 Terpenes and Steroids (21.1C) 21 - 8 Terpenes Steroids
21.2 Biosynthesis of Lipids 21 - 11
Acetyl-CoA (21.2A) 21 - 11 Fatty Acids (21.2B) 21 - 13 Palmitic Acid Types of Reactions in Palmitic Acid Biosynthesis Other Fatty Acids Fats, Oils and Phospholipids (21.2C) 21 - 16 Waxes (21.2D) 21 - 18 Prostaglandins (21.2E) 21 - 18 Terpenes (21.2F) 21 - 18 Steroids (21.2G) 21 - 22
Chapter Review 21 - 22
21: Lipids
- Structures of Lipids
- Biosynthesis of Lipids
Preview
Lipids are biological molecules soluble in organic solvents such as alcohols and ethers. They include fats , oils , waxes , terpenes , steroids , prostaglandins , and molecular components of membranes. We begin this chapter with an exploration of their structures and properties and conclude it with a description of their biosynthetic origins. Lipids have different types of functional groups so they are not a discrete "class" of organic molecules such as those we have studied in previous chapters or will study in the final chapters of this text. However, they share the common feature that their biosynthetic origin is the fundamental biological building block acetyl CoA. [Figure 21.01] Figure 21.
21.1 Structures of Lipids
The functional groups of fats , oils , waxes , and prostaglandins are significantly different from those of terpenes and steroids. Fats, Oils, and Related Compounds (21.1A) Fats and oils are triacylglycerols ( triglycerides ) with three ester (acyl) groups. [Figure 21.02] Figure 21. These groups R 1 C(=O), R 2 C(=O), and R 3 C(=O) have unbranched carbon chains (typically C 12 to C 24 ) that are saturated alkyl groups, or unsaturated groups with one or more C=C double bonds. [Figure 21.03] [next page].
Table 21.1. Some Common Fatty Acids Cn Structure Common Name Saturated C 12 CH 3 - (CH 2 ) 10 - CO 2 H lauric acid C 14 CH 3 - (CH 2 ) 12 - CO 2 H myristic acid C 16 CH 3 - (CH 2 ) 14 - CO 2 H palmitic acid C 18 CH 3 - (CH 2 ) 16 - CO 2 H stearic acid Unsaturated* C 18 CH 3 - (CH 2 ) 7 - (CH=CH-CH 2 ) 1 - (CH 2 ) 6 - CO 2 H oleic acid C 18 CH 3 - (CH 2 ) 4 - (CH=CH-CH 2 ) 2 - (CH 2 ) 6 - CO 2 H linoleic acid C 18 CH 3 - (CH 2 ) 1 - (CH=CH-CH 2 ) 3 - (CH 2 ) 6 - CO 2 H linolenic acid C 20 CH 3 - (CH 2 ) 4 - (CH=CH-CH 2 ) 4 - (CH 2 ) 2 - CO 2 H arachidonic acid
- Structural formulas are abbreviated for ease of comparison. Table 21.2. Comparative Acyl Group Composition of Beef Tallow and Peanut Oil Beef Tallow (a fat) Peanut Oil (an oil) Acyl Group %-Acyl Group %-Acyl Group** Saturated myristoyl (C 14 ) 4 0 palmitoyl (C 16 ) 30 10 stearoyl (C 18 ) 20 4 (total %-saturated) ( 54 ) ( 14 ) Unsaturated oleoyl (C 18 ) 40 45 linoleoyl (C 18 ) 2 30 (total %-unsaturated) ( 42 ) ( 75 ) *Total percentages are <100% because there are other acyl groups not listed. Hydrogenation of Fats and Oils. Hydrogenation of fats or oils transforms unsaturated acyl side chains into saturated acyl side chains as we show below for conversion of liquid trioleoylglycerol ( triolein , m.p. - 5 °) into solid tristearoylglycerol ( tristearin , m.p. 55°). Figure 21. Since oleoyl, linoleoyl, and linolenoyl groups are all C 18 , complete hydrogenation transforms each of them into C 18 stearoyl groups. Industrial laboratories use hydrogenation to increase the melting points of fats and oils. You may have noticed that various "partially hydrogenated" oils are ingredients in many food products. The major difference between "soft margarines" sold in plastic tubs, and the harder margarine sold in wrapped "sticks" is the extent of hydrogenation of the vegetable oils that are their primary ingredients.
Partially Hydrogenated Vegetable Oils and Your Health. Nutritional experts tell us that highly unsaturated triglycerides are better for our health than saturated triglycerides. Since hydrogenation reduces unsaturation, we can conclude that partially hydrogenated fats and oils are less nutritionally desirable than those that are not hydrogenated. During hydrogenation, cis C=C bonds isomerize to trans C=C bonds and recent studies suggest that fats and oils with trans C=C bonds have the same disadvantages with respect to our health as those with saturated side chains. Soaps. Base catalyzed hydrolysis of fats or oils ( saponification ) gives sodium or potassium salts of fatty acids that we call soaps [Figure 21.06]. Figure 21. Soaps are not lipids! We discuss them here because they come from lipids and are of significant historical and commercial importance. Early Soap Making. The art of making soap is thousands of years old. The Romans made soap by heating animal fat with ashes from wood fires and early immigrants and pioneers in this country made soap in the same way. Animal fat contains water, and ashes contain strong bases, so this soap-making process is akin to saponification carried out in a laboratory. Soaps work as cleansing agents in water because they combine with grease to form micelles. Figure 21. These micelles are water miscible aggregates with polar ( hydrophilic ) exteriors and nonpolar ( hydrophobic ) interiors. The long chain hydrocarbon "tails" of soap molecules dissolve in grease droplets in water forming a micelle with the CO 2 -^ groups of the fatty acid carboxylates on its surface. The negatively charged surface of the spherical micelle hydrogens bonds to water molecules. This solubilizes grease so that the rinsing process removes it along with the micelles.
Naturally occurring "waxes" can be mixtures of different esters ( waxes ) and they can also include small amounts of high molecular mass alkanes such as CH 3 (CH 2 ) 19 CH 3 and CH 3 (CH 2 ) 27 CH 3 found in beeswax. Glycerophospholipids. Biological membranes contain a number of different types of molecular species including glycerophospholipids that we also call phosphoglycerides or simply phospholipids. They are structurally similar to triglycerides except that they have an organophosphate group as shown for phosphatidyl choline. Figure 21. The Biological Origin of Fatty Acids. We have mentioned that fatty acids found as acyl groups in fats, oils, and waxes, have an even number of C atoms because they form from C 2 acyl groups of acetyl-CoA (CH 3 - C(=O)-SCoA). We will explore the reactions and intermediates of these biosynthetic pathways later in this chapter. Figure 21. Prostaglandins (21.1B) Prostaglandins are C 20 carboxylic acids containing a C 5 ring substituted with a C 8 chain, and an adjacent C 7 chain with a terminal CO 2 H group. Figure 21. Their common biosynthetic precursor is the C 20 unsaturated fatty acid arachidonic acid (four C=C bonds) that comes from linoleic acid (two C=C bonds) (see earlier Table 21.1 and Figure 21.14)[next page]. Neither humans nor animals biosynthesize linoleic acid, so it is an essential nutrient that we and they must obtain from sources such as plant fats and oils. Prostaglandins such as those shown in Figure 21.15 [next page] have dramatic influences on biological processes including inflammation, blood clotting, blood pressure, pain, fever, and reproduction. We will outline their biosynthesis later in this chapter.
Figure 21. Figure 21. Terpenes and Steroids (21.1C) Lipids that we have described so far all have ester or carboxylic acid functional groups and even numbers of C atoms. None of these features characterize terpenes or steroids. Terpenes. We find terpenes or derivatives of terpenes in both plants and animals. They are branched polyenes with isolated C=C bonds, that often have no other functional groups. Figure 21. We can dissect the carbon skeletons of terpenes into C 5 fragments called isoprene units (Figure 21.17) [next page]. You can think of an isoprene unit as a C 2 "ethyl" fragment bonded to the central C of a C 3 "isopropyl" fragment. This "ethyl-isopropyl" skeleton is called an isoprene unit because isoprene (2-methyl-1,3-butadiene) has the same C 5 σ- skeleton.
One of the terminal C's of the C 3 fragment is its "head" (C1), while the terminal C of its C 2 fragment is the end of its "tail" (C5). You can see in the structures in Figure 21.19 that C (the tail C) of one isoprene unit bonds to C1 (the head C) of another with the exception of the central C5-C5 (tail to tail) bond in squalene. While terpenes are structurally different from fats, oils, waxes, and prostaglandins, they are lipids because of their solubility in organic solvents and their biosynthetic origin from acetyl- CoA described below. Steroids. Steroids are lipids that arise from the terpene squalene. Squalene cyclizes in three steps to give lanosterol that then gives cholesterol via a sequence of 19 reactions. Figure 21. Cholesterol is the biosynthetic precursor of all steroid hormones (Figure 21.21)[next page]. While biochemists group steroids into five major categories as shown along with these examples, they all have a number of common features. Each steroid has four fused rings (three 6-membered rings and one 5-membered ring) labeled A, B, C, and D. They also have an OH or keto group at C3, a methyl (or carbonyl) group at C10 and/or C13, and a side-chain or OH group on C17. When you consider the contrasting biological effects of testosterone and estradiol , it seems amazing that there are such small differences in their chemical structures!
Figure 21.
21.2 Biosynthesis of Lipids
We can include all lipids in a general biosynthetic scheme that begins with acetyl-CoA. Figure 21. Acetyl-CoA (21.2A) Acetyl-CoA is the product of reaction sequences in which carbohydrates (Chapter 20), proteins (Chapter 22), and even fats and oils , break down via catabolic pathways. Figure 21.
Fatty Acids (21.2B) Fatty acid biosynthesis occurs by several different pathways. Palmitic Acid. One of these pathways combines eight C 2 fragments from acetyl-CoA (CH 3 - C(=O)-S-CoA) in a stepwise manner to give the C 16 fatty acid palmitic acid. Figure 21. It first transforms a C 2 acetyl group (CH 3 C(=O)) into a C 4 butryl group (CH 3 (CH 2 ) 2 C(=O)), then into a C 6 hexanoyl group (CH 3 (CH 2 ) 4 C(=O)), and so on in C 2 increments until it reaches the C 16 palmitoyl group (CH 3 (CH 2 ) 14 C(=O)). In each of the 7 cycles of the pathway, the molecule that provides the C 2 fragment to the growing chain of the acyl group is not acetyl-CoA , but is one derived from its carboxylated derivative malonyl-CoA (see Figure 21.27)[next page]. Before it provides the C 2 fragment to the growing acyl chain, the protein containing group ACP ( acyl-carrier protein ) replaces the CoA group of malonyl-CoA. The resultant malonyl-ACP transfers the C 2 fragment to the growing acyl chain increasing its length from Cn to Cn+2. ACP. If we represent the general structure of CoA as [nucleotide]-[pyrophosphate]-["R"] , then we can represent the general structure of ACP as [protein]-[phosphate]-["R"]. In CoA and ACP, the large organic sequence ["R"] is the same and it binds to acyl groups via a terminal S atom (see Figure 21.24). The growing acyl chain that receives the C 2 fragment from malonyl-ACP in Step (3) is in bold font in the general structure CH 3 - (CH 2 )n-C(=O) - S-ACP (an "acyl"-ACP) (Figure 21.27). In the first reaction cycle, the "acyl"-ACP that reacts with malonyl-ACP in Step (3) is
Other Fatty Acids. Palmitic acid (C 16 ) is the shortest fatty acid biosynthesized by humans and higher animals. Elongase enzymes convert it into higher saturated fatty acids. One elongase pathway uses the same sequence of steps as palmitic acid biosynthesis (Figure 21.27) except CoA rather than ACP binds to all acyl groups. Another elongase pathway also uses CoA rather than ACP , but acetyl-CoA (rather than malonyl-CoA ) provides the enolate- type ion in the chain-lengthening step (Step (3)). So Step (3) in this second elongase pathway is a "Claisen-type" reaction (Chapter 18), not a "malonic ester-type" synthesis. Desaturase enzymes dehydrogenate CH 2 CH 2 groups in saturated or certain unsaturated fatty acids to give cis CH=CH groups. Figure 21. These "desaturase" reactions have specific restrictions on the values of "a" and "b" in the structures shown in Figure 21.28, and on the positions of new C=C double bonds with respect to existing C=C bonds. A major consequence is that many animals as well as humans cannot biosynthesize linoleic acid (a crucial precursor of arachidonic acid and hence prostaglandins ). We and they must obtain it from external sources such as plant fats and oils in order to sustain proper metabolism. Fats, Oils and Phospholipids (21.2C) The biosynthesis of fats, oils, and phospholipids requires not only fatty acids, but also the glycerol portion of these glycerides. Figure 21. This glycerol portion comes from fructose which organisms enzymatically convert to dihydroxyacetone phosphate. Figure 21.
Dihydroxyacetone phosphate (or its C=O reduction product glycerol phosphate ) reacts with fatty acid acyl-CoA molecules to give triglycerides (fats and oils) or phospholipids. Figure 21.
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