Lipids Part 1 PDF

Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. COMMON THEMES Structure determines function. The structure of different lipids partially dictates t...

Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. COMMON THEMES Structure determines function. The structure of different lipids partially dictates their function; for example, amphipathic phospholipids form bilayers, whereas nonpolar triacylglycerols provide an anhydrous energy source. The hydrophobic effect causes amphipathic lipids to form structures such as bilayers, micelles, and vesicles. The structure of many enzymes in these pathways is conserved both within a single organism and between different species. A common set of chemical reactions are used in the metabolism of lipids and in biosynthesis of different lipids. Biochemical information is transferred, exchanged, and stored. Lipids can serve as important signaling molecules and second messengers. Biomolecules are altered through pathways involving transformations of energy and matter. Reactions that are energetically unfavorable become favorable when coupled to reactions with large, negative ΔG values; we often think of this in terms of ATP hydrolysis, but other bonds with a large negative ΔG of hydrolysis, such as thioesters, are common in lipid biochemistry. Likewise, as we have seen elsewhere, reactions that have ΔG° values close to zero can be tipped to move in the forward or reverse direction by changing the concentrations of substrates or products. Fatty acids and ketone bodies are synthesized and degraded using similar reactions. Different biochemical pathways and compartmentalization result in biosynthesis of fatty acids or βoxidation. This is similar to the situation seen with glycolysis and gluconeogenesis. Similar chemical patterns are found in β-oxidation, fatty acid synthesis, and ketone body synthesis and breakdown. Some of these reactions were discussed in the citric acid cycle. in ag ia nn ot ti1 3 @g m ai l.c om Evolution's outcomes are conserved. Comparison of the major enzymes of fatty acid biosynthesis (acetyl-CoA carboxylase ACC and fatty acid synthase FAS) gives insights as to how this pathway has evolved. Analysis of different species reveals that genes coding for the proteins and enzymes responsible for the synthesis of certain molecules, such as some steroids and eicosanoids, have evolved more recently; hence, these molecules may address needs that are lacking in simpler species. ar 9.1 Properties, Nomenclature, and Biological Functions of Lipid Molecules m This section provides an overview of the main categories of lipids: fatty acids, neutral lipids, phospholipids, steroids, bile salts, and eicosanoids. 9.1.1 Fatty acids are a common building block of many lipids Fatty acids are components of neutral lipids, phospholipids, and eicosanoids. Typically, fatty acids are unbranched long-chain carboxylic acids, containing an even number of carbon atoms, generally between 12 and 26 with 16 and 18 carbons being the most common (Figure 9.2). Fatty acids are either saturated (no double bonds), monounsaturated (one double bond), or polyunsaturated (multiple double bonds). Unless otherwise noted, all double bonds in fatty acids can be assumed to be in the cis conformation. Figure 9.2 illustrates the notation system used to describe fatty acids. The number of carbon atoms and the number of double bonds are separated by a colon. The position of a double bond is designated by a superscript Greek delta, Δ, followed by the position of the first carbon of each double bond; for example, a double bond between C-9 and C-10 would be denoted Δ9. Common polyunsaturated fatty acids do not have conjugated double bonds and instead have a single methylene unit between double bonded carbons. ia nn ot ti1 3 @g m ai l.c om Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. FIGURE 9.2 Structure of oleic acid and major categories of fatty acids. A. Fatty acids are long-chain, even-numbered carboxylic acids. They may be unsaturated (like oleic acid) in which instance they almost always have cis double bonds. B. Palmitic, palmitoleic, and linolenic acids are all fatty acids. Palmitic (16:0) is a saturated fatty acid, palmitoleic (16:1Δ9) is a monounsaturated fatty acid, and linolenic (18:2Δ9,12) is a polyunsaturated fatty acid. Note how the inclusion of cis double bonds introduces kinks into the carbon chain. Highly polyunsaturated fatty acids are kinked to the point of being almost circular in shape. ar in ag Because of the way fatty acids are synthesized, elongated (extended beyond 16 carbons in length), and desaturated (how double bonds are incorporated), many fatty acids are also characterized by the position of the double bond farthest from the carboxyl group. The carbon farthest from the carboxyl group within that double bond is given the designation ω (omega, the last letter in the Greek alphabet); hence, we find ω-3, ω-6, ω-7, or ω-9 fatty acids, counting from the terminal carbon. The melting points of fatty acids vary widely, but some common trends emerge. As the fatty acid gets longer, the melting point increases. However, incorporation of double bonds greatly decreases the melting point (Table 9.1). m TABLE 9.1 Some Common Fatty Acids Melting point ωnomenclature Description Common name IUPAC name Source 12:0 Lauric acid Dodecanoic acid Bay leaves 44.2 – 14:0 Myristic acid Tetradecanoic acid Coconut oil 53.9 – 16:0 Palmitic acid Hexadecanoic acid Palm oil 63.1 – 18:0 Stearic acid Octadecanoic acid Beef tallow 69.6 – 20:0 Arachidic acid Eicosanoic acid Peanut oil 76.5 – 1 ω-7 16:1Δ9 Palmitoleic acid cis-9-hexadecenoic acid Animal fats 18:1Δ9 Oleic acid cis-9-octadecaenoic acid Olive oil 13.4 ω-9 18:2Δ9, 12 Linoleic acid Safflower cis-, cis-9, 12-octadecadienoic acid oil −5 ω-6 18:3Δ9, 12, 15 α-Linolenic cis-, cis-, cis-9, 12, 15 octadecadienoic acid −11 ω-3 Safflower oil Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. Description Common name IUPAC name Source Melting point ωnomenclature 20:3Δ8, 11, 14 DGLA, dihomogamma linolenic acid Eicosatrienoic acid Fish oil (rare) ∼−5 ω-6 20:4Δ5,8,11,14 Arachidonic acid Eicosatetraenoic acid −49.5 ω-6 20:5Δ5,8,11,14,17 EPA Eicosapentaenoic acid ∼−50 ω-3 Docosahexaenoic acid −44 ω-3 22:6Δ4,7,10,13,16,19 DHA Fatty acids are typically known by their common name (stearic acid), or an International Union of Pure and Applied Chemistry [IUPAC] name such as octadecanoic acid, 18:0. Fatty acids: acid-base properties ai l.c om Because fatty acids contain a carboxyl group, they act as weak acids. The pKa of the carboxyl group is about 4.5, similar to other carboxylic acids such as amino acids or the intermediates of the citric acid cycle. Fatty acids are generally ionized (approximately 99.9%) at pH 7.4. In the laboratory, to increase the percentage of ionized species and thus make the molecule more soluble in aqueous solutions, the acid is often reacted with a strong base to generate a carboxylic acid salt or a carboxylate. Carboxylic acids, the protonated species, are distinct from carboxylates, the deprotonated species, and it is not surprising that the two forms have different properties and reactivity. WORKED PROBLEM 9.1 | Characterizing a fatty acid @g m Fatty acids are typically found as esters or amides. Amides are referred to as peptides in protein chemistry but as amides in the lipid literature. Both ester and amide linkages are frequently observed in lipid chemistry. The term free fatty acid is reserved for fatty acids that are not part of an ester or an amide. Strategy ia nn ot ti1 3 Linolenic acid is 18:3Δ9, 12, 15. What is the omega designation of linolenic acid? Examine Figure 9.2. Think about what the omega designation tells us. It may be useful to draw the structure of linolenic acid. Solution ar in ag Linolenic acid has the following structure: m The omega carbon is the final carbon. The double bond closest to that carbon is on the third carbon from the end, making this a ω-3 fatty acid. Follow-up question Using Table 9.1, identify this fatty acid: With a long, nonpolar hydrophobic tail at one end of the molecule and a polar carboxylate group at the other end, fatty acids are considered amphipathic, literally meaning having “both feelings,” because they contain both hydrophobic and hydrophilic groups and are thus both water-loving and water-hating. The term amphipathic is also used to describe α helices having both a polar and a nonpolar face. Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. Fatty acids: assembly into higher order structures ar in ag ia nn ot ti1 3 @g m ai l.c om Like other amphipathic molecules, fatty acids exhibit interesting properties when dissolved in water. At a very low concentration, fatty acids are found as single molecules in solution, but at higher concentrations, they aggregate into a roughly spherical structure known as a micelle (Figure 9.3). Within the micelle, the hydrophobic tails are oriented toward the waterless core of the sphere and the hydrophilic head groups are oriented toward the solvent. This structure maximizes the interaction of the polar head groups with water while shielding the hydrophobic tails. Similar to protein folding, when hydrophobic molecules are clustered together, water molecules are effectively freed from having to cage these molecules. Formation of a micelle appears to result in an apparent gain of organization and loss of entropy; however, the reverse is true when looking at the larger picture. The gain in entropy as a result of liberating water makes the overall ΔS positive. The free energy (ΔG) of the assembly of micelles is therefore negative, and the process is spontaneous. This is another illustration of the hydrophobic effect. m FIGURE 9.3 Micelle formation results in increased degrees of freedom for water. A. In a micelle, amphipathic molecules are oriented with their hydrophobic tails toward the center of the micelle and their hydrophilic head groups oriented toward the aqueous surroundings. Note that micelles have no water in their core. Micelles form as a result of the hydrophobic effect. B. Shown is an isolated amphipathic molecule surrounded by water molecules. Water molecules can hydrogen bond to the polar head group but form a cage around the hydrophobic tail. C. Fewer water molecules are organized around a micelle than around a similar number of monomeric amphiphiles. Despite the apparent gain of organization and loss of entropy in the organization of a micelle, the gain in entropy as a result of liberating water makes the overall ΔS positive. The free energy (ΔG) of the assembly of micelles is therefore negative, and the process is spontaneous. The concentration at which micelles form is called the critical micellar concentration (or CMC). The CMC is a property of all amphipathic molecules, including all detergents and many lipids and differs for each molecule. For fatty acids, the CMC is typically in the range of 1 to 6 μM; below this concentration, fatty acids are found as monomers in solution, and above it, they spontaneously assemble into micelles. The micelles exist in a dynamic equilibrium with monomeric (single) fatty acids in solution. Increasing the concentration of fatty acids above the CMC increases the number of micelles found in a sample but not the size of the micelles. When working with an amphipathic molecule in the laboratory, it is useful to know the CMC. Below the CMC, amphipathic molecules will partition into membranes; at concentrations above the CMC, membranes will begin to dissolve. 9.1.2 Neutral lipids are storage forms of fatty acids or cholesterol Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. Because fatty acids are carboxylic acids and are amphipathic, they can act as detergents and affect pH—properties that could be harmful to the organism. Therefore, fatty acids are stored as neutral lipids, which lack charged groups because the carboxylic acid group is esterified to either glycerol or cholesterol. ai l.c om Fatty acids esterified to glycerol form a monoacylglycerol (one fatty acid), diacylglycerol (two fatty acids), or triacyclglycerol (three fatty acids); these molecules are also referred to as a monoglyceride, diglyceride, or triglyceride, respectively (Figure 9.4). @g m FIGURE 9.4 Other examples of glycerides. A. Shown is an example of a monoglyceride and a diglyceride, or monoacylglycerol and diacylglycerol. B. The second example shows saturated, monounsaturated, and polyunsaturated triacylglycerols. ia nn ot ti1 3 If a glyceride contains only saturated fatty acids (no double bonds), it is called a saturated fat; if it contains at least one fatty acid that has a single double bond, it is termed a monounsaturated fat; and if it contains at least one fatty acid with multiple double bonds, it is known as a polyunsaturated fat. Glycerides can be crudely categorized as fats or oils based on their melting points; fats are solid at room temperature, whereas oils are liquid at room temperature. A saturated fat is solid at room temperature because the straight chains of the fatty acids allow the molecules to pack tightly together. Saturated fats commonly come from animal origins, such as tallow from cows or lard from pigs. Unsaturated fats are liquid at room temperature because the kinked cis double bonds prevent the molecules from packing tightly together. Common sources of unsaturated fats are vegetables, such as corn, soy, or canola, and cold-water fish, such as salmon. m ar in ag Although plant oils are readily available, they are less desirable for some food and cooking applications than solid fats because of their physical properties. For example, solid fats are preferred for baked and fried products. Hence, vegetable oils used in processed foods are often hydrogenated, that is, chemically treated with hydrogen gas and a catalyst to convert some of the unsaturated fatty acids into saturated ones. The complete conversion of all the cis double bonds in a vegetable oil results in a product that has too high a melting point and a waxy taste and feel. Therefore, oils are often partially hydrogenated. This partial reaction can result in the naturally occurring cis fatty acids being isomerized to trans fatty acids, forming trans fats. The properties of trans fats are similar to those of saturated fats; they are metabolized differently than either saturated or cis fatty acids. Although low quantities of trans fatty acids do occur naturally in some bacteria and ruminants, organisms have not evolved to metabolize large quantities of trans fats. It is possible to remove most of the trans fatty acids from processed foods. This is done by replacing the partially hydrogenated fats with blends of saturated and unsaturated fats that have similar physical properties to the hydrogenated fats. Fatty acids esterified to cholesterol yield cholesteryl esters, the main storage form of cholesterol. Cholesteryl esters form a large part of the plaque in arteries that leads to atherosclerosis and heart disease. Certain cells in the body also store cholesteryl esters as a cholesterol reserve, used to produce steroid hormones. Fatty acids and cholesterol are not the only molecules stored as esters. Retinyl palmitate, a storage form of vitamin A, is an ester of retinol and palmitic acid. The esterification protects the hydroxyl group of retinol from oxidation to an aldehyde or carboxylic acid until the vitamin is needed in the body. Vitamin E is often administered in an esterified form, as tocopheryl acetate or succinate; the esterification prevents oxidation of the alcohol and thus helps to stabilize the compound (Figure 9.5). The ester bond is cleaved in the stomach, and the free alcohol (tocopherol) is absorbed and used by the body. ai l.c om Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. @g m FIGURE 9.5 Structures of vitamins A and E. Both vitamins A and E are fat-soluble vitamins and contain an ester moiety. For vitamin A (retinyl palmitate), this serves to protect the hydroxyl group from oxidation to an aldehyde or carboxylic acid until needed in the body. Vitamin E (tocopheryl acetate) is also frequently found as an ester—in this case an acetate. Again, this form of the vitamin is protected from premature oxidation by the ester. Vitamin A is found in foods such as tomatoes. Nuts are a rich source of vitamin E. ar in ag ia nn ot ti1 3 Another class of neutral lipids worth mentioning is the waxes. These molecules are long-chain fatty acids esterified to a long-chain alcohol termed a fatty alcohol; waxes are lengthy and hydrophobic (Figure 9.6). Organisms produce and secrete these molecules as protective coatings that provide some degree of natural waterproofing and prevent desiccation. The best known include bee's wax; carnauba wax; jojoba oil; lanolin; and spermaceti, the wax found in the head of sperm whales. Lanolin, also known as wool grease, is secreted by sheep to assist in waterproofing their wool and skin. Found in many skin creams and lotions, lanolin differs from the other waxes mentioned above because the alcohol forming the ester is cholesterol rather than a fatty alcohol. m FIGURE 9.6 Structure of waxes. Waxes, such as triaconatyl palmitate, consist of long-chain fatty acids esterified to longchain alcohols. The highly hydrophobic nature of these molecules makes them useful as protectants against desiccation. 9.1.3 Phospholipids are important in membrane formation Phospholipids are lipid molecules that contain a phosphate moiety. The hydrophilic nature of the phosphate group and the hydrophobic nature of the aliphatic lipid make these molecules amphipathic. Phospholipids are considered polar lipids and are the main structural component of membranes. Many phospholipids are also involved in signal transduction and act as lipid signaling mediators. Phospholipids can broadly be divided into two categories, depending on the type of backbone they contain. Glycerophospholipids have a glycerol backbone, whereas sphingolipids have a sphingosine backbone. Because of their structure and geometry, some of these molecules can form micelles, but many will form other more complex structures, such as lipid bilayers, monolayers, or vesicles, a bilayer surrounding an aqueous compartment (Figure 9.7). Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. ai l.c om FIGURE 9.7 Examples of phospholipid structures. Phospholipids can form a variety of structures, depending on the head group, type of acyl chain, and number of acyl chains (one versus two). These structures can include some of the ones we have seen previously, for example, micelles and bilayers. Phospholipids can also form structures such as vesicles, a bilayer structure containing an aqueous core. Glycerophospholipids m ar in ag ia nn ot ti1 3 @g m Glycerophospholipids feature two fatty acyl chains and a phosphoalcohol esterified to a glycerol backbone. Fatty acids are found esterified in the first two positions on the glycerol backbone. A phosphate group is in the final position, attached through a phosphoester linkage. Also connected to the phosphate is an alcohol, again through a phosphoester linkage. The alcohol gives rise to the common names of these lipids (Figure 9.8). ar in ag ia nn ot ti1 3 @g m ai l.c om Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. m FIGURE 9.8 Structure of glycerophospholipids. A. and B. Glycerophospholipids feature two fatty acyl chains and a phosphoalcohol esterified to a glycerol backbone. The length and degree of unsaturation of the fatty acids may vary. The most significant difference between these molecules occurs in the head group alcohol. C. The five common most head groups of phospholipids. At physiological pH both the amine on the choline head group as well as the phosphate moiety are ionized making the molecule zwitterionic. D. Cardiolipin is a glycerophospholipid comprised of two molecules of phosphatidate connected by a central glycerol molecule. It is prevalent in mitochondrial membranes. Phosphatidylcholine and phosphatidylethanolamine both contain positively charged amine groups in addition to the negatively charged phosphate, and like amino acids they are zwitterionic at neutral pH. Phosphatidylinositol, phosphatidylserine, and phosphatidate are all important in signaling cascades. These molecules are termed anionic phospholipids or acidic phospholipids due to their net negative charge. Phosphatidylcholine and phosphatidylethanolamine also form the bulk of the lipid in cell membranes (typically over 50%), although membrane composition varies within the cell (such as the plasma membrane compared to mitochondrial or ER membrane), within the organism (a hepatocyte compared to a rod or cone cell in the eye), and between different species (erythrocytes from a human and a cow) (Figure 9.9). Phospholipid concentrations are also different in the two bilayers of the plasma membrane. Typically, the cytosolic leaflet of the plasma membrane is enriched in acidic phospholipids: phosphatidate, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol. Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. ai l.c om FIGURE 9.9 Composition of lipid bilayers. The lipid composition of a membrane varies from cell type to cell type and from organelle to organelle. Shown is the composition for three different membranes. The lipids shown are cholesterol, phosphatidylethanolamine (PE), phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylserine (PS), cardiolipin (CL), and all other lipids. A phospholipid containing a single acyl chain and a free hydroxyl group is termed a lysophospholipid. @g m Sphingolipids ia nn ot ti1 3 Sphingolipids contain many of the same fatty acids and alcohols as glycerophospholipids but differ in the backbone alcohol. The backbone alcohol in sphingolipids is sphingosine, which has two free hydroxyls, an amine, and a long (16 carbons) monounsaturated hydrocarbon tail. The phosphate is esterified through the primary hydroxyl most distal from the fatty-acid tail. Fatty acids are attached via an amide linkage to the amine, and the secondary hydroxyl between the amide and the fatty tail is not usually modified in any way. m ar in ag One of the first sphingolipids identified was sphingomyelin, a major component of the myelin sheath coating some neurons. More recently, sphingolipids have gained attention for their possible involvement in the regulation of apoptosis and anoikis (programmed cell death). In particular, ceramide, sphingosine, and sphingosine-1-phosphate generated from sphingomyelin have been shown to promote apoptosis (Figure 9.10). FIGURE 9.10 Structural examples of sphingophospholipids. A. The backbone of these lipids is formed by the long-chain fatty alcohol sphingosine. B. A single fatty acid is attached via an amide linkage to sphingosine. C. Phosphoalcohols are attached via a phosphoester bond to the more terminal hydroxyl of sphingosine. WORKED PROBLEM 9.2 | Candy chemistry Some candy bar labels state that the product contains soy lecithin, an emulsifier. Lecithin is another term for phosphatidylcholine. Describe how soy lecithin works as an emulsifier. Printed by: [email protected]. Printing is for personal use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted. Strategy To answer this question we need to know what lecithin is and how an emulsifier works. We discussed phosphatidylcholine in this section, and we previously discussed emulsifiers. Drawing the structure of phosphatidylcholine will help us to explain which properties of this molecule make it an emulsifier. Solution Emulsifiers are amphipathic molecules that act to suspend one type of molecule in another, such as a hydrophobic molecule in an aqueous solvent. Soy lecithin is phosphatidylcholine obtained from soybeans. This phospholipid forms a micelle-like structure, coating and solubilizing fatty molecules in chocolate, for example. Within the micelles, the hydrophobic acyl chains are oriented toward the core of the particle and the polar head groups to the outside. ai l.c om Follow-up question m ar in ag ia nn ot ti1 3 @g m We have previously seen how other amphipathic molecules can act as detergents. Could soy lecithin be used as a detergent?