Lipid biochemistry,5ed.

1 Lipids: definition, isolation, separation and detection, 1 1.1 Introduction, 1 1.2 Definitions, 1 1.3 Structural chemistry and nomenclature, 1 1.4 Extraction of lipids from natural samples, 3 1.5 Likely components of the crude lipid extract, 4 1.6 General features of lipids important for their analysis, 4 1.7 Chromatographic techniques for separating lipids, 5 1.7.1 The two phases can be arranged in a variety of ways, 5 1.7.2 Gas±liquid chromatography is a particularly useful method for volatile derivatives of lipids, 6 1.7.3 Absorption column chromatography is used for the separation of large amounts of lipids, 8 1.7.4 Thin layer absorption chromatography can achieve very good separation of small lipid samples, 10 1.8 Other useful methods, 10 1.9 Summary, 10 Further Reading, 12 2 Fatty acid structure and metabolism, 13 2.1 Structure and properties, 13 2.1.1 Saturated fatty acids, 13 2.1.2 Branched-chain fatty acids, 15 2.1.3 Unsaturated fatty acids, 15 2.1.3.1 Monoenoic (monounsaturated) fatty acids, 15 2.1.3.2 Polyenoic (polyunsaturated) fatty acids, 16 2.1.4 Cyclic fatty acids, 16 2.1.5 Oxy acids, 16 2.1.6 Conjugated unsaturated fatty acids, 18 2.1.7 Fatty aldehydes and alcohols, 18 2.1.8 Some properties of fatty acids, 19 2.1.9 Quantitative and qualitative fatty acid analysis, 19 2.1.9.1 General principles, 19 2.1.9.2 Determination of the structure of an unknown acid, 21 2.2 The biosynthesis of fatty acids, 21 2.2.1 Conversion of fatty acids into metabolically active thiolesters is often a prerequisite for their metabolism, 21 2.2.1.1 Acyl-CoA thiolesters were the first types of activated fatty acids to be discovered, 22 2.2.1.2 Acyl±acyl carrier proteins can be formed as distinct metabolic intermediates in some organisms, 24 2.2.2 The biosynthesis of fatty acids can be divided into de novo synthesis and modification reactions, 25 2.2.3 De novo biosynthesis, 26 2.2.3.1 Acetyl-CoA carboxylase, 26 2.2.3.2 Fatty acid synthase, 27 2.2.3.3 Termination, 37 2.2.3.4 Elongation, 39 2.2.3.5 Branched-chain fatty acids, 40 2.2.4 The biosynthesis of hydroxy fatty acids results in hydroxyl groups in different positions along the fatty chain, 41 2.2.5 The biosynthesis of unsaturated fatty acids is mainly by oxidative desaturation, 42 2.2.5.1 Monounsaturated fatty acids, 42 v 2.2.5.2 Polyunsaturated fatty acids, 46 2.2.5.3 Formation of polyunsaturated fatty acids in animals, 49 2.2.6 Biohydrogenation of unsaturated fatty acids takes place in rumen micro- organisms, 51 2.2.7 The biosynthesis of cyclic acids provided one of the first examples of a complex lipid substrate for fatty acid modifications, 52 2.2.8 The control of fatty acid synthesis can take place at a number of enzyme steps, 53 2.2.8.1 Acetyl-CoA carboxylase (ACC) regulation in animals, 53 2.2.8.2 Acetyl-CoA carboxylase regulation in other organisms, 56 2.2.8.3 Regulation of fatty acid synthase, 56 2.2.8.4 Control of animal desaturases, 58 2.3 Degradation of fatty acids, 59 2.3.1 b-Oxidation is the most common type of biological oxidation of fatty acids, 59 2.3.1.1 Cellular site of b-oxidation, 59 2.3.1.2 Transport of acyl groups to the site of oxidation: the role of carnitine, 60 2.3.1.3 Control of acylcarnitine formation is very important, 61 2.3.1.4 Enzymes of mitochondrial b- oxidation, 61 2.3.1.5 Other fatty acids containing branched chains, double bonds and an odd number of carbons atoms can also be oxidized, 62 2.3.1.6 Regulation of mitochondrial b- oxidation, 64 2.3.1.7 Fatty acid oxidation in E. coli, 66 2.3.1.8 b-Oxidation in microbodies, 67 2.3.2 a-Oxidation of fatty acids is important when structural features prevent b-oxidation, 68 2.3.3 o-Oxidation uses mixed-function oxidases, 69 2.3.4 Chemical peroxidation is an important reaction of unsaturated fatty acids, 70 2.3.5 Peroxidation catalysed by lipoxygenase enzymes, 71 2.3.6 Lipoxygenases are important for stress responses and development in plants, 72 2.4 Essential fatty acids and the biosynthesis of eicosanoids, 75 2.4.1 The pathways for prostaglandin synthesis are discovered, 77 2.4.2 Cyclic endoperoxides can be converted into different types of eicosanoids, 80 2.4.3 New eicosanoids are discovered, 81 2.4.4 The cyclooxygenase products exert a range of activities, 82 2.4.5 Prostaglandins and other eicosanoids are rapidly catabolized, 83 2.4.6 Instead of cyclooxygenation, arachidonate can be lipoxygenated or epoxygenated, 84 2.4.7 Control of leukotriene formation, 84 2.4.8 Physiological action of leukotrienes, 86 2.4.9 For eicosanoid synthesis an unesterified fatty acid is needed, 88 2.4.10 Essential fatty acid activity is related to double bond structure and to the ability of such acids to be converted into a physiologically active eicosanoid, 89 2.5 Summary, 90 Further Reading, 91 3 Lipids as energy stores, 93 3.1 Introduction, 93 3.2 The naming and structure of the acylglycerols (glycerides), 93 3.2.1 Triacylglycerols are the major components of natural fats and oils; partial acylglycerols are usually intermediates in the breakdown or synthesis of triacylglycerols, 93 3.2.2 All natural oils are complex mixtures of molecular species, 95 3.3 The storage of triacylglycerols in animals and plants, 97 3.3.1 Adipose tissue depots are the sites of TAG storage in animals, 97 3.3.2 Milk triacylglycerols provide a supply vi Contents of energy for the needs of the new-born, 99 3.3.3 Some plants use lipids as a fuel, stored as minute globules in the seed, 100 3.4 The biosynthesis of triacylglycerols, 102 3.4.1 Pathways for complete (de novo) synthesis build-up TAG from small basic components, 104 3.4.1.1 The glycerol 3-phosphate pathway in mammalian tissues provides a link between TAG and phospholipid metabolism, 104 3.4.1.2 The dihydroxyacetone phosphate pathway in mammalian tissues is a slight variant to the main glycerol 3- phosphate pathway and provides an important route to ether lipids, 107 3.4.1.3 Formation of triacylglycerols in plants involves the co- operation of different subcellular compartments, 108 3.4.2 The monoacylglycerol pathway is important mainly in rebuilding TAG from absorbed dietary fat, 111 3.5 The catabolism of acylglycerols, 113 3.5.1 The nature and distribution of lipases, 113 3.5.2 Animal triacylglycerol lipases play a key role in the digestion of food and in the uptake and release of fatty acids by tissues, 114 3.5.3 Plant lipases break down the lipids stored in seeds in a specialized organelle, the glyoxysome, 115 3.6 The integration and control of animal acylglycerol metabolism, 116 3.6.1 Fuel economy: the interconversion of different types of fuels is hormonally regulated to maintain blood glucose concentration within the normal range and ensure storage of excess dietary energy in triacylglycerols, 116 3.6.2 The control of acylglycerol biosynthesis is important, not only for fuel economy but for membrane formation, requiring close integration of storage and structural lipid metabolism, 117 3.6.3 Mobilization of fatty acids from fat stores is regulated by hormonal balance, which in turn is responsive to nutritional and physiological states, 121 3.7 Wax esters, 122 3.7.1 Occurrence and characteristics, 123 3.7.2 Biosynthesis of wax esters involves the condensation of a long-chain fatty alcohol with fatty acyl-CoA, 123 3.7.3 Digestion and utilization of wax esters is poorly understood, 124 3.7.4 Surface lipids include not only wax esters but a wide variety of lipid molecules, 125 3.8 Summary, 125 Further Reading, 126 4 Dietary lipids, 127 4.1 Lipids in food, 127 4.1.1 The fats in foods are derived from the structural and storage fats of animals and plants, 127 4.1.2 The fatty acid composition of dietary lipids depends on the relative contributions of animal and plant structural or storage lipids, 128 4.1.3 Industrial processing may influence the chemical and physical properties of dietary fats either beneficially or adversely, 129 4.1.3.1 Catalytic hydrogenation, 129 4.1.3.2 Heating, 131 4.1.3.3 Irradiation, 131 4.1.3.4 Interesterification, 131 4.1.3.5 Fractionation, 132 4.1.3.6 Structured fats, 132 4.1.4 A few dietary lipids may be toxic, 132 4.1.4.1 Cyclopropenes, 133 4.1.4.2 Long-chain monoenes, 133 4.1.4.3 Trans-unsaturated fatty acids, 133 4.1.4.4 Lipid peroxides, 134 4.2 Roles of dietary lipids, 134 Contents vii 4.2.1 Triacylglycerols provide a major source of metabolic energy especially in affluent countries, 134 4.2.2 Lipids supply components of organs and tissues for membrane synthesis and other functions, 135 4.2.2.1 Foetal growth, 135 4.2.2.2 Post-natal growth, 138 4.2.3 Dietary lipids supply essential fatty acids that are essential to life but cannot be made in the animal body, 140 4.2.3.1 Historical background: discovery of essential fatty acid deficiency, 140 4.2.3.2 Biochemical basis for EFA deficiency, 141 4.2.3.3 Functions of essential fatty acids, 143 4.2.3.4 Which fatty acids are essential? 146 4.2.3.5 What are the quantitative requirements for essential fatty acids in the diet? 147 4.2.4 Dietary lipids supply fat-soluble vitamins, 150 4.2.4.1 Vitamin A, 151 4.2.4.2 Vitamin D, 153 4.2.4.3 Vitamin E, 156 4.2.4.4 Vitamin K, 158 4.2.5 Lipids play an important role in enhancing the flavour and texture and therefore the palatability of foods, 159 4.2.5.1 Odour, 159 4.2.5.2 Taste, 160 4.2.5.3 Texture, 161 4.3 Dietary lipids in relation to immune function, 161 4.3.1 Components of the immune system and their functional assessment, 161 4.3.2 Summary of lipid effects on different components of immunity, 162 4.3.2.1 Influence on target cell composition, 162 4.3.2.2 Influence on lymphocyte functions ex vivo, 163 4.3.2.3 Influence on antibody production, 163 4.3.2.4 Influence on delayed-type hypersensitivity, 163 4.3.2.5 Graft versus host and host versus graft reactions and organ transplants, 163 4.3.2.6 Survival after infection, 163 4.3.2.7 Influence on autoimmune and inflammatory disease processes, 163 4.3.3 Mechanisms, 164 4.3.3.1 Membrane properties, 164 4.3.3.2 Availability of eicosanoid precursors, 164 4.3.3.3 Availability of vitamin E, 164 4.3.3.4 Gene expression, 164 4.3.3.5 Implications for dietary advice, 165 4.4 Lipids and cancer, 165 4.4.1 Dietary lipids and cancer, 165 4.4.2 Cellular lipid changes in cancer, 166 4.4.3 Lipids and the treatment of cancer, 167 4.5 Summary, 167 Further Reading, 168 5 Lipid transport, 170 5.1 Digestion and absorption, 170 5.1.1 Intestinal digestion of dietary fats involves breakdown into their component parts by a variety of digestive enzymes, 170 5.1.2 The intraluminal phase of fat absorption involves passage of digestion products into the absorptive cells of the small intestine, 173 5.1.3 The intracellular phase of fat absorption involves recombination of absorbed products in the enterocytes and packaging for export into the circulation, 174 5.1.4 Malassimilation of lipids, through failure to digest or absorb lipids properly, can arise from defects in the gut or other tissues but may also be induced deliberately, 175 5.2 Transport of lipids in the blood: plasma lipoproteins, 177 5.2.1 Lipoproteins can be conveniently viii Contents divided into groups according to density, 177 5.2.2 The apolipoproteins are the protein moieties that help to stabilize the lipid; they also provide specificity and direct the metabolism of the lipoproteins, 179 5.2.3 The different classes of lipoprotein particles transport mainly triacylglycerols or cholesterol through the plasma, 179 5.2.4 Specific lipoprotein receptors mediate the cellular removal of lipoproteins and of lipids from the circulation, 182 5.2.4.1 Membrane receptors, 183 5.2.4.2 The LDL-receptor, 183 5.2.4.3 The LDL-receptor-related protein and other members of the LDL-receptor family, 184 5.2.4.4 Scavenger receptors, 186 5.2.5 The lipoprotein particles transport lipids between tissues but they interact and are extensively remodelled in the plasma compartment, 186 5.2.6 Species differ quantitatively in their lipoprotein profiles, 190 5.3 The co-ordination of lipid metabolism in the body, 190 5.3.1 The sterol regulatory element binding protein (SREBP) system controls pathways of cholesterol accumulation in cells and may also control fatty acid synthesis, 191 5.3.2 The peroxisome proliferator activated receptor (PPAR) system regulates fatty acid metabolism in liver and adipose tissue, 194 5.3.3 Other nuclear receptors are activated by fatty acids and affect gene expression, 196 5.3.4 Adipose tissue secretes hormones and other factors that may themselves play a role in regulation of fat storage, 197 5.4 Diseases involving changes or defects in lipid metabolism, 199 5.4.1 Atherosclerosis, 200 5.4.2 Risk factors for CHD and the effects of diet, 205 5.4.3 Hyperlipoproteinaemias (elevated circulating lipoprotein concentrations) are often associated with increased incidence of cardiovascular disease, 208 5.4.4 Obesity and diabetes are associated with increased risk of cardiovascular diseases, 208 5.4.5 Hypolipoproteinaemias are rare conditions of abnormally low plasma lipoprotein concentrations, 212 5.5 Summary, 212 Further Reading, 213 6 Lipids in cellular structures, 215 6.1 Cell organelles, 215 6.2 Glycerolipids, 217 6.2.1 Phosphoglycerides are the major lipid components of most biological membranes, 217 6.2.2 Phosphonolipids constitute a rare class of lipids found in few organisms, 218 6.2.3 Glycosylglycerides are particularly important components of photosynthetic membranes, 219 6.2.4 Betaine lipids are important in some organisms, 221 6.2.5 Ether-linked lipids and their bioactive species, 221 6.3 Sphingolipids, 222 6.4 Sterols, 228 6.4.1 Major sterols, 228 6.4.2 Other steroids, 229 6.5 Membrane structure, 229 6.5.1 Early models already envisaged a bilayer of lipids but were uncertain about the location of the proteins, 229 6.5.2 The lipid±globular protein mosaic model now represents the best overall picture of membrane structure, 230 6.5.3 Membrane structure is not static but shows rapid movement of both lipid and protein components, 232 6.5.4 Further remarks on the lipid composition of membranes, 234 6.5.5 Transbilayer asymmetry is an essential feature of all known biological membranes, 234 Contents ix 6.5.6 Lateral heterogeneity is probably important in some membranes at least, 237 6.5.7 Physical examination of membranes reveals their fluid properties, 238 6.5.8 General functions of membrane lipids, 238 6.5.9 Membrane lipids are modified to maintain fluidity at low temperatures, 242 6.5.10 Lipids and membrane fusion, 245 6.5.11 Lipids and proteins interact in order to determine membrane structure and shape, 247 6.5.12 Why are there so many membrane lipids? 249 6.5.13 Liposomes and drug delivery systems, 250 6.5.14 Lipid anchors for proteins, 251 6.5.14.1 Acylation, 252 6.5.14.2 Prenylation, 252 6.5.14.3 GPI-anchors, 252 6.6 Lipids as components of the surface layers of different organisms, 254 6.6.1 Cutin, suberin and waxes ± the surface coverings of plants, 254 6.6.2 Mycobacteria contain specialized cell- wall lipids, 255 6.6.3 Lipopolysaccharide forms a major part of the cell envelope of Gram-negative bacteria, 256 6.6.4 Gram-positive bacteria have a completely different surface structure, 258 6.6.5 Insect waxes, 259 6.6.6 Lipids of the skin ± the mammalian surface layer and skin diseases, 259 6.7 Summary, 261 Further Reading, 263 7 Metabolism of structural lipids, 267 7.1 Phosphoglyceride biosynthesis, 267 7.1.1 Tracer studies revolutionized concepts about phospholipids, 267 7.1.2 Formation of the parent compound, phosphatidate, is demonstrated, 267 7.1.3 A novel cofactor for phospholipid synthesis was found by accident, 268 7.1.4 The core reactions of glycerolipid biosynthesis are those of the Kennedy pathway, 268 7.1.5 The zwitterionic phosphoglycerides can be made using CDP-bases, 270 7.1.6 CDP-diacylglycerol is an important intermediate for phosphoglyceride formation in all organisms, 271 7.1.7 Phospholipid formation in E. coli is entirely via CDP-diacylglycerol, 271 7.1.8 Differences between phosphoglyceride synthesis in different organisms, 272 7.1.9 Plasmalogen biosynthesis, 273 7.1.10 Platelet activating factor (PAF): a biologically active phosphoglyceride, 276 7.2 Degradation of phospholipids, 277 7.2.1 General features of phospholipase reactions, 278 7.2.2 Phospholipase A activity is used to remove a single fatty acid from intact phospholipids, 280 7.2.3 Phospholipase B and lysophospholipases, 281 7.2.4 Phospholipases C and D remove water- soluble moieties, 282 7.2.5 Phospholipids may also be catabolized by non-specific enzymes, 283 7.3 Metabolism of glycosylglycerides, 283 7.3.1 Biosynthesis of galactosylglycerides takes place in chloroplast envelopes, 283 7.3.2 Catabolism of glycosylglycerides, 284 7.3.3 Relatively little is known of the metabolism of the plant sulpholipid, 284 7.4 Metabolism of sphingolipids, 285 7.4.1 Biosynthesis of the sphingosine base and ceramide, 285 7.4.2 Cerebroside biosynthesis, 286 7.4.3 Formation of neutral glycosphingolipids, 286 7.4.4 Ganglioside biosynthesis, 286 7.4.5 Sulphated sphingolipids, 288 7.4.6 Sphingomyelin is both a sphingolipid and a phospholipid, 289 7.4.7 Catabolism of the sphingolipids, 289 x Contents 7.5 Cholesterol biosynthesis, 291 7.5.1 Acetyl-CoA is the starting material for terpenoid as well as fatty acid synthesis, 291 7.5.2 Further metabolism generates the isoprene unit, 293 7.5.3 Higher terpenoids are formed by a series of condensations, 293 7.5.4 A separate way of forming the isoprene unit occurs in plants, 293 7.5.5 Sterol synthesis requires cyclization, 294 7.5.6 Cholesterol is an important metabolic intermediate, 294 7.5.7 It is important that cholesterol concentrations in plasma and tissues are regulated within certain limits and complex regulatory mechanisms have evolved, 296 7.6 Specific roles, 297 7.7 Pulmonary surfactant, 298 7.8 Lipid storage diseases (lipidoses), 300 7.9 The `phosphatidylinositol cycle' in cell signalling, 302 7.10 A wider range of lipid signalling molecules, 304 7.11 Phospholipase D in cell signalling, 305 7.12 Role of sphingolipids in cellular signalling, 307 7.13 Summary, 310 Further Reading, 312