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Chapter 27. Biomolecules: Lipids Why this Chapter? Lipids are the largest and most diverse class of biomolecules To examine lipid structure, function, and metabolism About Lipids Natural materials that preferentially extract into nonpolar organic solvents Includes fats, oils, waxes, some vitamins and hormones, some components of membrane General types: esters (“saponifiable”) and those that can’t be hydrolyzed 2 27.1 Waxes, Fats, and Oils Waxes - contain esters formed from long-chain (C16C36) carboxylic acids and long-chain alcohols (C24-C36) Triacontyl hexadecanoate is in beeswax 3 Triacylglycerol Tri-esters of glycerol with three longchain carboxylic acids, fatty acids. 4 Fatty Acids (from Fats and Oils) Straight-chain (C12 - C20) carboxylic acids Double bonds are cis-substituted but trans-fatty acids also occur A fat or oil in nature occurs as a mixture of many different triacylglycerols The most abundant saturated fatty acids are palmitic (C16) and stearic (C18) 5 Unsaturated and Polyunsaturated Fatty Acids Oleic (C18 with one C=C) and linolenic (C18 with 3 C=C) are the most abundant unsaturated 6 27.2 Soap A mixture of sodium or potassium salts of long-chain fatty acids produced by alkaline hydrolysis (saponification) of animal fat with alkali 7 Cleansing Action of Soap The carboxylate end of the long-chain molecule is ionic and therefore is preferentially dissolved in water The hydrocarbon tail is nonpolar and dissolves in grease and oil Soaps enable grease to be dissolved into water 8 Detergents “Hard” water contains Mg+2 and Ca+2 that form insoluble salts with soaps Synthetic detergents are alkylbenzene sulfonates that dissolve dirt like soaps but do not form scums with Mg+2 and Ca+2 . 9 27.3 Phospholipids Phospholipids are diesters of H3PO4, phosphoric acid Phosphoric acid can form monoesters, diesters and triesters In general these are known as “phosphates” 10 Phosphoglycerides Contain a glycerol backbone linked by ester bonds to two fatty acids and phosphoric acid Fatty acid residues with C12–C20 The phosphate group at C3 has an ester link to an amino alcohol 11 Sphingolipids The other major group of phospholipids Sphingosine or a dihydroxyamine backbone Constituents of plant and animal cell membranes Abundant in brain and nerve tissue, as coating around nerve fibers. 12 Phosphoglyceride Membranes Phosphoglycerides comprise the major lipid component of cell membranes Nonpolar tails aggregate in the center of a bilayer Ionic head is exposed to solvent 13 27.4 Prostaglandins & Other Eicosanoids C20 lipids that contain a fivemembered ring with two long side chains Present in small amounts in all body tissues and fluids Many physiological effects 14 Prostaglandin Sources Biosynthesized from arachidonic acid (C20 unsaturated fatty acid) 15 27.5 Terpenoids Steam distillation of plant extracts produces “essential oils” Chemically related to compounds in turpentine (from pine sap) called terpenes and thus called terpenoids Mostly hydrocarbons (some oxygens) that do not contain esters (stable to hydrolysis) 16 Biosynthesis of Terpenoids Isopentenyl pyrophosphate (IPP) forms higher isoprenoids in reactions catalyzed by prenyl transferase Monoterpenoids, diterpenoids, and tetraterpoids arise from 1-deoxyxylulose 5-phosphate (DXP). 17 Mevalonate Pathway to Isopentenyl Diphosphate Begins with the conversion of acetate to acetyl CoA followed by Claisen condensation to yield acetoacetyl CoA Catalyzed by acetoacetyl-CoA acetyltransferase 18 Aldol Condensation Carbonyl condensation reaction of acetoacetyl CoA with acetyl CoA Produces 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) 19 Reduction HMG CoA is reduced to mevalonate Catalyzed by HMG CoA reductase utilizing NADPH 20 Phosphorylation & Decarboxylation Pyrophosphorylation gives mevalonyl-PP Addition of phosphate from ATP followed by loss of CO2 and phosphate 21 Conversion of Isopentenyl Diphosphate to Terpenoids For triterpenes and larger, head-to-head coupling of farnesyl diphosphates gives squalene 22 Mechanism of Isomerization Isomerization of IPP to DMAPP is catalyzed by IPP isomerase through a carbocation pathway 23 Coupling Mechanism Nucleophilic substitution reaction in which the double bond of IPP behaves as a nucleophile in displacing diphosphate ion leaving group (PPO) 24 Conversions of Monoterpenoids Typically involves carbocation intermediates and multistep reaction pathways catalyzed by a terpene cyclase 25 27.6 Steroids Steroids,are another class of nonsaponifiable lipid, defined by structure Has four fused rings A, B, C, and D, beginning at the lower left Carbon atoms are numbered beginning in the A ring The six-membered rings are in fixed chair conformations 26 Functions of Steroids In humans as hormones, steroids are chemical messengers secreted by glands and carried through the bloodstream to target tissues Also widely distributed as cholesterol 27 Male Sex Hormones Testosterone and androsterone are the two most important male sex hormones, or androgens Androstanedione is a precursor 28 Female Sex Hormones Estrone and estradiol are the two most important female sex hormones, or estrogens Progesterone is the most important progestin, steroids that function in pregnancy 29 Adrenocortical Hormones Adrenocortical steroids: secreted by the adrenal glands near the upper end of each kidney Mineralocorticoids: control tissue swelling by regulating cellular salt balance Glucocorticoids: regulation of glucose metabolism and in the control of inflammation 30 Synthetic Steroids Made in pharmaceutical laboratories as new drugs Includes oral contraceptives and anabolic agents Methandrostenolone is an anabolic steroid used for tissue-building 31 27.7 Steroid Biosynthesis Enzyme-catalyzed addition of oxygen atom to squalene Stereospecific formation of an oxirane from an alkene 32 Which fatty acid is not commonly found in nature? 20% 1. x 2. x 3. x 4. None of these is commonly found in nature. All of these are commonly found in nature. 5. 1 20% 2 20% 20% 3 4 20% 5 Which of the following would not be classified as a lipid? 20% 1. 1 2. 2 3. 3 4. All of these are lipids. None of these are lipids. 5. 1 20% 2 20% 20% 3 4 20% 5 Which of the following describes how a soap cleans? 1. The fatty acids react with grease molecules to form water soluble compounds that are subsequently washed away. 2. The fatty acids form micelles which encapsulate grease molecules. These micelles are soluble in water and consequently washed away. 3. The fatty acids react with grease molecules to generate new compounds that can form micelles in water. These micelles are soluble in water and consequently washed away. 4. All of these statements accurately describe how soap works. 5. None of these statements accurately describe how soap works. 20% 1 20% 2 20% 20% 3 4 20% 5 The following molecule would be expected to form micelles. 50% 1. 2. 50% True False 1 2 What forces are responsible for lipid bilayer and micelle formation? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% covalent bonding and electrostatic interactions hydrophobic effects and hydrogen bonding electrostatic interactions and hydrophobic effects electrostatic interactions and fluid mosaics hydrophobic effects and fluid mosaics 1 2 5 The figure shown is: 20% 1. 2. 3. 4. 5. phosphatidylethanolamine phosphatidylcholine phosphatidylserine sphingosine sphingomyelin 1 20% 2 20% 20% 3 4 20% 5 What do these two lipids have in common? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% They are eicosanoids. They form micelles in water. They are prostaglandins. They are steroids. They are used as detergents. 1 2 5 Which is a role of the prostaglandins? 1. 2. 3. 4. 5. They stimulate the contraction of uterine muscle. They lower blood pressure in the blood stream. They suppress gastric ulceration. All of these None of these 20% 1 20% 2 20% 20% 3 4 20% 5 The molecule shown below is a: 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% thromboxane prostaglandin leukotriene eicosanoid thromboxane and eicosanoid 1 2 5 The molecule shown below is: 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% lanosterol camphor β-carotene patchouli alcohol None of these 1 2 5 The first step of mevalonate synthesis, shown below, is a: The 1st step of mevalonate synthesis, shown is a: 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% Claisen condensation aldol-like condensation reduction reaction substitution reaction Schiff base formation 1 2 5 Why can the molecule below not be classified as a steroid? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% Steroids cannot have double bonds. Steroids cannot contain branched hydrocarbon chains. Steroids have a different arrangement of rings. Steroids are usually comprised of benzene rings, making them mostly flat. All of these 1 2 5 How is cholesterol transported through the blood stream? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% through micelles through ion channels by prostaglandins by lipoproteins as water solvated ions 1 2 5 In which organ of the body is cholesterol synthesized? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% the lungs the gallbladder the liver the kidneys the pancreas 1 2 5 Which of the following is not a good indicator of health? 20% 1. 2. 3. 4. 5. 20% 20% 20% 3 4 20% cholesterol levels below 200 mg/dL LDL levels greater than 100 mg/dL HDL levels greater than 60 mg/dL active, athletic lifestyle diets rich in seafood 1 2 5