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Chapter 27. Biomolecules: Lipids. Based on McMurry’s Organic Chemistry, 6th edition. About Lipids. Naturally materials that preferentially extract into nonpolar organic solvents Includes fats, oils, waxes, some vitamins and hormones, some components of membrane

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chapter 27 biomolecules lipids

Chapter 27. Biomolecules: Lipids

Based on McMurry’s Organic Chemistry, 6th edition

about lipids
About Lipids
  • Naturally 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
27 1 waxes fats and oils
27.1 Waxes, Fats, and Oils
  • Waxes - contain esters formed from long-chain (C16-C36) carboxylic acids and long-chain alcohols (C24-C36)
  • Triacontyl hexadecanoate is in beeswax
fats and oils
Fats and Oils
  • Animal fats and vegetable are triacylglycerols oil
    • Triesters of glycerol with three long-chain carboxylic acids
  • Saponification of a fat or oil with (reaction with aqueous NaOH) yields glycerol and three fatty acids
fatty acids from fats and oils
Fatty Acids (from Fats and Oils)
  • Straight-chain (C12 - C20) carboxylic acids
  • Double bonds are cis-substituted but trans-fatty acid 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)
unsaturated and polyunsaturated fatty acids
Unsaturated and Polyunsaturated Fatty Acids
  • Oleic (C18 with one C=C) and linolenic (C18 with 3 C=C) are the most abundant unsaturated
27 2 soap
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
cleansing action of soap
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
detergents
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 .
27 3 phospholipids
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”
phosphoglycerides
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
phosphoglyceride membranes
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
sphingolipids
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.
27 4 prostaglandins
27.4 Prostaglandins
  • C20 lipids that contain a five-membered ring with two long side chains
  • Present in small amounts in all body tissues and fluids
  • Many physiological effects
prostaglandin sources
Prostaglandin Sources
  • Biosynthesized from arachidonic acid (C20 unsaturated fatty acid)
  • Catalyzed by cyclooxygenase (COX)
27 5 terpenoids
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)
terpenoids are isoprenoids
Terpenoids Are Isoprenoids
  • Terpenoid structures are derived from isoprene (2-methyl-1,3-butadiene) units connected head-to-tail (Isoprene Rule)
  • The term “head-to-tail” means that dissimilar ends are connected (you can choose which is a “head” and which is a “tail”)
classification of terpenoids
Classification of Terpenoids
  • Classified according to the number of di-isoprenoid (C10) units they contain
  • Monoterpenes have 10-carbons, sesquiterpenes (sesqui = 1 1/2) have 15-carbons from three isoprene units, and so on
some important terpenoids
Some Important Terpenoids
  • The triterpene lanosterol is the precursor of steroid hormones
  • The tetraterpene b-carotene is a source of vitamin A
27 6 biosynthesis of terpenoids
27.6 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.
mevalonate pathway to isopentenyl diphosphate
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
aldol condensation produces hmg coa
Aldol Condensation Produces HMG-CoA
  • Carbonyl condensation reaction of acetoacetyl CoA with acetyl CoA
  • Produces 3-hydroxy-3-methylglutaryl CoA (HMG-CoA)
reduction forms mevalonate
Reduction Forms Mevalonate
  • HMG CoA is reduced to mevalonate
  • Catalyzed by HMG CoA reductase utilizing NADPH
conversion of mevalonate to ipp
Conversion of Mevalonate to IPP
  • Pyrophosphorylation gives mevalonyl-PP
  • Addition of phosphate from ATP followed by loss of CO2 and phosphate
conversion of isopentenyl diphosphate to terpenoids
Conversion of Isopentenyl Diphosphate to Terpenoids
  • For triterpenes and larger, head-to-head coupling of farnesyl diphosphates gives squalene
mechanism of isomerization
Mechanism of Isomerization
  • Isomerization of IPP to DMAPP is catalyzed by IPP isomerase through a carbocation pathway
coupling mechanism
Coupling Mechanism
  • Nucleophilic substitution reaction in which the double bond of IPP behaves as a nucleophile in displacing diphosphate ion leaving group (PPO)
conversions of monoterpenoids
Conversions of Monoterpenoids
  • Typically involves carbocation intermediates and multistep reaction pathways catalyzed by a terpene cyclase
27 7 steroids
27.7 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
functions of steroids
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
male sex hormones
Male Sex Hormones
  • Testosterone and androsterone are the two most important male sex hormones, or androgens
  • Androstanedione is a precursor
female sex hormones
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
adrenocortical hormones
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
synthetic steroids
Synthetic Steroids
  • Made in pharmaceutical laboratories as new drugs
  • Includes oral contraceptives and anabolic agents
  • Methandrostenolone is an anabolic steroid used for tissue-building
27 8 stereochemistry of steroids
27.8 Stereochemistry of Steroids
  • Two cyclohexane rings can be joined in either a cis or a trans manner
  • In cis-decalin, both groups at the ring-junction positions are on the same side of the two rings
  • In trans-decalin, the groups at the ring junctions are on opposite sides
a b junction in steroids
A/B Junction in Steroids
  • Steroids can have either a cis or a trans fusion of the A and B ring
  • (B–C and C–D) are usually trans
27 9 steroid biosynthesis
27.9 Steroid Biosynthesis
  • Enzyme-catalyzed addition of oxygen atom to squalene
  • Stereospecific formation of an oxirane from an alkene
cyclization to steroid
Cyclization to Steroid
  • Stork-Eschenmoser mechanism
  • Enzyme-catalyzed
  • See Figure 27.6