Chapter 27' Biomolecules: Lipids

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Chapter 27. Biomolecules Lipids
Based on McMurrys Organic Chemistry, 7th edition
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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 cant be hydrolyzed

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Why this Chapter?

• Lipids are the largest and most diverse class of
  biomolecules
• To examine lipid structure, function, and
  metabolism

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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

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Triacylglycerol

• Tri-esters of glycerol with three long-chain
  carboxylic acids, fatty acids.

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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)

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Unsaturated and Polyunsaturated Fatty Acids

• Oleic (C18 with one CC) and linolenic (C18 with
  3 CC) are the most abundant unsaturated

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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

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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

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Detergents

• Hard water contains Mg2 and Ca2 that form
  insoluble salts with soaps
• Synthetic detergents are alkylbenzene sulfonates
  that dissolve dirt like soaps but do not form
  scums with Mg2 and Ca2 .

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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

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Phosphoglycerides

• Contain a glycerol backbone linked by ester bonds
  to two fatty acids and phosphoric acid
• Fatty acid residues with C12C20
• The phosphate group at C3 has an ester link to an
  amino alcohol

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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.

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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

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27.4 Prostaglandins and Other Eicosanoids

• 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

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Prostaglandin Sources

• Biosynthesized from arachidonic acid (C20
  unsaturated fatty acid)

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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)

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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).

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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

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Aldol Condensation

• Carbonyl condensation reaction of acetoacetyl CoA
  with acetyl CoA
• Produces 3-hydroxy-3-methylglutaryl CoA (HMG-CoA)

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Reduction

• HMG CoA is reduced to mevalonate
• Catalyzed by HMG CoA reductase utilizing NADPH

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Phosphorylation and Decarboxylation

• Pyrophosphorylation gives mevalonyl-PP
• Addition of phosphate from ATP followed by loss
  of CO2 and phosphate

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Conversion of Isopentenyl Diphosphate to
Terpenoids

• For triterpenes and larger, head-to-head coupling
  of farnesyl diphosphates gives squalene

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Mechanism of Isomerization

• Isomerization of IPP to DMAPP is catalyzed by IPP
  isomerase through a carbocation pathway

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Coupling Mechanism

• Nucleophilic substitution reaction in which the
  double bond of IPP behaves as a nucleophile in
  displacing diphosphate ion leaving group (PPO?)

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Conversions of Monoterpenoids

• Typically involves carbocation intermediates and
  multistep reaction pathways catalyzed by a
  terpene cyclase

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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

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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

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Male Sex Hormones

• Testosterone and androsterone are the two most
  important male sex hormones, or androgens
• Androstanedione is a precursor

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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

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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

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Synthetic Steroids

• Made in pharmaceutical laboratories as new drugs
• Includes oral contraceptives and anabolic agents
• Methandrostenolone is an anabolic steroid used
  for tissue-building

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27.7 Steroid Biosynthesis

• Enzyme-catalyzed addition of oxygen atom to
  squalene
• Stereospecific formation of an oxirane from an
  alkene