Chapter 27. Chapter 27. Biomolecules: LipidsBiomolecules: Lipids

Why this Chapter?Why this Chapter?

Lipids are the largest and most diverse class of biomolecules

To examine lipid structure, function, and metabolism

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About LipidsAbout 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

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27.1 Waxes, Fats, and Oils27.1 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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TriacylglycerolTriacylglycerolTri-esters of glycerol with three

long-chain carboxylic acids, fatty acids.

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Fatty Acids (from Fats and Oils)Fatty Acids (from Fats and Oils)

Straight-chain (C12 - C20) carboxylic acidsDouble bonds are cis-substituted but trans-

fatty acids also occurA fat or oil in nature occurs as a mixture of

many different triacylglycerolsThe most abundant saturated fatty acids are

palmitic (C16) and stearic (C18)

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Unsaturated and Polyunsaturated Unsaturated and Polyunsaturated Fatty AcidsFatty Acids

Oleic (C18 with one C=C) and linolenic (C18 with 3 C=C) are the most abundant unsaturated

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27.2 Soap27.2 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 SoapCleansing 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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DetergentsDetergents

“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 .

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27.3 Phospholipids27.3 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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PhosphoglyceridesPhosphoglyceridesContain a glycerol backbone linked by ester

bonds to two fatty acids and phosphoric acidFatty acid residues with C12–C20The phosphate group at C3 has an ester link to

an amino alcohol

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SphingolipidsSphingolipidsThe other major group of phospholipids Sphingosine or a dihydroxyamine backbone Constituents of plant and animal cell membranesAbundant in brain and nerve tissue, as coating

around nerve fibers.

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Phosphoglyceride MembranesPhosphoglyceride MembranesPhosphoglycerides comprise the major lipid

component of cell membranesNonpolar tails aggregate in the center of a bilayer Ionic head is exposed to solvent

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27.4 Prostaglandins & Other Eicosanoids27.4 Prostaglandins & 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 SourcesProstaglandin SourcesBiosynthesized from arachidonic acid (C20

unsaturated fatty acid)

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27.5 Terpenoids27.5 TerpenoidsSteam 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 TerpenoidsBiosynthesis 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 Mevalonate Pathway to Isopentenyl DiphosphateIsopentenyl 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 CondensationAldol Condensation Carbonyl condensation reaction of acetoacetyl CoA with

acetyl CoA Produces 3-hydroxy-3-methylglutaryl CoA (HMG-CoA)

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ReductionReductionHMG CoA is reduced to mevalonateCatalyzed by HMG CoA reductase utilizing NADPH

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Phosphorylation & DecarboxylationPhosphorylation & DecarboxylationPyrophosphorylation gives mevalonyl-PPAddition of phosphate from ATP followed by

loss of CO2 and phosphate

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Conversion of Isopentenyl Diphosphate Conversion of Isopentenyl Diphosphate to Terpenoidsto Terpenoids

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

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Mechanism of IsomerizationMechanism of Isomerization

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

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Coupling MechanismCoupling MechanismNucleophilic 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 MonoterpenoidsConversions of MonoterpenoidsTypically involves carbocation intermediates

and multistep reaction pathways catalyzed by a terpene cyclase

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27.6 Steroids27.6 SteroidsSteroids,are another class of nonsaponifiable lipid,

defined by structureHas four fused rings A, B, C, and D, beginning at the

lower leftCarbon atoms are numbered beginning in the A ringThe six-membered rings are in fixed chair

conformations

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Functions of SteroidsFunctions 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 HormonesMale Sex HormonesTestosterone and androsterone are the two most

important male sex hormones, or androgensAndrostanedione is a precursor

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Female Sex HormonesFemale Sex HormonesEstrone and estradiol are the two most

important female sex hormones, or estrogensProgesterone is the most important progestin,

steroids that function in pregnancy

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Adrenocortical HormonesAdrenocortical HormonesAdrenocortical steroids: secreted by the adrenal

glands near the upper end of each kidneyMineralocorticoids: control tissue swelling by

regulating cellular salt balance Glucocorticoids: regulation of glucose

metabolism and in the control of inflammation

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Synthetic SteroidsSynthetic SteroidsMade in pharmaceutical laboratories as new

drugs Includes oral contraceptives and anabolic agentsMethandrostenolone is an anabolic steroid used

for tissue-building

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27.7 Steroid Biosynthesis27.7 Steroid Biosynthesis

Enzyme-catalyzed addition of oxygen atom to squalene

Stereospecific formation of an oxirane from an alkene

1. x

2. x

3. x4. None of these is

commonly found in nature.

5. All of these are commonly found in nature.

Which fatty acid is Which fatty acid is notnot commonly commonly found in nature?found in nature?

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1. 1

2. 2

3. 34. All of these are

lipids. 5. None of these are

lipids.

Which of the following would not be Which of the following would not be classified as a lipid?classified as a lipid?

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Which of the following describes how a soap Which of the following describes how a soap cleans?cleans?

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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.

The following molecule would The following molecule would be expected to form micelles.be expected to form micelles.

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1. True2. False

What forces are responsible for lipid What forces are responsible for lipid bilayer and micelle formation?bilayer and micelle formation?

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1. covalent bonding and electrostatic interactions

2. hydrophobic effects and hydrogen bonding

3. electrostatic interactions and hydrophobic effects

4. electrostatic interactions and fluid mosaics

5. hydrophobic effects and fluid mosaics

The figure shown is:The figure shown is:

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1. phosphatidylethanolamine

2. phosphatidylcholine3. phosphatidylserine4. sphingosine5. sphingomyelin

What do these two lipids have in What do these two lipids have in common?common?

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1. They are eicosanoids.

2. They form micelles in water.

3. They are prostaglandins.

4. They are steroids.5. They are used as

detergents.

Which is a role of the Which is a role of the prostaglandins?prostaglandins?

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20% 20% 20%20%20%1. They stimulate the contraction of uterine muscle.

2. They lower blood pressure in the blood stream.

3. They suppress gastric ulceration.

4. All of these5. None of these

The molecule shown below is a:The molecule shown below is a:

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1. thromboxane2. prostaglandin3. leukotriene4. eicosanoid5. thromboxane and

eicosanoid

The molecule shown below is:The molecule shown below is:

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1. lanosterol2. camphor3. β-carotene4. patchouli alcohol5. None of these

The first step of mevalonate The first step of mevalonate synthesis, shown below, is a:synthesis, shown below, is a:

The 1The 1stst step of mevalonate synthesis, step of mevalonate synthesis, shown is a:shown is a:

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1. Claisen condensation2. aldol-like

condensation3. reduction reaction4. substitution reaction5. Schiff base formation

Why can the molecule below Why can the molecule below notnot be be classified as a steroid?classified as a steroid?

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1. Steroids cannot have double bonds.

2. Steroids cannot contain branched hydrocarbon chains.

3. Steroids have a different arrangement of rings.

4. Steroids are usually comprised of benzene rings, making them mostly flat.

5. All of these

How is cholesterol transported How is cholesterol transported through the blood stream?through the blood stream?

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1. through micelles2. through ion

channels3. by prostaglandins4. by lipoproteins5. as water solvated

ions

In which organ of the body is In which organ of the body is cholesterol synthesized?cholesterol synthesized?

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1. the lungs2. the gallbladder3. the liver4. the kidneys5. the pancreas

Which of the following is Which of the following is notnot a a good indicator of health?good indicator of health?

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1. cholesterol levels below 200 mg/dL

2. LDL levels greater than 100 mg/dL

3. HDL levels greater than 60 mg/dL

4. active, athletic lifestyle

5. diets rich in seafood