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Chapter 27
Organic Chemistry
6th Edition
Paula Yurkanis Bruice
The Organic
Chemistry of
Lipids
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Lipids are soluble in nonpolar organic compounds and
have a variety of structures and functions:
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Common Naturally Occurring Fatty Acids
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• Fatty acids, a major group of lipids, are carboxylic
acids with long hydrocarbon chains.
• Polyunsaturated fatty acids have more than one
double bond.
• The double bonds in naturally occurring fatty acids
have the cis configuration.
• Double bonds in naturally occurring unsaturated
fatty acids are separated by one methylene group.
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Unsaturated fatty acids have lower melting points than
saturated fatty acids
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Waxes Are High-Molecular-Weight Esters
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Fats and Oils
• The three OH groups of glycerol are esterified with
fatty acids.
• A simple triacylglycerol has three identical fatty
acid components.
• A mixed triacylglycerol has two or three different fatty
acid components.
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Fats are triacylglycerols existing as solid or semisolid
state at room temperature:
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Oils are liquid triacylglycerols:
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Some or all of the double bonds of polyunsaturated oils
can be reduced by catalytic hydrogenation (Section 4.12)
• Catalytic hydrogenation is used to convert oils to fats
(e.g., vegetable oil to margarine).
• This process also isomerizes double bonds from cis to
trans, resulting in unhealthy trans fats.
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Oxidation of Polyunsaturated
Fats and Oils by O2
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Phosphoacylglycerols
• They are the major components of cell membranes.
• The terminal OH group of glycerol is esterified with
phosphoric acid.
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• The most common phosphoacylglycerols in
membranes are phosphodiesters.
• The alcohols most commonly used to form the
second ester group are ethanolamine, choline, and
serine.
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Phosphoacylglycerols form membranes by arranging
themselves into a lipid bilayer:
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Sphingolipids
They contain an amino alcohol called sphingosine
instead of glycerol:
Sphingolipids are the major lipid
component in the myelin sheath, the
insulator of nerve cells axons
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Two of the Most Common Kinds
of Sphingolipids
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Prostaglandins
• Prostaglandins are synthesized from arachidonic acid.
• They are responsible for regulating a variety of
physiological responses.
• They are classified using the formula PGX, where X
designates the functional groups of the five-membered
ring.
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Examples of prostaglandins and physiological
processes they regulate:
• Blood pressure
• Clotting
• Fever
• Pain
• Labor
• Sleep–wake cycle
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• Prostaglandin endoperoxide synthase catalyzes the
conversion of arachidonic acid to PGH2.
• There are two forms of this enzyme: production under
normal physiological conditions and in response to
inflammation.
• The enzyme has a cyclooxygenase activity and a
hydroperoxidase activity.
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Aspirin (acetylsalicyclic acid) inhibits cyclooxygenase
activity:
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Other anti-inflammatory drugs also compete with
arachidonic acid or the peroxy radical for the enzyme’s
binding site:
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Terpenes
• Terpenes contain carbon atoms in multiples of five.
• They are made by joining five-carbon isoprene units.
• Oxygen-containing terpenes are sometimes called
terpenoids.
• They are a diverse class of lipids.
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The Isoprene Rule
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Recognizing the Isoprene
Subunit
• Zingiberene, oil of ginger
• Has three isoprene units
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Squalene, a triterpene, is a precursor of steroid
molecules:
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Lycopene and b-carotene are tetraterpenes called
carotenoids:
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Vitamin A Is a Terpene
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Isopentenyl pyrophosphate is the starting point for
biosynthesis of terpenes:
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The Conversion of Mevalonic Acid into
Mevalonyl Phosphate
an SN2 reaction
a second SN2
Review Sections 26.2 and 26.3
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Isoprene Biosynthesis: Claisen
Condensation
• Starred carbon can be 14C
• Claisen condensation joins
two acetates
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Isoprene Biosynthesis: Second
Claisen Condensation
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Isoprene Biosynthesis: Leaving
Group Elaboration
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Isoprene Biosynthesis:
Elimination and Isomerization
Isopentenyl
pyrophosphate
Dimethylallyl
pyrophosphate
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Both isopentyl pyrophosphate and dimethylallyl
pyrophosphate are needed for the biosynthesis of
terpenes
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Terpene Biosynthesis
Formation of geranyl pyrophosphate:
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Geranyl Pyrophosphate
Formation with 14C-Labels
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Many monoterpenes can be synthesized from geranyl
pyrophosphate:
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Formation of Farnesyl Pyrophosphate
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Formation of Squalene,
the Precursor of Cholesterol
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Steroids
• Steroids are nonpolar compounds and are lipids.
• Hormones are chemical messengers.
• Many hormones are steroids.
• Steroids contain a tetracyclic ring system.
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Structural Features of Steroids
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In steroids, the B, C, and D rings are trans fused:
Methyl groups at C-10 and C-13 are called angular
methyl groups
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The A and B rings are also trans fused in most naturally
occurring steroids:
• Substituents on the same side of the steroid ring
system as the angular methyl groups are
b-substituents.
• Those on the opposite side of the plane of the ring
system are a-substituents.
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Cholesterol Is a Steroid
• The most abundant member of the steroid family in
animals
• The precursor of all other steroids
• More rigid than other membrane lipids
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Examples of Steroids
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Examples of Synthetic Steroids
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