John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 27
Biomolecules: Lipids
About Lipids
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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
Why this Chapter?
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Lipids are the largest and most diverse class of
biomolecules
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To examine lipid structure, function, and
metabolism
27.1 Waxes, Fats, and Oils
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Waxes - contain esters formed from long-chain
(C16-C36) carboxylic acids and long-chain
alcohols (C24-C36)
Triacontyl hexadecanoate is in beeswax
Triacylglycerol
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Tri-esters of glycerol with three long-chain
carboxylic acids, fatty acids.
Fatty Acids (from Fats and
Oils)
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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)
Unsaturated and Polyunsaturated
Fatty Acids
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Oleic (C18 with one C=C) and linolenic (C18 with 3 C=C)
are the most abundant unsaturated
27.2 Soap
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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
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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
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“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
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Phospholipids are diesters of H3PO4, phosphoric acid
Phosphoric acid can form monoesters, diesters and
triesters
In general these are known as “phosphates”
Phosphoglycerides
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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
Sphingolipids
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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.
Phosphoglyceride Membranes
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Phosphoglycerides comprise the major lipid component of
cell membranes
Nonpolar tails aggregate in the center of a bilayer
Ionic head is exposed to solvent
27.4 Prostaglandins and Other
Eicosanoids
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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
Prostaglandin Sources
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Biosynthesized from arachidonic acid (C20
unsaturated fatty acid)
27.5 Terpenoids
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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)
Biosynthesis of Terpenoids
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Isopentenyl pyrophosphate (IPP) forms higher isoprenoids
in reactions catalyzed by prenyl transferase
Monoterpenoids, diterpenoids, and tetraterpinoids arise
from 1-deoxyxylulose 5-phosphate (DXP).
Mevalonate Pathway to
Isopentenyl Diphosphate
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Begins with the conversion of acetate to acetyl
CoA followed by Claisen condensation to yield
acetoacetyl CoA
Catalyzed by acetoacetyl-CoA acetyltransferase
Aldol Condensation
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Carbonyl condensation reaction of acetoacetyl CoA with
acetyl CoA
Produces 3-hydroxy-3-methylglutaryl CoA (HMG-CoA)
Reduction
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HMG CoA is reduced to mevalonate
Catalyzed by HMG CoA reductase utilizing NADPH
Phosphorylation and
Decarboxylation
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Pyrophosphorylation gives mevalonyl-PP
Addition of phosphate from ATP followed by loss
of CO2 and phosphate
Conversion of Isopentenyl
Diphosphate to Terpenoids
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For triterpenes and larger, head-to-head coupling of
farnesyl diphosphates gives squalene
Mechanism of Isomerization
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Isomerization of IPP to DMAPP is catalyzed by
IPP isomerase through a carbocation pathway
Coupling Mechanism
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Nucleophilic substitution reaction in which the
double bond of IPP behaves as a nucleophile in
displacing diphosphate ion leaving group (PPO-)
Conversions of
Monoterpenoids
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Typically involves carbocation intermediates and multistep
reaction pathways catalyzed by a terpene cyclase
27.6 Steroids
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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
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In humans as hormones, steroids are chemical
messengers secreted by glands and carried
through the bloodstream to target tissues
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Also widely distributed as cholesterol
Male Sex Hormones
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Testosterone and androsterone are the two most
important male sex hormones, or androgens
Androstanedione is a precursor
Female Sex Hormones
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Estrone and estradiol are the two most important
female sex hormones, or estrogens
Progesterone is the most important progestin,
steroids that function during pregnancy
Adrenocortical Hormones
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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
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Made in pharmaceutical laboratories as new
drugs
Includes oral contraceptives and anabolic agents
Methandrostenolone is an anabolic steroid used
for tissue-building
27.7 Biosynthesis of Steroids
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Enzyme-catalyzed addition of oxygen atom to
squalene
Stereospecific formation of an oxirane from an
alkene
Let’s Work a Problem
The plasmologens are a group of lipids found in
nerve and muscle cells. How do plasmalogens
differ from fats?
Answer
This problem asks you to recall the different lipid
structural features. The plasmologens differ from
the fats in that, in plasmalogens, the C3 is a vinyl
ether as opposed to a fat, which has a carboxylic
acid ester at the corresponding position