Chemistry 2223b Winter 2013–14 Western University
Topic 5: Lipids
This section discusses the properties of triglycerides and fatty acids, the metabolism
and biosynthesis of fatty acids and other lipids, soaps and detergents, and vitamins.
Background material that is important includes:
o Reactions of carboxylic acids and acid derivatives
o Reactions of carbonyl compounds
o Reactions of the -carbon
o Dehydration reactions of alcohols
o Substitution reactions
o Reactions of alkenes and aromatic compounds Lipids 2
A. Classification of Lipids
Lipids are a mixed collection of compounds that are water-insoluble, yet are soluble
in less-polar solvents such as dichloromethane, acetone, and diethyl ether.
Therefore, lipids encompass a large assortment of biomolecules. They are divided
into two general categories based on what they contain:
o A relatively large hydrophobic region and a small hydrophilic region
Triglycerides (fats and oils)
o A common structural element
Steroids and derivatives: fused tetracyclic system
Terpenes: isoprene building blocks Lipids 3
B. Fats, Oils, and Fatty Acids
1. Fats and Oils: Triesters of Glycerol
Fats are solids, while oils are liquids, at room temperature. Aside from this, both fats
and oils are triesters of glycerol. Scientists and the medical community refer to these
collectively as triglycerides (or triacylglycerides).
In the laboratory, triglycerides can be hydrolyzed under acidic or basic conditions
(review the hydrolysis of esters):
CH 2 C R CH 2H
O H or OH –
HC O C R HC OH
CH O C R CH OH
The R groups are long alkyl chains (same or different) that may contain sites of
unsaturation (C=C double bonds). These R groups are part of the fatty acids.
Fatty acids on their own (not esterified to glycerol) are not found in substantial
amounts in the body. Lipids 4
2. Properties of Triglycerides and Fatty Acids
The presence of unsaturation (especially cis C=C bonds) in one or more alkyl chains
lowers the melting point of the triglyceride by making the triglyceride harder to pack
together. Naturally occurring fatty acids are almost always cis and rarely trans.
Completely saturated Cis alkene makes big kink
compared to trans alkene
This is why highly saturated fats (such as butter) are solids at room temperature,
while highly unsaturated oils (such as olive oil) are liquids at room temperature. Lipids 5
The effect of unsaturation on melting point can be seen even in the individual fatty
Stearic acid (18:0) MP = 70°C
Some trans (18:1) MP = 52°C
Oleic acid (18:1) MP = 16°C
OH Linoleic acid (18:2) MP = –5°C
A monounsaturated fatty acid has one C=C
bond, while a polyunsaturated fatty acid has
two or more C=C bonds. (See food labels) Lipids 6
Fatty acids have both a hydrophilic (polar) head and a hydrophobic (non-polar) tail.
Because of the long, hydrophobic tails, fatty acids are practically insoluble in water
unless they are ionized (RCOO ). Although fatty acids are essentially carboxylic
acids, we don’t call the short ones, such as acetic acid, fatty acids.
The pK oa the carboxyl group is about 5, similar to that acetic acid. When fatty acids
are added to water, the majority of the molecules remain unionized. Only a tiny
amount is ionized because the ionization decreases the pH of the water, stopping
further ionization from taking place. Lipids 7
B. In-the-lab Preparation and Biosynthesis of Fatty Acids
In natural systems, fatty acids almost always have an even number of carbon
atoms, usually in the range of 12-24 and most commonly 16 or 18. Furthermore,
they are biosynthesized not as acids, but as esters.
This even-number phenomenon arises from an abundant carbon source used for
biosynthesis: a derivative of acetic acid. Long fatty acids are generated by joining
multiple acetyl units together, e.g. 18C fatty acid has nine2C units.
# new C-C bonds?
A crucial step in fatty-acid synthesis is the formation of a new C-C bond between
two acetate-ester units. How many C-C bond forming reactions do you know of?
o Cyanide as a nucleophile adds a single carbon (as a cyano group)
o Grignard reagent adds an alkyl group
o Aldol condensation a Nu addition reaction of aldehydes and ketones
o Claisen condensation a Nu acyl substitution reaction using carbanions
We’ll first determine how we could hypothetically synthesize a fatty acid in the lab
and then see how these in-the-lab concepts could also be extended to biosynthesis. Lipids 8
1. Hypothetical In-the-lab Synthesis of Fatty Acids
The key step is the formation of a new C-C bond by joining two acetate esters
together via a Claisen condensation. This is a nucleophilic acyl substitution where
the enolate of one ester acts as a carbon nucleophile, replacing the OR group.
O Na OR (strong base) O O
H 3 OR H3C OR
Very good nucleophile
Was electrophilic Was nucleophilic Lipids 9
A series of chemical conversions takes us to a saturated chain.
O O multiple steps O
Add another C2unit and
do the series of reduction,
oxidation, and reduction
reactions to a give C ester
6 Lipids 10
The four types of reactions that must occur biologically are exactly the same as
those required in the lab synthesis. However, the biosynthesis cannot be identical to
the laboratory synthesis, because:
o NaBH , 4onc H SO ,2and4H /Pt do2’t exist biologically;
o Claisen reactions also have very small equilibrium constants; and
In the lab, we can make the products more favourable by increasing the
amount of enolate; this can be done by using stoichiometric amounts of
strong base. However, this is difficult to do at physiological pH because the
-H is not very acidic.
Biological solution: perform a “modified” Claisen-type condensation using a
more-reactive acid derivative and carbanion equivalents.
o We’re adding acetate units one at a time, and we need a way to prevent the
growing chain from diffusing away until the desired length is reached. Lipids 11
2. Biosynthesis of Fatty Acids
a. Modified Claisen condensation
A modified Clasien condensation is used to make the reaction more favourable
under physiological conditions.
Modification #1: use thioesters instead of oxygen esters
Thioesters are more electrophilic than oxygen esters.
RS is a better leaving group than is RO . (The pKa of a thiol is about 8, while that of
an alcohol is about 16.)
O O O
SR SR Lipids 12
Modification #2: use carbanion equivalents
We rely on the decarboxylation of malonyl thioester to generate the enolate and
drive the Claisen reaction to completion in the absence of a strong base.
O O –CO 2 O O
O SR SR SR
malonyl thioester enolate of acetyl thioester
Although we have made the enolate without the use of a base,2the loss of CO is
concomitant with nucleophilic attack. In other words, we avoid the enolate
completely. i.e. malonyl thioester is the carbanion equivalent.
O acetyl2C O O O O
SR O O SR SR
O SR C4-keto
malonyl3C Lipids 13
Malonyl thioester is made from acetyl thioester using 2TP and biotin, a CO carrier.
This enzymatic reaction 2ses CO dissolved as bicarbonate. (Note: This is the same
CO2that is lost in the modified Claisen reaction).
O Biotin O O
SR ATP ADP, Pi O SR
o ATP first phosphorylat3sto form carboxyphosphate (mechanism is
similar to other phosphorylations), which is then used to form carboxybiotin.
O P O
HO O P O
O O O
N NH HO N NH
R R R R Lipids 14
o The carboxyl group is subsequently transferred to acetyl CoA.
HO N NH HN NH
R R R R
O O O
SR HO SR Lipids 15
In one cycle, acetyl-CoA grows to a four-carbon acyl chain:
(made from acetyl thioester)
Once at the butyryl thioester,
use it instead of acetyl thioester O O
for the next cycle. And repeat...
O OH O
unsaturated thioester -hydroxythioester Lipids 16
b. The diffusion problem
Fatty acids are typically synthesized16molecule (additional modifications are
possible, such as extension 18 C and other reactions). Because the fatty acid
grows 2 C at a time, it must not be allowed to diffuse away until16t reaches C .
The reaction involves the use of an acyl carrier protein (ACP) with a long “arm”
made from pantothenic acid and other components, and with a thiol group.
O OH C CH O C i
3 3 O p
HS O O CH p n
N N P l a
H H NH p c
from pantothenic acid
The reactions are done on a multi-enzyme complex, fatty acid synthase (FAS),
which also contains a thiol group. ACP is strongly bound to FAS, so the growing
chain does not float away until it is deliberately detached.
Diagrammatically, we have…. Lipids 17
O O C
H 3 SCoA The acetyl group is loaded onto
- HS the fatty acid synthase
O CH2 SCoA
P FAS complex
The malonyl-ACP component is
bound to an ACP-binding site
O Both reactions are catalyzed by
O O H3C S enzymes with transacylase
O CH2 S C
The long arm of ACP permits
movement for the Claisen-
type condensation reaction
- H3C C and allows the product to
O access other units of FAS for
O CH 2 subsequent transformations
S Lipids 18
O O HS
An enzyme catalyzes the
H3C CH2 S P FAS Claisen-type condensation
Reduction: NADPH + H
The reduction is
OH O HS enzymatically performed
H3C CH2 S P FAS
Dehydration is also
O HS enzymatic
H C S P FAS
-unsaturated thioester Lipids 19
H 3 S C of C=C bond to give
A a fully reduced tail
H C S
3 growing chain moves
HS FAS to spot of original
C electrophile for next
ACP controls reaction cycle
add new malonyl
thioester Nu S Ready for next cycle
O O FAS
O CH2 S
Same as first step Lipids 20
Note that the growing fatty acid is always attached to either ACP, which is in turn
attached to FAS, or directly to one of the enzymes in the FAS complex.
o The electrophile is always the growing chain, and it grows at the thioester end.
o The nucleophile is always malonyl thioester, regardless of the length of the
growing chain. Remember that the use of malonyl thioes3er, a C compound,
only adds an acetate uni2 (C ), because one carbon is l2st as CO .
o There are two reductions and one dehydration per cycle.
After seven cycles, we will have made seven new C-C bonds a16 a C fatty acid.
7 new C-C bonds O
C C C C C C C C
C C C C C C C C SR
acetyl malonyl-derived2C units
Because the malonyl-derived units are made from acetyl thioe2ter + CO , and the
CO 2s later lost, we can say 16e C compound originates from eight acetyl units. Lipids 21
c. Reduction and dehydration reagents
How do we perform the reduction, dehydration, and reduction reactions?
1. Reduction of ketone to alcohol by NADPH
O O OH O
2. Dehydration of alcohol to alkene
OH O O
-hydroxythioester -unsaturated thioester
Compare this reaction to the formation of phosphoenolpyruvate in glycolysis. Lipids 22
3. Reduction of alkene to alkane by NADPH
-unsaturated thioester saturated tail
Alkenes normally cannot be reduced by hydride reagents (Na4H , NADH, NADPH,
etc.) because hydrides do not attack nonpolar C=C bonds. However, it works in this
case because the alkene is conjugated to the C=O. A resonance structure shows
that the -carbon has positive character and is therefore electrophilic.
Polarized to be more
O because of resonanceO
Reaction with NADPH would generate the enol form, which tautomerizes to the
product. The addition of a nucleophile to the carbon of an ,-unsaturated
carbonyl is termed conjugate addition and has the name Michael Addition. Lipids 23
d. Other fatty acids
The normal product of fatty-acid biosynthesis in animals is palmitic acid (16).
Separate elongase and desaturase enzymes are responsible for elongating this fatty
acid and producing monounsaturated fatty acids, respectively. Desaturase enzymes
are remarkable in that they introduce a cis alkene using atmospheric oxygen as the
oxidizing agent. (We won’t look at these details).
Palmitic acid (C16 O
Stearic acid (C18 Oleic acid C 18 9
We can’t desaturate any further than delta 9 Lipids 24
There are some polyunsaturated fatty acids (PUFA’s) that humans cannot
synthesize and must obtain in their diet. These two are the “parent” PUFA’s, as they
can be converted to other necessary PUFA’s by the liver when consumed.
They have desaturation further than 9
Linoleic acid 189,12
An omega-6 fatty acid, which has a double
bond six carbons from the omega end (methyl)
Alpha linolenic acid (ALA) 18
An omega-3 fatty acid, which has a double
bond three carbons from the omega end (methyl) Lipids 25
C. Catabolism of Triacylglycerides and Fatty Acids
1. Hydrolysis of Triacylglycerides
Triacylglycerides are poorly absorbed from the digestive tract, but the hydrolysis
products can be absorbed. Hydrolysis is performed by lipase enzymes.
Lipases use the same catalytic triad that is found in chymotrypsin (Asp-His-Ser), and
may act on one, two, or all three of the esters. The reaction for the hydrolysis of all
three esters is shown below.
CH2O C R CH2OH
HC O C R 3 H2O HC OH 3 RCOOH
CH2O C R CH2OH Lipids 26
2. Phosphorylation and Oxidation of Glycerol
Glycerol, a three-carbon compound, is readily converted into a glycolysis
intermediate via a phosphorylation followed by an oxidation.
CH OH CH OH CH OH
2 ATP 2 NAD + 2
H OH H OH O
CH 2H CH 2PO 32- CH 2PO 32-
3. -Oxidation of Fatty Acids
Fatty acids are degraded into acetyl CoA via a pathway known as -oxidation,
where the carbon is oxidized from an alkyl carbon to a ketone. The pathway is
quite similar to the reverse of fatty-acid biosynthesis, but with some differences. Lipids 27
First, the fatty acid is enzymatically converted to the CoA thioester.
HS-CoA If we start wi18 a C
O fatty acid, we will obtain
9 acetyl CoA. Where are
the 9 units, and which
SCoA carbons need to be
oxidized? Lipids 28
FAD then oxidizes the saturated tail, resulting in an ,-unsaturated thioester. (Note
that in biosynthesis, NADPH was used to reduce the C=C).
N N O
N N O
R H Lipids 29
Conjugate addition (Michael addition) of water to the ,-unsaturated thioester forms
a -hydroxy thioester, which is then oxidized by NAD to give a -keto thioester.
These two steps are mechanistically opposite to those in biosynthesis.
SCoA Lipids 30
The last step is a C-C bond-breaking reaction that uses cysteine residue to expel
acetyl CoA as a leaving group. The enolate (-carbanion) of acetyl CoA is
protonated as it departs (compare this to a retro-aldol reaction).
O and repeat....
SCoA oxidation, cleavage Lipids 31
Summary up to now
Textbook Chapter 19:
o Pages covered: 649–653
o Suggested Quick Quiz questions: 2, 5, 10, 14, 16
o Suggested problems: 3, 4, 5, 6, 9, 10, 12
Textbook Chapter 21:
o Pages covered: 704–706, 713–716
o Suggested Quick Quiz questions: 1, 5, 9, 10
o Suggested problems: 18, 20
o Practice problems: 1–21
o 2009 Final Exam: 20–26
o Suggestions for the 2011 Final Exam are not included – save that exam to
evaluate your performance when preparing for the Final Exam. Lipids 32
D. Soaps and Detergents
Both soaps and detergents perform the same task, but soaps are made from
naturally occurring materials, while detergents are considered to be synthetic.
Soaps are metal salts of fatty acids, prepared by the base hydrolysis of triglycerides.
CH O C R CH OH
O NaOH O
CH O C R HC OH 3R C O Na
CH2O C R CH 2H
The sodium salts are soluble at low concentrations. However, at very high [Na ], the
equilibrium below is forced to the left, causing the salt to precipitate (this is useful for
isolating and purifying soap).
R C O Na R C O Na Lipids 33
Individual soap molecules aggregate to form spherical structures known as micelles.
These structures form because the hydrophobic tails are attracted to each other via
However, the tails do not pack together tightly. This is because the heads (which
also electrostatically repel each other if they have a net charge) are solvated and
require a certain amount of space. Micelles are therefore leaky; molecules can enter
and exit the micelle. Yet, as a cleaning agent, this is essential so that hydrophobic
grease and dirt can enter into the hydrophobic interior. Lipids 34
Soaps are easily made from natural sources, but there is one major problem with
them. The Na and K salts of carboxylates are q2+te s2+ubl3+ but the carboxylates
precipitate in the presence of polyvalent ions (Ca , Mg , Fe , etc.).
R C O M2+
These polyvalent ions are those found in hard water. Soaps therefore precipitate out
and perform poorly in hard water.
One solution is to use synthetic detergents that do not precipitate out in the
presence of polyvalent ions. These detergents contain a hydrophilic head that is not
a carboxylate group.
Many different detergents are commercially available, and they can be categorized
according to the nature of the hydrophilic head. Lipids 35
One example would be the first detergents developed in the 1940s, and these are
the alkylbenzene sulfonate