Chapter 26
Lipids
Lipids
Lipids are naturally occurring substances
grouped together on the basis of a common
property—they are more soluble in nonpolar
solvents than in water.
Some of the most important of them—the ones
in this chapter—are related in that they have
acetic acid (acetate) as their biosynthetic origin.
In many biosynthetic pathways a substance
called acetyl coenzyme A serves as the source
of acetate.
26.1
Acetyl Coenzyme A
Structure of Coenzyme A
O O
HO
P
O
HO P
HO
P
O
O
O
HO
O
N
OH
OH
H
H
N
N
O
SR
H3C
CH3 O
O
N
NH2
N
N
R = H; Coenzyme A
O
R = CCH3; Acetyl coenzyme A
Reactivity of Coenzyme A
Nucleophilic acyl substitution
O
CH3CSCoA
HY ••
O
CH3C
Y •• + HSCoA
Acetyl coenzyme A is a source of an acetyl
group toward biological nucleophiles; it is an
acetyl transfer agent.
Reactivity of Coenzyme A
can react via enol
O
OH
H2C
CH3CSCoA
CSCoA
E+
Acetyl coenzyme A reacts
with biological
electrophiles at its a
carbon atom.
O
E
CH2CSCoA
26.2
Fats, Oils, and Fatty Acids
Fats and Oils
O
O CH2OCR'
RCOCH
CH2OCR"
O
Fats and oils are naturally occurring mixture of
triacylglycerols (also called triglycerides).
Fats are solids; oils are liquids.
Fats and Oils
O
O CH2OC(CH2)16CH3
CH3(CH2)16COCH
CH2OC(CH2)16CH3
O
Tristearin; mp 72°C
Fats and Oils
O
O CH2OC(CH2)16CH3
CH3(CH2)6CH2
C
H
CH2(CH2)6COCH
C
CH2OC(CH2)16CH3
H
O
2-Oleyl-1,3-distearylglycerol; mp 43°C
Fats and Oils
2-Oleyl-1,3-distearylglycerol
mp 43°C
H2, Pt
Tristearin
mp 72°C
Fatty Acids
O
O
O CH2OCR
O
H2O
R'COCH
CH2OCR"
R'COH
CH2OH
HOCR
HOCH
CH2OH
HOCR"
O
O
Acids obtained by the hydrolysis of fats and oils are called
fatty acids.
Fatty acids usually have an unbranched chain with an even
number of carbon atoms.
If double bonds are present, they are almost always cis.
Table 26.1 (p 1018)
Systematic name Common name
O
CH3(CH2)10COH
Dodecanoic acid
Lauric acid
Tetradecanoic acid
Myristic acid
O
CH3(CH2)12COH
O
CH3(CH2)14COH
Hexadecanoic acid Palmitic acid
Table 26.1 (p 1018)
Systematic name Common name
O
CH3(CH2)16COH
Octadecanoic acid
Stearic acid
Icosanoic acid
Arachidic acid
O
CH3(CH2)18COH
Table 26.1 (p 1018)
O
CH3(CH2)7
(CH2)7COH
C
H
C
H
Systematic name: (Z)-9-Octadecenoic acid
Common name: Oleic acid
Table 26.1 (p 1018)
O
CH3(CH2)4
C
H
(CH2)7COH
CH2
C
C
H H
C
H
Systematic name: (9Z, 12Z)-9,12-Octadecadienoic acid
Common name: Linoleic acid
Table 26.1 (p 1018)
O
CH3CH2
C
H
C
C
H H
Systematic name:
(CH2)7COH
CH2
CH2
C
C
H H
C
H
(9Z, 12Z, 15Z)-9,12,15Octadecatrienoic acid
Common name: Linolenic acid
Table 26.1 (p 1018)
O
OH
H
H
H
H
H
H
H
Systematic name:
H
(5Z, 8Z, 11Z, 14Z)-5,8,11,14Icosatetraenoic acid
Common name: Arachidonic acid
26.3
Fatty Acid Biosynthesis
Fatty Acid Biosynthesis
Fatty acids are biosynthesized via acetyl
coenzyme A.
The group of enzymes involved in the overall
process is called fatty acid synthetase.
One of the key components of fatty acid
synthetase is acyl carrier protein (ACP—SH).
Fatty Acid Biosynthesis
An early step in fatty acid biosynthesis is the
reaction of acyl carrier protein with acetyl
coenzyme A.
O
CH3CSCoA + HS—ACP
O
CH3CS—ACP
+ HSCoA
Fatty Acid Biosynthesis
A second molecule of acetyl coenzyme A reacts
at its a carbon atom with carbon dioxide (as
HCO3–) to give malonyl coenzyme A.
O
–
+
HCO3
CH3CSCoA
Acetyl
coenzyme A
O
O
–
OCCH2CSCoA
Malonyl
coenzyme A
Fatty Acid Biosynthesis
Malonyl coenzyme A then reacts with acyl
carrier protein.
O
O
O
–
OCCH2CS—ACP
ACP—SH
O
–
OCCH2CSCoA
Malonyl
coenzyme A
Fatty Acid Biosynthesis
Malonyl—ACP and acetyl—ACP react by
carbon-carbon bond formation, accompanied by
decarboxylation.
O
CH3C
S—ACP
O
CH3C
– ••
•• O
••
O
C
O
CH2CS—ACP
O
CH2CS—ACP
S-Acetoacetyl—ACP
Fatty Acid Biosynthesis
In the next step, the ketone carbonyl is reduced
to a secondary alcohol.
OH
CH3C
H
O
CH2CS—ACP
NADPH
O
CH3C
O
CH2CS—ACP
S-Acetoacetyl—ACP
Fatty Acid Biosynthesis
The alcohol then dehydrates.
OH
CH3C
H
O
CH2CS—ACP
O
CH3CH
CHCS—ACP
Fatty Acid Biosynthesis
Reduction of the double bond yields
ACP bearing an attached butanoyl group.
Repeating the process gives a 6-carbon acyl
group, then an 8-carbon one, then 10, etc.
O
CH3CH2CH2CS—ACP
O
CH3CH
CHCS—ACP
26.4
Phospholipids
Phospholipids
Phospholipids are intermediates in the
biosynthesis of triacylglycerols.
The starting materials are L-glycerol 3phosphate and the appropriate acyl coenzyme A
molecules.
CH2OH
HO
H
O
+
O
RCSCoA
+ R'CSCoA
CH2OPO3H2
The diacylated
species formed
in this step is
O
called a
phosphatidic
R'CO
acid.
O
CH2OCR
H
CH2OPO3H2
The
phosphatidic
acid then
O
undergoes
hydrolysis of its R'CO
phosphate ester
function.
O
CH2OCR
H
CH2OPO3H2
O
O
R'CO
CH2OCR
H
CH2OH
H2O
O
O
R'CO
Reaction with a
third acyl
coenzyme A
molecule yields
the
triacylglycerol.
CH2OCR
H
CH2OPO3H2
O
O
R'CO
CH2OCR
H
CH2OH
O
R"CSCoA
O
R'CO
O
CH2OCR
H
O
CH2OCR"
Reaction with a
third acyl
coenzyme A
molecule yields
the
triacylglycerol.
Phosphatidylcholine
Phosphatidic acids are intermediates in the
formation of phosphatidylcholine.
O
O
O
R'CO
CH2OCR
H
CH2OPO3H2
O
R'CO
CH2OCR
H
CH2OPO2–
+
(CH3)3NCH2CH2O
Phosphatidylcholine
O
O
R'CO
hydrophobic "tail"
CH2OCR hydrophobic "tail"
H
CH2OPO2–
+
(CH3)3NCH2CH2O
polar "head group"
Phosphatidylcholine
hydrophobic
(lipophilic) "tails"
hydrophilic "head group"
Cell Membranes
water
Cell membranes are
"lipid bilayers." Each
layer has an assembly
of phosphatidyl
choline molecules as
its main structural
component.
water
Cell Membranes
water
The interior of the cell
membrane is
hydrocarbon-like.
Polar materials cannot
pass from one side to
the other of the
membrane.
water
26.5
Waxes
Waxes
Waxes are water-repelling solids that coat the
leaves of plants, etc.
Structurally, waxes are mixtures of esters. The
esters are derived from fatty acids and longchain alcohols.
Waxes
Waxes are water-repelling solids that coat the
leaves of plants, etc.
Structurally, waxes are mixtures of esters. The
esters are derived from fatty acids and longchain alcohols.
O
CH3(CH2)14COCH2(CH2)28CH3
Triacontyl hexadecanoate: occurs in beeswax
26.6
Prostaglandins
Prostaglandins
Prostaglandins are involved in many biological
processes.
Are biosynthesized from linoleic acid (C18) via
arachidonic acid (C20). (See Table 26.1)
Examples: PGE1 and PGF1a
O
O
OH
HO
OH
O
HO
OH
HO
PGE1
OH
PGF1a