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Chapter 24Chapter 24LipidsLipids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Lipids
Lipids are naturally occurring substances grouped together on the basis of a common property—solubility. They are extracted from cellular material using a nonpolar solvent.
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 a carrier of acetate.
Lipids
Since lipids are defined by solubility rather than structure, they include a variety of molecules having different structures. Some are:
1. Fats & Oils (Triacylglycerols)
2. Fatty acids
3. Phospholipids - glycerol based - sphinogsine based
4. Waxes
5. Prostaglandins
6. Terpenes - Cholesterol
24.124.1Acetyl Coenzyme A:Acetyl Coenzyme A:
Not a lipid, it is a coenzyme Not a lipid, it is a coenzyme used in fatty acid metabolismused in fatty acid metabolism
Structure of Coenzyme A P
OP
ON N
SR
H3C CH3
OH
O O
H H
O
HO O O OH
O
HO
OPHO
HO
O
NN
NN
NH2
R = H; Coenzyme A
R = CCH3; Acetyl coenzyme A
O
Reactivity of Coenzyme A
Nucleophilic acyl substitution
CH3CSCoA
OHY ••
CH3C
O
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
CH3CSCoA
O
Acetyl coenzyme A reacts with biological electrophiles at its carbon atom.
CSCoA
OH
H2C
E+
CH2CSCoA
O
E
24.224.2Fats, Oils, and Fatty AcidsFats, Oils, and Fatty Acids
Fats and Oils
Fats and oils are naturally occurring mixture of triacylglycerols (also called triglycerides).
Fats are solids; oils are liquids.
RCOCH
CH2OCR'O
CH2OCR"
O
O
CH3(CH2)16COCH
CH2OC(CH2)16CH3O
CH2OC(CH2)16CH3
O
O
Fats and Oils
Tristearin; mp 72°C
Fats and Oils
2-Oleyl-1,3-distearylglycerol; mp 43°C
CH2OC(CH2)16CH3
CH2OC(CH2)16CH3
O
O
CH2(CH2)6COCH
O
C
H
C
H
CH3(CH2)6CH2
Fats and Oils
2-Oleyl-1,3-distearylglycerolmp 43°C
H2, Pt
Tristearinmp 72°C
Fatty Acids
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.
H2OHOCH
CH2OH
CH2OH
O
R'COCH
CH2OCRO
CH2OCR"
O
O
R'COH
O
HOCR
O
HOCR"
Table 24.1
O
CH3(CH2)10COH Dodecanoic acid Lauric acid
Systematic name Common name
O
CH3(CH2)12COH Tetradecanoic acid Myristic acid
O
CH3(CH2)14COH Hexadecanoic acid Palmitic acid
Table 24.1
O
CH3(CH2)16COH Octadecanoic acid Stearic acid
O
CH3(CH2)18COH Eicosanoic acid Arachidic acid
Systematic name Common name
Table 24.1
O
C
H
C
H
CH3(CH2)7 (CH2)7COH
Systematic name: (Z)-9-Octadecenoic acid
Common name: Oleic acid
Table 24.1
Systematic name: (9Z, 12Z)-9,12-Octadecadienoic acid
Common name: Linoleic acid
C
H
C
H
CH3(CH2)4
O
(CH2)7COHCH2
C
H
C
H
Table 24.1
Systematic name: (9Z, 12Z, 15Z)-9,12,15-Octadecatrienoic acid
Common name: Linolenic acid
C
H
C
H
O
(CH2)7COHCH2
C
H
C
H
CH3CH2
C
H
C
H
CH2
Table 24.1
Systematic name: (5Z, 8Z, 11Z, 14Z)-5,8,11,14-Icosatetraenoic acid
Common name: Arachidonic acid
H
H H
H
H HH
H
OH
O
trans-Fatty Acids
Are formed by isomerization that can occur when esters of cis-fatty acids are hydrogenated. O
OR
H O
ORH
H2, cat
O
OR
H O
ORH
H
O
OR
H
H2, cat
24.424.4PhospholipidsPhospholipids
A major component of cell A major component of cell membranes.membranes.
Phospholipids
Phospholipids may be glycerol based or sphingosine based.
Sphingosine itself is derived from serine and palmitic acid and leads to ceramide-P.
Phosphatidic acid is derived from glycerol, fatty acids and Pi. It is used to form triacylglycerols (TAG) and glycerol based phospholipids.
The starting materials for TAG are L-glycerol 3-phosphate and the appropriate acyl coenzyme A (CoASH is an acyl group carrier).
Phosphatidic Acid
Phosphatidic acid is central to the synthesis of triacylglycerols and glycerol based phospholipids.
Common glycerol based phospholipids are phosphatidylcholine (lecithin), phosphatidylethanolamine (cephalin), phosphatidylserine and phosphatidylinositol.
Phospholipids contain a head group (polar) and a tail section (nonpolar) and are major components of cell membranes.
CH2OPO3H2
CH2OH
HHO + R'CSCoA
O
RCSCoA
O
+
The diacylated species formed in this step is called a phosphatidic acid.
CH2OPO3H2
CH2OCR
HR'CO
O
O
Phosphatidic Acid Synthesis
The phosphatidic acid then undergoes hydrolysis of its phosphate ester function.
H2O
CH2OPO3H2
CH2OCR
HR'CO
O
O
CH2OH
CH2OCR
HR'CO
O
O
TAG Synthesis
Reaction with a third acyl coenzyme A molecule yields a triacylglycerol (TAG).
R"CSCoA
OCH2OH
CH2OCR
HR'CO
O
O
CH2OCR"
CH2OCR
HR'CO
O
O
O
This shows triglyceride (TAG) formation.
TAG is not a phospholipid.
TAG Synthesis
Phosphatidic acids are intermediates in the formation of the phospholipid phosphatidylcholine.
Phosphatidylcholine
CH2OPO3H2
CH2OCR
HR'CO
O
O
CH2OPO2
CH2OCR
HR'CO
O
O
–
(CH3)3NCH2CH2O+
Phosphatidylcholine
Phosphatidylcholine
CH2OPO2
CH2OCR
HR'CO
O
O
–
(CH3)3NCH2CH2O+
polar "head group"
hydrophobic "tail"
hydrophobic "tail"
Phosphatidylcholine
hydrophilic "head group"
hydrophobic(lipophilic) "tails"
Cell Membranes
Cell membranes are "lipid bilayers."
Each layer has an assembly of phospholipid molecules as the main structural component.
water
water
Cell Membranes
The interior of the cell membrane is hydrocarbon-like.
Polar materials cannot pass from one side to the other of the membrane.
water
water
Ceramide Phosphate with a head group attached (usually choline) are sphingomyelins.
Sphingomyelins
CH2OPO3H2
CHCH=CH(CH2)12CH3
HR'CNH
O
CH2OPO2
HR'CNH
O
–
(CH3)3NCH2CH2O+
A sphingomyelin
HO
HO
CHCH=CH(CH2)12CH3
Ceramide phosphate
24.524.5WaxesWaxes
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 long-chain alcohols.
CH3(CH2)14COCH2(CH2)28CH3
O
Triacontyl hexadecanoate: occurs in beeswax
24.624.6ProstaglandinsProstaglandins
Eicosanoids include:prostaglandinsthromboxanesprostacyclinsleukotrienes
Eicosanoids are compounds related to eicosanoic acidCH3(CH2)18CO2H.
Eicosanoids
Prostaglandins are involved in many biological processes.
One group of prostaglandins is made from arachidonic acid (C20) which is derived from linoleic acid (C18). (See Table 24.1)
Prostaglandins
O
HO
O
OH
OH HO
HO OH
O
OH
PGE1
PGF1
Examples: PGE1 and PGF1
PGE2 is biosynthesized from arachidonic acid.
The oxygens come from O2.
The enzyme involved (prostaglandin endoperoxide synthase) has cyclooxygenase (COX) acitivity.
Prostaglandin Biosynthesis
Arachidonic acid
O2
fatty acid cyclooxygenase
CO2H
CH3
PGG2
O
HOO
CO2H
CH3
O
Prostaglandin Biosynthesis
reduction ofhydroperoxide
PGG2
O
HOO
CO2H
CH3
O
PGH2
O
HO
CO2H
CH3
O
Prostaglandin Biosynthesis
OHO HO
CO2H
CH3 PGE2
PGH2
O
HO
CO2H
CH3
O
Prostaglandin Biosynthesis
Cleavage of cyclic peroxide
PGH2
O
HO
CO2H
CH3
O
TXA2
O
OHO
CO2H
CH3
Thromboxane A2 is biosynthesized from PGH2
TXA2 promotes platelet
aggregation and blood clotting
Thromboxane A2 (TXA2)
PGI2
Like thromboxane A2, prostacyclin I2 is
biosynthesized from PGH2.
PGI2 inhibits platelet
aggregation and relaxescoronary arteries.
OH
HO2C
CH3
HO
O
Prostacyclin I2 (PGI2)
Leukotrienes arise from arachidonic acid viaa different biosynthetic pathway.
They are the substances most responsible for constricting bronchial passages during asthma attacks.
Leucotriene C4 (LTC4)
CO2H
OH
CH3
SCH2CHCNHCH2CO2H
O
NH
C OO2CCHCH2CH2
NH3+
–
Leucotriene C4 (LTC4)
24.724.7Terpenes: The Isoprene RuleTerpenes: The Isoprene Rule
Terpenes
Terpenes are natural products that are structurally related to isoprene.
H2C C
CH3
CH CH2
or
Isoprene(2-methyl-1,3-butadiene)
Terpenes
Myrcene (isolated from oil of bayberry) is a typical terpene.
CH2
CH3
CH3C CHCH2CH2CCH
CH2 or
The Isoprene Unit
An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds). Myrcene contains two isoprene units.
The Isoprene Unit
The isoprene units of myrcene are joined "head-to-tail". head tail
tail head
Table 24.2
Class Number of carbon atoms
Monoterpene 10
Sesquiterpene 15
Diterpene 20
Sesterpene 25
Triterpene 30
Tetraterpene 40
Classification of Terpenes
Figure 24.7
Representative Monoterpenes -Phellandrene
(eucalyptus)Menthol
(peppermint)Citral
(lemon grass)
O
H
OH
Figure 24.7
Locating the isoprene units. -Phellandrene
(eucalyptus)Menthol
(peppermint)Citral
(lemon grass)
OH O
H
Figure 24.7
-Phellandrene(eucalyptus)
Menthol(peppermint)
Citral(lemon grass)
Locating the isoprene units.
Figure 24.7
Representative Sesquiterpenes
-Selinene(celery)
H
Figure 24.7
-Selinene(celery)
H
Locating the isoprene units.
Figure 24.7
-Selinene(celery)
Locating the isoprene units.
Figure 24.7
Representative Diterpene
Vitamin A
OH
Figure 24.7
Vitamin A
OH
Locating the isoprene units.
Figure 24.7
Vitamin A
Locating the isoprene units.
Figure 24.7
Representative Triterpene
Squalene (shark liver oil)
tail-to-tail linkage of isoprene units
Locating the isoprene units.
24.824.8Isopentenyl Pyrophosphate: Isopentenyl Pyrophosphate:
The Biological Isoprene UnitThe Biological Isoprene Unit
The Biological Isoprene Unit
The isoprene units in terpenes do not come from isoprene.
They come from isopentenyl pyrophosphate.
Isopentenyl pyrophosphate (5 carbons) comes from acetate (2 carbons) via mevalonate (6 carbons).
The Biological Isoprene Unit
CH3COH
O
3 HOCCH2CCH2CH2OH
CH3
OH
O
Mevalonic acid
H2C CCH2CH2OPOPOH
CH3 O O
Isopentenyl pyrophosphate
Isopentenyl Diphosphate
H2C CCH2CH2OPOPOH
CH3 O O
Isopentenyl pyrophosphate
or
OPP
Isopentenyl and Dimethylallyl Diphosphate
Isopentenyl diphosphate is interconvertible with2-methylallyl diphosphate.
OPP
OPP
Dimethylallyl pyrophosphate has a leaving group (pyrophosphate) at an allylic carbon; it is reactive toward nucleophilic substitution at this position.
Isopentenyl pyrophosphate Dimethylallyl pyrophosphate
24.924.9Carbon-Carbon Bond FormationCarbon-Carbon Bond Formation
in Terpene Biosynthesisin Terpene Biosynthesis
Carbon-Carbon Bond Formation
The key process involves the double bond of isopentenyl pyrophosphate acting as a nucleophile toward the allylic carbon of dimethylallyl pyrophosphate.
+OPP
OPP
Carbon-Carbon Bond Formation
+OPP
OPP–
+OPP
OPP
After C—C Bond Formation...
+
OPP
The carbocation can lose a proton to give a double bond.
OPP
H–+
After C—C Bond Formation... OPP
This compound is called geranyl pyrophosphate. It can undergo hydrolysis of its pyrophosphate to give geraniol (rose oil).
After C—C Bond Formation... OPP OH
Geraniol
H2O
From 10 Carbons to 15 +
OPP
OPP
Geranyl pyrophosphate +
OPPOPP
+
OPP
From 10 Carbons to 15
H–+
OPP
From 10 Carbons to 15 OPP
This compound is called farnesyl pyrophosphate.
Hydrolysis of the pyrophosphate ester gives the alcohol farnesol (Figure 24.7).
From 15 Carbons to 20 OPP
Farnesyl pyrophosphate is extended by another isoprene unit by reaction with isopentenyl pyrophosphate.
OPP
Cyclization
Rings form by intramolecular carbon-carbon bond formation.
OPP
OPP
+
E double bond
Z double bond
+
OH
H–+
H2O
Limonene
-Terpineol
Bicyclic Terpenes +
+
+
-Pinene
+
-Pinene
24.1024.10The Pathway from AcetateThe Pathway from Acetate
to Isopentenyl Diphosphateto Isopentenyl Diphosphate
Recall
CH3COH
O
3 HOCCH2CCH2CH2OH
CH3
OH
O
Mevalonic acid
H2C CCH2CH2OPOPOH
CH3 O O
Isopentenyl pyrophosphate
Biosynthesis of Mevalonic Acid
CH3CCH2CSCoA
OO
S-Acetoacetylcoenzyme A
In a sequence analogous to the early steps of fatty acid biosynthesis, acetyl coenzyme A is converted to S-acetoacetyl coenzyme A.
Biosynthesis of Mevalonic Acid
CH3CSCoA
O
+CH3CCH2CSCoA
OO
In the next step, S-acetoacetyl coenzyme A reacts with acetyl coenzyme A.
Nucleophilic addition of acetyl coenzyme A (probably via its enol) to the ketone carbonyl of S-acetoacetyl coenzyme A occurs.
Biosynthesis of Mevalonic Acid
CH3CSCoA
O
CH3CCH2CSCoA
CH2COH
HO O
O
+CH3CCH2CSCoA
OO
3-hydroxy-3-methylglutarylCoA
Biosynthesis of Mevalonic Acid
Next, the acyl coenzyme A function is reduced.
The product of this reduction is mevalonic acid.
CH3CCH2CSCoA
CH2COH
HO O
O
CH3CCH2CH2OH
CH2COH
HO
OMevalonic acid
Conversion of Mevalonic Acid to Isopentenyl Pyrophosphate
CH3CCH2CH2OH
CH2COH
HO
O
The two hydroxyl groups of mevalonic acid undergo phosphorylation.
CH3CCH2CH2OPP
CH2COH
O
OPO3
2–
Conversion of Mevalonic Acid to Isopentenyl Pyrophosphate
Phosphorylation is followed by a novel elimination involving loss of CO2 and PO4
3– giving isopentenyl pyrophosphate.
CH3CCH2CH2OPP
CH2
O
OPO3
2–
C O••
••••–
CH3CCH2CH2OPP
CH2
OPO3
3–
O C O
Biosynthetic Pathway is Based on Experiments with 14C-labeled Acetate
CH3COH
O
Citronellal biosynthesized using 14C-labeled acetate as the carbon source has the labeled carbons in the positions indicated.
H2C CCH2CH2OPOPOH
CH3 O O O
H
•
• • •
•
•
24.1124.11Steroids: CholesterolSteroids: Cholesterol
Structure of Cholesterol Fundamental framework of steroids is the tetracyclic unit shown.
Structure of Cholesterol
Cholesterol has the fundamental steroid skeleton modified as shown.
Cholesterol has 27 carbons.
HO
CH3
H
H
H
CH3
CH3 CH3
CH3
Structure of Cholesterol
Some parts of the cholesterol molecule are isoprenoid. Other parts don't obey the isoprene rule, it does not have a multiple of 5 carbons.
HO
CH3
H
H
H
CH3
CH3 CH3
CH3
Biosynthesis of Cholesterol
Cholesterol is biosynthesized from the triterpene squalene. In the first step, squalene is converted to its 2,3-epoxide. O
O2, NADH, enzyme
Biosynthesis of Cholesterol
To understand the second step, we need to look at squalene oxide in a different conformation, one that is in a geometry suitable for cyclization.
O
O
H+
Biosynthesis of Cholesterol
Cyclization is triggered by epoxide ring opening. O
+
HO
H
Biosynthesis of Cholesterol
The five-membered ring expands to a six-membered one.
+
HO
H
H
+
HO
H
Biosynthesis of Cholesterol
Cyclization to form a tetracyclic carbocation.
H
+
HO
H
+
H
H
HO
protosteryl cation
Biosynthesis of Cholesterol
Deprotonation and multiple migrations.
H
+HO
OH2
••••
H
H HO
H
H
Biosynthesis of Cholesterol
The product of this rearrangement is a triterpene called lanosterol. A number of enzyme-catalyzed steps follow that convert lanosterol to cholesterol.
HO
H
H
Molecules Made from Cholesterol
Cholesterol (C27) is the biosynthetic precursor to a large number of important steroids:
Vitamin D (27 carbons)
Bile acids (24 carbons)
Adrenocortico hormones + progesterone (21 carbons)
Sex hormones: Male (19 carbons)
Female (18 carbons)
24.1224.12Vitamin DVitamin D
Cholesterol HO
CH3
H
H
H
CH3
CH3 CH3
CH3
Cholesterol is the precursor to vitamin D.
Enzymes dehydrogenate cholesterol to introduce a second double bond in conjugation with the existing one. The product of this reaction is called 7-dehydrocholesterol.
7-Dehydrocholesterol
HO
CH3
H H
CH3
CH3 CH3
CH3
Sunlight converts 7-dehydrocholesterol on the skin's surface to vitamin D3.
Vitamin D3
CH3
HO
H
CH3 CH3
CH3
Insufficient sunlight can lead to a deficiency of vitamin D3, interfering with Ca2+ transport and bone development. Rickets can result.
24.1324.13Bile AcidsBile Acids
Cholesterol HO
CH3
H
H
H
CH3
CH3 CH3
CH3
Oxidation in the liver degrades the cholesterol side chain and introduces OH groups at various positions on the steroid skeleton. Cholic acid (next slide) is the most abundant of the bile acids.
Cholic Acid
Salts of cholic acid amides (bile salts), such as sodium taurocholate (next slide), act as emulsifying agents to aid digestion.
HO
CH3
H
H
H
CH3
CH3
HOH
HO
O
OH
Sodium Taurocholate HO
CH3
H
H
H
CH3
CH3
HOH
HO
O
NHCH2CH2SO3Na
24.1424.14Cortiosteroids and Cortiosteroids and
ProgesteroneProgesterone
Cholesterol HO
CH3
H
H
H
CH3
CH3 CH3
CH3
Enzymatic degradation of the side chain and oxidation of various positions on the steroid skeleton convert cholesterol to corticosteroids.
Cortisol
Cortisol is the most abundant of the corticosteroids. Enzyme-catalyzed oxidation of cortisol gives cortisone.
O
CH3
H
H
H
CH3OH
HO
O
OH
Cortisone
Corticosteroids are involved in maintaining electrolyte levels, in the metabolism of carbohydrates, and in mediating the allergic response.
O
CH3
H
H
H
CH3OH
O
O
OH
Progesterone
Supresses ovulation during pregnancy.
O
H
H
H
H3C
H3CO
24.1524.15Sex HormonesSex Hormones
Testosterone
Testosterone is the main male sex hormone.
O
H
H
H
H3C
H3COH
Estradiol
Estradiol is a female sex hormone involved in regulating the menstrual cycle and in reproduction.
HO
H
H
H
H3COH
24.1624.16CarotenoidsCarotenoids
Carotenoids
Carotenoids are naturally occurring pigments.
Structurally, carotenoids are tetraterpenes. They have 40 carbons. Two C20 units are linked in a tail-to-tail fashion.
Examples are lycopene and -carotene.
Carotenoids
Lycopene (tomatoes) -Carotene (carrots)
End of Chapter 24End of Chapter 24LipidsLipids
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