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Lipids
• Fatty Acids• Triacylglycerols• Glycerophospholipids
Lipids consist of hydrophobic molecules that have great structural variability and are not polymeric.
In general, the lipids perform three biological functions:
1. Essential components of biological membranes.
2. Contain hydrocarbon chains that serve as energy stores.
3. Involved in many intracellular and intercellular signaling.
• Sphingolipids• Steroids, Eicosanoids• Lipid Bilayers
Fatty Acids
Fatty acids are carboxylic acids with long-chain hydrocarbon side groups. In plants and animals, the predominant fatty acid residues have C16 or C18 side chains.
Type of Fatty Acid
Saturated Fatty Acids
Unsaturated Fatty Acids
Polyunsaturated Fatty Acids
Content of Side Chain
no double bonds
contain double bonds
contain 2 or more double bonds
Common C18 Fatty Acids
HOOCCH2
CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH2CH2
CH3
HOOC
1
18
HOOC
1189
HOOC
1
18
9
HOOC
118
9
12
12 15
Stearic Acid18:0
Oleic Acid18:1
Linoleic Acid18:2
-Linolenic Acid18:3
Fatty acid double bonds are almost always have a cis configuration.
Table 9-1. Common Biological Fatty Acids
Common Name Systematic Name Structure
Saturated fatty acids12:0 Lauric acid Dodecanoic acid CH3(CH2)10COOH14:0 Myristic acid Tetradecanoic acid CH3(CH2)12COOH16:0 Palmitic acid Hexadecanoic acid CH3(CH2)14COOH18:0 Stearic acid Octadecanoic acid CH3(CH2)16COOH20:0 Arachidic acid Eicosanoic acid CH3(CH2)15COOH22:0 Behenic acid Docosanoic acid CH3(CH2)20COOH24:0 Lignoceric acid Tetracosanoic acid CH3(CH2)22COOH
Unsaturated fatty acids (all double bonds are cis)16:1 Palmitoleic acid 9-Hexadecenoic acid CH3(CH2)5CH=CH(CH2)7COOH18:1 Oleic acid 9-Octadecenoic acid CH3(CH2)7CH=CH(CH2)7COOH18:2 Linoleic acid 9,12-Octadecadienoic acid CH3(CH2)4(CH=CHCH2)2(CH2)6COOH18:3 -Linolenic acid 9,12,15-Octadecatrienoic acid CH3CH2(CH=CHCH2)3(CH2)6COOH18:3 -Linolenic acid 6,9,12-Octadecatrienoic acid CH3(CH2)4(CH=CHCH2)3(CH2)3COOH20:4 Arachidonic acid 5,8,11,14-Eicosatetraenoic acid CH3(CH2)4(CH=CHCH2)4(CH2)2COOH20:5 EPA 5,8,11,14,17-Eicosapentaenoic
acidCH3CH2(CH=CHCH2)5(CH2)2COOH
24:1 Nervonic acid 15-Tetracosenoic acid CH3(CH2)7CH=CH(CH2)13COOH
Triacylglycerols
H2C
HC
H2C
OH
OH
OH
H2C
HC
H2C
O
O
O
C R1
O
C R2
O
C R3
O
Glycerol Triacylglycerol
CO
H2C
HC
H2C
O
O
O
CO
CO 18
18
16
1-palmitoleoyl-2-linoleoyl-3-stearoyl-glycerol
1
2
3
Fats and Oils - Energy Reserves
Fats and oils exist primarily as triacylglycerols (or triglycerides).
Fats are solids at room temperature, oils are liquids.
Fats and oils are complex mixtures of triacylglycerols (composition)varies with organism). Plant oils are richer in unsaturated fatty acids (lower melting point and thus are liquids at room temperature).
Fats are a highly efficient form of energy. Fats are less oxidized than proteins or carbohydrates and thus produce more energy per unit mass upon complete oxidation. Also, fats are stored anhydrous.
Fats provide about six times the metabolic energy of an equal weight of hydrated glycogen.
Adipocytes (fat cells) synthesize and store triacylglycerols and adipose tissue is the most abundant tissue source.
Glycerophospholipids (or phosphoglycerides)
• major component of biological membranes• esters of glycerol-3-phosphate
Amphiphilic molecules: nonpolar aliphatic (hydrocarbon) “tails”polar phosphoryl-X “heads”
H2C
C
H2C
OH
H
O
HO
P OH
OH
O
H2C
C
H2C
O
H
O
O
P O
O-
O
C R1
O
CR2
O
X
L-glycerol-3-phosphate general structure of a glycerophospholipid
Common Classes of Phospholipids
X-OH
water
ethanolamine
choline
serine
glycerol
Formula of -X
-H
-CH2CH2NH3+
-CH2CH2N(CH3)3+
-CH2CH(NH3+)COO-
-CH2CH(OH)CH2OH
Name
phosphatidic acid
phosphatidylethanolamine
phosphatidylcholine (lecithin)
phosphatidylserine
phosphatidylglycerol
Table 9-2. H2C
C
H2C
O
H
O
O
P O
O-
O
C R1
O
CR2
O
X
Structure of a Phospholipid
1-stearoyl-2-oleoyl-3-phosphatidylcholine
Fig. 9-4
H2C
C
H2C
O
H
O
O
P O
O-
O
C
O
(CH2)16CH3
C
O
(CH2)7
CH2CH2N(CH3)3+
HC
HC(CH2)7H3C
Phospholipases hydrolyze phospholipids.
phospholipase A2
phospholipase A2
phospholipase A1
phospholipase C phospholipase D
H2C
C
H2C
O
H
O
O
P O
O-
O
C R1
O
CR2
O
X
H2C
C
H2C
O
H
O
HO
P O
O-
O
C R1
O
X
H2O
R2COOH
H2C
C
H2C
O
H
O
O
P O
O-
O
C R1
O
CR2
O
X
Phospholipases A2 - phospholipid complex
Fig. 9-6
Sphingolipids
H2C C C H
CH
OH
sphingosine
OH H
+H3N
HC
(CH2)12
CH3
H2C C C H
CH
OH
ceramide(R = fatty acid residue)
OH H
HN
HC
(CH2)12
CH3
C
R
O
• a major membrane component
• derivatives of the C18 amino alcohol sphingosine
• N-acyl fatty acid derivatives are known as ceramides
Steroids
A B
C D
cyclopentanoperhydrophenanthrene
Cholesterol
HO
cholesterol
Central steroid in the animal kingdom
Two Functions:
• Serves as an integral component in biological membranes
• Serves as the biosynthetic precursor to other steroid molecules
Adrenal Steroidogenesis
HO
cholesterol HO
pregnenolone
O
side chain cleavage
HO
O
OH
O
OH HO
O
OH
O
O
cortisol aldosterone
Sex Hormone Steroidogenesis
HO
cholesterol HO
pregnenolone
O
side chain cleavage
O
progesterone
OOH
HO
OH
O
aromatase
testosteroneestradiol
Eicosanoids
• Eicosanoids (C20) are derived from arachidonic acid (20:4)
• Four classes of eicosanoids:– prostaglandins– prostacyclins– thromboxanes– leukotrienes
COOH
158
11 1420
arachidonic acid(5,8,11,14-eicosatetraenoic acid)
Eicosanoid Biosynthesis
COOHarachidonic acid
phospholipids
phospholipase A2
cyclooxygenase 5-lipooxygenase
O
O
COOH
OH
PGH2
COOH
OH OH
LTB4
(leukotriene)
COOH
OH
HO
HO
COOH
OH
O
HO
COOH
OHOHO
HO
PGF2
(prostaglandin) PGI2
(prostacyclin)
TxB2
(thromboxane)
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