Lipids
 Function of Lipids
 Energy storage molecules in animals [release 9 kcal/g versus 4
kcal/g by carbohydrates]
 Form membranes (cellular or sub-cellular) that separate
compartments in cells [lipids are insoluble in water so help
generate membranes]
 Serve as chemical messengers [steroid hormones are primary
messengers, while prostaglandins and thromboxanes are secondary
messengers mediating hormonal response]
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Lipids
 Classification of Lipids by Structure
 Lipids can be classified into four groups based on their structure
1) Simple lipids, e.g., fats;
2) Complex lipids
3) Steroids; 4) Prostaglandins, thromboxanes, and leukotrienes
Simple and Complex Lipids
Complex
Simple
Phospholipids
Fatty acid
Fatty acid
PO4 Alcohol
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Fatty acid
PO4 choline
Sphingosine
Fatty acid
Fatty acid
Sphingolipids
Sphingosine
Fatty acid
Glycerol
Glycerol
Glycerophospholipids
Glycolipids
Glup / Galp
Fatty acid
2
Lipids
 Triglycerides




Animal fats and vegetable oils are triglycerides
Triesters of glycerol and long chain carboxylic acids called fatty acids
Glycerol component cannot vary, but the fatty acid component may vary
Fatty acids are unbranched carboxylic acids having 10 to 20 carbons in even
number
 Some fatty acids have double bond(s). Typically these double bond(s) are cis
O
CH2OH
CHOH
OH
n
A triglyceride
+
CH2OH
Glycerol
O
m
palmitate (16:0)
OH
oleate (18:1)
H2C
CH3(CH2)7CH=CH(CH2)7
O CH
O
(CH2)14CH3
O
H2C O
O
(CH2)16CH3
O
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stearate (18:0)
3
Properties of Triglycerides
 Generally fats from animals are solids at room temperature. Most fats from
plants and fish are liquids. Liquid fats are referred to as oils.
 Presence or absence of unsaturation makes a triglyceride solid or liquid
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
HOOC
Saturated and trans fatty acids stack well
(greater hydrophobic interactions;
greater order in arrangement;
carries over to triglyceride structure)
Unsaturated cis fatty acids do not ….
(lesser hydrophobic interactions;
lesser order in arrangement;
carries over to triglyceride structure)
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Properties of Triglycerides
Saturated Fatty Acids
12:0 CH3(CH2)10COOH
14:0 CH3(CH2)12COOH
16:0 CH3(CH2)14COOH
18:0 CH3(CH2)16COOH
20:0 CH3(CH2)18COOH
lauric acid
myristic acid
palmitic acid
stearic acid
arachidic acid
Unsaturated cis Fatty Acids
16:1 CH3(CH2)5 CH=CH(CH2)7COOH
18:1 CH3(CH2)7 CH=CH(CH2)7COOH
18:2 CH3(CH2)4 (CH=CHCH2)2(CH2)6COOH
18:3 CH3CH2(CH=CHCH2)3(CH2)6COOH
20:4 CH3(CH2)4 (CH=CHCH2)4(CH2)2COOH
palmitoleic acid
oleic acid
linoleic acid
linolenic acid
arachidonic acid
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Tm (OC)
44
58
63
70
77
1
16
-5
-11
-49
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Properties of Triglycerides
 Pure fats and oils are colorless, odorless and tasteless.
 Fats are not soluble in water. They are hydrophobic molecules and soluble in organic
solvents
 The presence of double bonds, especially in plant fats, makes them susceptible to
oxidation. Heavily oxidized fats are not edible. These generate rancidity.
 The ester group in fats is susceptible to hydrolysis releasing acidic groups, e.g., butyric
acid, which have bad taste and odor.
H2C
R"
O CH
O
R
O
H2C O
NaOH
O
R'
O
H2C
OH
HO CH
+
_
O Na +
O
H2C OH
R
_
O Na +
O
R'
_
O Na +
O
R"
Mixture of fatty acid sodium salt
(Soap)
Saponification
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Complex Lipids
 Complex lipids constitute the main component of membranes.
 Two main types of complex lipids – phospholipids and glycolipids. Phospholipids
contain a phosphate group, while glycolipids contain a carbohydrate group
 Phospholipids can be made from glycerol or from sphingosine
O
(CH2)16CH3
O
H2C
O
(CH2)7CH=CHCH2CH=CH(CH2)4CH3
HC O
+
H2C
O
_
O
P
O CH2CH2N(CH3)
choline
O
Phosphatidylcholine (lecithin)
(A glycerophospholipid)
O
+
Me3N
A sphingomyelin (A sphingolipid)
O
OH
sphingosine
P+
O
_
O
choline
CH3
HN
CH3
O
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Membranes
 Complex lipids containing unsaturated fatty acids constitute cell membranes.
 Membranes are made up of lipid bi-layers.
The Fluid Mosaic Model of Cell Membranes
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Steroids
 Steroids constitute the third major class of lipids
 Steroids are cyclopentanoperhydrophenanthrenes.
 Cholesterol is the commonest steroid. It is present cell membranes. It is also
present in lipoprotein particles, e.g., LDL, HDL, and others
C
A
D
B
Cyclopentanoperhydrophenanthrene
(Steroid skeleton)
CH3
CH3
CH3
CH3
Cholesterol
CH3
HO
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Lipoprotein Particles
 Lipoprotein particles transport cholesterol in blood
 Contain a core of hydrophobic lipid molecules surrounded by a shell of hydrophilic
molecules such as proteins and phospholipids
 At least four kinds of particles: HDL, LDL, VLDL and Chylomicrons
Apolipoprotein
Cholesterol
Phospholipids
Composition
Density
Size
Chylomicrons
TG >> C, CE
Low
Large
VLDL
TG > CE
LDL
CE >> TG
HDL
CE > TG
High
Small
Cholesterol ester
A Model of a Lipoprotein Particle
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Prostaglandins, Thromboxanes and Leukotrienes
 Prostaglandins are derived from arachidonic acid
 They have a wide variety of effects of the body; they can lower or raise blood
pressure, cause inflammation, induce blood clotting and induce labor
 Aspirin and other NSAIDs inhibit the enzyme (cyclooxygenase COX) involved in
the synthesis of prostaglandins
HOOC
Arachidonic Acid
OH
O
HOOC
HOOC
PGE2
PGF2a
OH
OH
OH
OH
HOOC
Thromboxane A2
O
O
OH
OH
OH
HOOC
Leukotriene B4
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