Post on 11-Jan-2016
MEDC 527 Fall 2008 1
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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Classification of Lipids by Structure Lipids can be classified into four groups based on their structure
Lipids
Simple and Complex Lipids
Simple Complex
Phospholipids Glycolipids
Glycerophospholipids Sphingolipids
Fatty acid
Fatty acid
Fatty acidGly
cero
l Fatty acid
Fatty acid
AlcoholGly
cero
l
PO4
Fatty acid
choline
Sp
hin
gosin
e
PO4
Glup / Galp
Fatty acid
Sp
hin
gosin
e
1) Simple lipids, e.g., fats; 2) Complex lipids
3) Steroids; 4) Prostaglandins, thromboxanes, and leukotrienes
MEDC 527 Fall 2008 3
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
Lipids
CHOH
CH2OH
CH2OH
Glycerol
+
CH
CH2
CH2
O (CH2)14CH3
O
O(CH2)16CH3
O
OCH3(CH2)7CH=CH(CH2)7
O
OH
O
OH
O
A triglyceride
palmitate (16:0)
stearate (18:0)
oleate (18:1)
n
m
p
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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
Properties of Triglycerides
HOOCHOOCHOOCHOOCHOOCHOOC
HOOCHOOC
HOOCHOOC
HOOC
HOOC
HOOCHOOCHOOCHOOCHOOCHOOC
Saturated and trans fatty acids stack well Unsaturated cis fatty acids do not ….(greater hydrophobic interactions; (lesser hydrophobic interactions; greater order in arrangement; lesser order in arrangement; carries over to triglyceride structure) carries over to triglyceride structure)
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Saturated Fatty Acids Tm (OC)
12:0 CH3(CH2)10COOH lauric acid 44
14:0 CH3(CH2)12COOH myristic acid 58
16:0 CH3(CH2)14COOH palmitic acid 63
18:0 CH3(CH2)16COOH stearic acid 70
20:0 CH3(CH2)18COOH arachidic acid 77
Unsaturated cis Fatty Acids
16:1 CH3(CH2)5 CH=CH(CH2)7COOH palmitoleic acid 1
18:1 CH3(CH2)7 CH=CH(CH2)7COOH oleic acid 16
18:2 CH3(CH2)4 (CH=CHCH2)2(CH2)6COOH linoleic acid -5
18:3 CH3CH2(CH=CHCH2)3(CH2)6COOH linolenic acid -11
20:4 CH3(CH2)4 (CH=CHCH2)4(CH2)2COOH arachidonic acid -49
Properties of Triglycerides
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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.
Properties of Triglycerides
CH
CH2
CH2
O R
O
OR'
O
OR"
O
CH
CH2
CH2
OH
OH
OH
O NaO
R
O NaO
R'
O NaO
R"
NaOH _ + _ + _ ++
SaponificationMixture of fatty acid sodium salt
(Soap)
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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
Complex Lipids
Phosphatidylcholine (lecithin)(A glycerophospholipid)
CH
CH2
CH2
O
O
O
P
(CH2)16CH3
O
O
(CH2)7CH=CHCH2CH=CH(CH2)4CH3
O
O
O
CH2CH2N(CH3)+
_
choline
A sphingomyelin (A sphingolipid)
OP
O
O
ONH
OH
O
CH3
CH3
Me3N +_
sphingosine
choline
+
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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
A B
C DCyclopentanoperhydrophenanthrene
(Steroid skeleton)
CH3
CH3
CH3
CH3
CH3
OH
Cholesterol
MEDC 527 Fall 2008 10
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
A Model of a Lipoprotein Particle
Cholesterol
Cholesterol ester
Phospholipids
Apolipoprotein
SmallHighCE > TGHDL
CE >> TGLDL
TG > CEVLDL
LargeLowTG >> C, CEChylomicrons
SizeDensityComposition
SmallHighCE > TGHDL
CE >> TGLDL
TG > CEVLDL
LargeLowTG >> C, CEChylomicrons
SizeDensityComposition
MEDC 527 Fall 2008 11
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
OH
O
OH
HOOC
OH
OH
OH
HOOC
O
OHOOC
OH
HOOC
OH OH
Arachidonic Acid
PGE2 PGF2
Thromboxane A2 Leukotriene B4