Lecture 2 : Lipids
Lipids are compounds, present in plants and animals, that are soluble in non polar solvent, such as chloroform or eter, but not soluble in water.
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Five (5) types of lipids:
• Energy-storage lipids: triacylglycerols
• Membrane lipids: phospholipids
• Emulsification lipids: bile
• Messenger lipids: steroid hormones
• Protective lipids: biological waxes
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FATTY ACIDS:SaturatedUnsaturated
GLYCERIDES:NeutralPhosphoglycerides
COMPLEX LIPID:LipoproteinsGlycolipids
NONGLYCERIDES:SphingolipidsSteroidWaxes
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Fatty acids: a component of many lipids• Different types of fatty acids:
– Saturated fatty acid
• Biosynthesized fatty acids are further classified as:– Long-chain: contain 12-26 carbons– Medium-chain: contain 6-10 carbons– Short-chain: contain 4-6 carbons
– Unsaturated fatty acid• Monounsaturated fatty acid: exp (20:1).• Polyunsaturated fatty acids (PUFA): exp (20:3).
• Monocarboxylic acids.
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Physical properties
of fatty acids• the length and degree of unsaturation.
Solubility in H2O
Chain Length
Melting Point
• chain > > solubility < <
• unsaturation > > melting point < < as – due to the lack of compact packing as a result of ‘kinks’ introduced by the double bonds
melting point > >
7D.B.0 1 2 3 4
meltingtemperature All unsaturated
fats are oils at room temperature
16:0 6016:1 1
18:0 6318:1 1618:2 -518:3 -1120:0 75
F. A. M. P. (0C)
20:4 -50 1 D.B.
Cis D.B.
2 D.B.
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Some fatty acids and their common names:14:0 myristic acid; CH3(CH2)12-COOH
16:0 palmitic acid; CH3(CH2)14-COOH
18:0 stearic acid; CH3(CH2)16-COOH
18:1 cis9 oleic acid CH3(CH2)7CH=CH(CH2)7-COOH
Double bonds in fatty acids usually have the cis configuration. Most naturally occurring fatty acids have an even number of carbon atoms.
CO
O 1
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fatty acid with a cis-9 double bond
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18:2 cis9,12 linoleic acidCH3(CH2)4CH=CHCH2CH=CH(CH2)7-COOHis used to synthesize arachidonic acid, which regulates blood pressure, clotting et
18:3 cis9,12,15 a-linonenic acidCH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7-COOHis used to synthesize eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA) which are required for normal brain function and development (an omega-3)
20:4 cis5,8,11,14 arachidonic acid
20:5 cis5,8,11,14,17 eicosapentaenoic acid(an omega-3)
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Waxes
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ester of fatty acids and O
C30H61 O C C15H31
O
C16H33 O C C15H31
Important in fruits:1. Natural protective layer in fruits, vegetables, etc.
2. Added in some cases for appearance and protection.Beeswax (myricyl palmitate),Spermaceti (cetyl palmitate)
O || R - C - O - R1
Waxes :long chain
alcohol
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C-OHCHC-OH
HH
HH
HO-
glycerol
Monoacylglycerol/monoglyceride
H-O-C-CH2C-R1
O
Diacylglycerol/diglyceride
R2-CH2-C-OHO
H-O-C-CH2C-R3
O
Triacylglycerol = Triglyceride
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The chemical reactions of triacylglycerols are similar to those of alkenes and esters.
Hydrogenation of double bonds in unsaturated fatty acids by H2 (compare to alkenes).
Chemical Properties of Triacylglycerols
Hydrolysis of ester bonds by water in the presence of an acid, a base, or an enzyme (compare to esters).
TAG : * energy reservoirs * insulator* digested in the small intestine by the enzyme pancreatic lipase
* saponification : formation of soapsMonoacylglycerols are absorbed through the intestinal cells, re-converted to TG and assembled into lipoproteins
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Triglycerides• Stored as droplets of
adipose tissue in body fat.
Fat stored in this cell
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Hydrogenated Fats• Hydrogenation of
vegetable oils to margarine or shortening, not only convert unsaturated bond to saturated, but also may converts some of cis unsaturated bonds to trans configuration
• Diet high in trans fatty acids risks heart disease, cancer.
cis trans
*
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PYROLYSIS
Hydrogenation: addition of two hydrogen atoms over a double bond. Some food products are partially hydrogenated
O O // // CH2 - O - C - R1 3 CH2 - O - C - R
O // CH - O - C - R2 O
O // // CH2 = CH - C - H CH2 - O - C - R3
fatty acids
acrolein-o2
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- COOH ( fatty acid)
- C = C - + O2 - C - C - - CHO - C=O O - O
(Peroxide)
H2O
Oxidation
Oxidation: addition of oxygen (derived from air), lead to oxidize the double bonds present in a fatty acid to produce carboxylic acid, and other carbonyl compound (keton, aldehyd) through peroxide intermediate.
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Rancidification
O //Fat R - CH2 - CH2 - C - O - H
O O // //R - C - CH2 - C - O - H R - C - C + CO2
// O Keton
Lipase
-Oksidase
+
the decomposition of fats and other lipids by hydrolysis and or oxidation
Keton or acids produced from lipid oxidation /rancidification process have low-molecular-mass and volatile. This volatile compounds are reponsible for rancid odor and flavor in some food products. Antioxidant is usually added to prevent lipid oxidation which lead to rancidity.
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• Glycerol esterified with 2 fatty acids.
• Third position is phosphate
• Other end of phosphate can combine with other molecules to form various derivatives
• Amphipathic –emulsifying agents
• component of cell membrane
PHOSPHATE
OH
H
OH
H
H
OHH
OH
H
H OHPhosphatidylinositol
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O P O
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
X
glycerophospholipid
a polar region: glycerol, carbonyl O of fatty acids, Pi, & the polar head group (X)
non-polar hydrocarbon tails of fatty acids (R1, R2).
Each glycerophospholipid / phosphoacylglycerol possess:
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O P O
O
O
H2C
CH
H2C
OCR1
O O C
O
R2
H
H
Phosphatidic acidPhosphatidyl
-OH
H
OH
H
H
OHH
OH
H
H OHPhosphatidylinositol
Phosphatidylinositol has roles in cell signaling, as well as component of cell membrane
CH2 CH2 N CH3
CH3
CH3
+
Phosphatidylcholine/lechitin It is a common membrane lipid
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phosphatidyl choline = lechitinLecithin, a common food additive, is a phospholipid,
Which embedded in cell membranes.
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Cell Membrane
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Structure of Phosphatidyl choline with 2 fatty acids, oleate and palmitate
OCH3
N+
CH3
CH3
O
(CH2)14 CH3O
O
O
(CH2)7 CH
CH
(CH2)7 CH3
CH2
CH
CH2
OP
O
O
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Sphingolipids
• Sphingosine instead of glycerol
• Derivatization at ester C1
• Fatty acids added at amine
• Different types are found in plasma membrane and myelin sheaths
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2
H2CHC
O H
C H
N C H
C
CH2
CH 3
H
( )12
sphingosine
Sphingolipids/ceramide
H
O H
C
R
O
OH
The amino group of sphingosine can form an amide bond with carboxylate group of a fatty acid, to yield a ceramide Ceramides usually include a polar
head group, esterified to the terminal OH of the sphingosine
PO O
OH2C
H2CN+
CH3
H3C
CH3
OH
Sphingomyelin, a ceramide with a phosphocholine or phosphethanolamine head group, is a common constituent of plasma membranes
Sphingomyelin
Sphingomyelins are found abundantly in the myelin sheath that surrounds the nerve fibers
PO O
OH2C
H2CN+
CH3
H3C
CH3
OH O
H H
H
OHH
OH
CH2OH
H
A cerebroside is a sphingolipid (ceramide) with a monosaccharide such as glucose or galactose as polar head group.
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Steroid Nucleus
Consists of:• 3 cyclohexane rings.• 1 cyclopentane ring.• no fatty acids.
steroid nucleussteroid nucleus
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CH3
CH3CH3
CH3
HO
CH3
Cholesterol• is the most abundant steroid in the
body. • has methyl CH3- groups, alkyl chain,
and -OH attached to the steroid nucleus.
OH
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Cholesterol:-important constituent of cell
membranes-has a rigid ring system and
a short branched hydrocarbon tail.
OH
hydrophobicamphipathic.
Keeps the lipids in membrane from aggregrating, eliminate of excess cholesterol in intestinal tract and artery.
- Precursor to Bile Acids:- Crusial role in lipid
digestion- break up large lipid to
smaller one, make it can be broken easily by enzymes Precursor to steroid hormones
that regulate gene expression
Precursor to Vitamin D
A normal, open artery.
An artery clogged by cholesterol plaque
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Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids.
OH
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. The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups.
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Lipoproteins: LDL, HDL, Etc…
• combine lipids with proteins & phospholipids.
• are soluble in water because its surface consists of polar lipids.
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Types of Lipoproteins
• differ in density, composition, and function.• include low-density lipoprotein (LDLs) and high-
density lipoprotein (HDLs).
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Transport of Lipoproteins in the Body
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LDL• » circulate in the blood• Build up in arteries;
lead to heart attack
HDL
• level ≥ 130 mg/dL, reflects an increased risk of heart disease
• level < 40 mg/dL, is thought to increase the risk for heart disease.
• cholesterol lipoprotein, that returns to the liver ; removes cholesterol out of the bloodstream;
• believed to prevent heart attacks
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