Lipids and Membranes - كلية الطب · 2020. 1. 22. · Lipids • Lipids: a heterogeneous...
Transcript of Lipids and Membranes - كلية الطب · 2020. 1. 22. · Lipids • Lipids: a heterogeneous...
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Lipids and
Membranes
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Learning Objectives 1. What Is the Definition of a Lipid?
2. What are the functions of lipids ?
3. What Are the Chemical Natures of the Lipid
Types?
4. What are phospholipids & what are their
functions ?
5. Which Are the Lipid-Soluble Vitamins, and What
Are Their Functions?
6. What are waxes, sphingolipids & glycolipids ?
7. What Are Steroids & their derivatives ?
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Lipids • Lipids: a heterogeneous class of naturally occurring
organic compounds classified together on the basis of common solubility properties • insoluble in water, but soluble in nonpolar organic solvents
including diethyl ether, chloroform, methylene chloride, and acetone
• Amphipathic in nature
• Lipids include
Open Chain forms
• fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, glycolipids,
• lipid-soluble vitamins
• prostaglandins, leukotrienes, and thromboxanes
Cyclic forms
• cholesterol, steroid hormones, and bile acids
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What Are Functions of Lipids ?
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Functions of Lipids • Source of energy
• Cellular membranes
• Excellent insulators
• Thermal insulators
• Electrical insulators (in membranes)
• Special roles:
• Signals
• Hormones, Mediators, or growth factors.
• Coenzymes
• Vitamins
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Fatty Acids • Fatty acid: an unbranched-chain carboxylic acid,
most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or phospholipids of biological membranes
• Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized)
• FA that contain C=C, are unsaturated: If they contain only C-C bonds, they are saturated
• Length of fatty acid plays a role in its chemical character
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• In most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare
• Unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point
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Fatty Acids
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Fatty Acids
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Fatty acid composition of three food fats
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In the shorthand notation for fatty acids • saturated FA: the number of carbons then (:) then (0)
are shown
• unsaturated FA: the number of carbons then (:) then the number and location of double bonds in the chain raised above delta symbol (Δ) are shown, separated by a colon
• Note: Counting starts from C of COOH
• Examples: palmitic (16:0), stearic (18:0), and oleic acids (18:1Δ9) = double bond between C9 and C10
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Essential Fatty Acids
• They have to be supplied in the diet without
exception.
• They are all polyunsaturated fatty acids: the C20
fatty acid arachidonic acid (20:4 Δ 5,8,11,14) and the
two C18 acids linoleic acid (18:2 Δ 9,12) and
linolenic acid (18:3 Δ 9,12 ,15 ).
• The animal organism requires arachidonic acid to
synthesize eicosanoids.
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• Nutritional fats contain palmitic,
stearic, oleic acid, and linoleic
acid particularly often.
Unsaturated fatty acids are usually
found at the central C atom of
glycerol.
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Triacylglycerols • Triacylglycerol (triglyceride): an ester of glycerol
with three fatty acids
• natural soaps are prepared by boiling triglycerides
(animal fats or vegetable oils) with NaOH, in a
hydrolysis reaction called saponification (Latin, sapo,
soap)
A triacylglycerol(a triglyceride)
CH2 O- CR
O
O
O
CH2 O- CR''
NaOH, H2 O
CH2 OH
CH2 OH
1,2,3-Propanetri ol(Gl ycerol, gl yceri n)
+
RCOO-Na+
Sodi umSoaps
HOCH
R' CO-CH
R' COO- Na+
R' ' COO-Na+
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Soaps • Soaps form water-insoluble salts when used in
water containing Ca(II), Mg(II), and Fe(III) ions
(hard water)
+
+
A sodium soap(soluble in water as mi cell es)
Cal ci um salt of a fatty aci d(i nsol uble i n water)
2 CH3 ( CH2 ) 1 4 COO-Na
+Ca
2 +
[ CH3 ( CH2 ) 1 4 COO-] 2 Ca
2 +2 Na
+
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Hydrolysis of Triglycerides • Chemical hydrolysis,
by saponification as
mentioned earlier
• Enzymatic digestion
by lipase enzyme
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Phosphoacylglycerols • Phosphoacylglycerols (= phospholipids) are the
second most abundant group of naturally
occurring lipids
• found almost exclusively in plant and animal cell
membranes, which typically consist of 40% -50%
phosphoacylglycerols and 50% - 60% proteins
• the most abundant phosphoacylglycerols are derived
from phosphatidic acid, a molecule in which glycerol is
esterified with two molecules of fatty acid and one of
phosphoric acid
• the three most abundant fatty acids in
phosphatidic acids are palmitic (16:0), stearic
(18:0), and linoleic (18:1)
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Phosphatidic acid
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Phosphatidic acid
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Phosphatidic acid
CH2
CH
CH2 -O- P-O-
O
O
O
O gl ycerol
palmi tic aci d
steari c acid
O
O-
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Phosphatidyl esters • further esterification with a low-molecular-
weight alcohol with phosphate group gives a phosphoacylglycerol or phosphatidyl ester
• the most common of these low-molecular-weight alcohols are:
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Phosphatidyl esters
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Phosphatidyl esters
Name and Formula Name of Phospholipid
ethanolamine
+choline phosphatidylchol ine
(l ecithin)
phosphatidylethanolamine(cephal in)
seri ne phosphatidylseri ne
NH3+
HOCH2 CHCOO-
HOCH2 CH2 N( CH3 ) 3
HOCH2 CH2 NH2
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Phosphatidyl esters inositol phosphatidylinositol
HO
HOOH
OHOH
OH
HOCH2 CHCH2OH
gl ycerol
phosphatidylglycerol
OHphosphatidylgl ycerol
HOCH2 CHCH2OPOCH2 CHOCR3
OH
O-
O
CH2 OCR4
O
O
di phosphatidyl glycerol(cardiolipin)
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Phosphatidyl esters • A lecithin
CH2
CH
CH2
O
O
O
O
O P OCH2 CH2 N( CH3) 3
O
O-
+
palmi tic acid
steari c acid
gl ycerol
choline
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Pulmonary surfactant
Molecular Dynamics simulation of DPPC lipid bilayer
formation in two phase systems
reduces surface tension
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Waxes • A complex mixture of esters of long-chain
carboxylic acids and alcohols
• Found as protective coatings for plants and
animals
Biological wax Beeswax, is an ester of palmitic acid
with the alcohol triacontanol.
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Sphingolipids • contain sphingosine,
a long-chain (18 C)
aminoalcohol from
which this class is
named
• Found in plants and
animals
• Abundant in nervous
system
choline
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Glycolipids • Glycolipid: a compound in which a carbohydrate
is bound to an -OH of the lipid
• many glycolipids are derived from ceramides
• Ceramide + simple sugar= cereboside (found in nerve
and brain cells)
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Multiple sclerosis
It is the most common autoimmune disorder affecting
the central nervous system.
The insulating covers of nerve cells in the brain and spinal
cord are damaged.
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Steroids • Steroids: a group of plant and animal lipids that
have this tetracyclic ring (four fused rings) structure
• Composed from three six-membered rings (A, B and C) and one five-membered ring (D)
• The features common to the ring system of most
naturally occurring steroids are illustrated here
A B
C D
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Cholesterol
H3 C
HO
H3 C
H H
H
• The steroid of most interest is cholesterol
• Cholesterol is the major component of biological
membranes (especially animal membranes)
• The precursor of other steroids (sex hormones and
vitamin D)
cholesterol
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Androgens • Androgens: male sex hormones
• synthesized in the testes
• responsible for the development of male secondary
sex characteristics
AndrosteroneTestosterone
O
H
OH
H
H3 C H
H
H3 C H3 C
H
HH3 C
H
O
HO
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Estrogens • Estrogens: female sex hormones
• synthesized in the ovaries
• responsible for the development of female secondary
sex characteristics and control of the menstrual cycle
Progesterone Estradiol
H3 C
H
HH3 C
H
C= O
H
O
CH3
OH
HO
H
H
H
H3 C
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