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Basic Biochemistry
CLS 233
1st semester, 2014- 2015
Ch:10
Lipids
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Storage Lipids
The fats and oils used almost universally as stored
forms of energy in living organisms are derivatives of
fatty acids.
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Fatty Acids Are Hydrocarbon
Derivatives
Fatty acids are carboxylic acids with hydrocarbon chain
ranging from 4 to 36 carbons long (C4 to C36).
In some fatty acids, this chain is unbranched and fully
saturated (contains no double bonds); in others the
chain contains one or more double bonds (Table 10–1).
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Nomenclature of fatty acids
A simplified nomenclature for these compounds specifies
the chain length and number of double bonds, separated
by a colon; for example, the 16-carbon saturated palmitic
acid is abbreviated 16:0, and the 18-carbon oleic acid,
with one double bond, is 18:1.
The positions of any double bonds are specified by
superscript numbers following Δ (delta); a 20-carbon
fatty acid with one double bond between C-9 and C-10
(C-1 being the carboxyl carbon) and another between
C-12 and C-13 is designated 20:2(Δ 9,12).
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Properties of F.A.
The physical properties of the fatty acids, are largely determined
by the length and degree of unsaturation of the hydrocarbon
chain.
The nonpolar hydrocarbon chain accounts for the poor solubility of
fatty acids in water.
The longer the fatty acyl chain and the fewer the double bonds,
the lower is the solubility in water.
The carboxylic acid group is polar (and ionized at neutral pH) and
accounts for the slight solubility of short-chain fatty acids in
water.
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Properties of F.A.
At room temperature (25 C), the saturated
fatty acids from 12:0 to 24:0 have a waxy
consistency, whereas unsaturated fatty
acids of these lengths are oily liquids.
This difference in melting points is due to
different degrees of packing of the fatty
acid molecules
(Fig. 10–1).
In the fully saturated compounds, molecules
can pack together tightly in nearly
crystalline arrays.
In unsaturated fatty acids, a cis double bond
forces a kink in the hydrocarbon chain.
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Triacylglycerols Are Fatty Acid
Esters of Glycerol
Triacylglycerols are composed of three fatty
acids each in ester linkage with a single
glycerol (Fig. 10–2).
Most naturally occurring triacylglycerols are
mixed; they contain two or more different
fatty acids.
Because the polar hydroxyls of glycerol and
the polar carboxylates of the fatty acids
are bound in ester linkages,
triacylglycerols are nonpolar, hydrophobic
molecules, essentially insoluble in water.
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Triacylglycerols Provide Stored Energy and
Insulation
There are two significant advantages to using triacylglycerols as
stored fuels, rather than polysaccharides such as glycogen and
starch:
1- Because Oxidation of triacylglycerols yields more than twice as
much energy, gram for gram, as the oxidation of carbohydrates .
2- Because triacylglycerols are hydrophobic and therefore
unhydrated, the organism that carries fat as fuel does not have to
carry the extra weight of water of hydration that is associated
with stored polysaccharides
In some animals, triacylglycerols stored under the skin serve not only
as energy stores but as insulation against low temperatures.
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Many Foods Contain Triacylglycerols
- Vegetable oils such as corn (maize) and olive oil are composed
largely of triacylglycerols with unsaturated fatty acids and thus
are liquids at room temperature .
- Triacylglycerols containing only saturated fatty acids, such as
tristearin, the major component of beef fat, are white, greasy
solids at room temperature.
- Note:
When lipid-rich foods are exposed too long to the oxygen in air,
they may spoil and become rancid. The unpleasant taste and
smell associated with rancidity result from the oxidative
cleavage of the double bonds .
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PART 2
12
Structural Lipids in Membranes
Biological membranes is a double layer of lipids, which acts as a barrier
to the passage of polar molecules and ions.
Membrane lipids are amphipathic: one end of the molecule is
hydrophobic, the other hydrophilic.
Their hydrophobic interactions with each other and their hydrophilic
interactions with water direct their packing into sheets called
membrane bilayers.
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Structural Lipids in Membranes
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L-glycerol 3-phosphate:
The backbone of phospholipids
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Glycerophospholipids
1. The common glycerophospholipids are diacylglycerols
linked to head-group alcohols through a phosphodiester bond.
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Glycerophospholipids
Phosphatidic acid, is the parent compound (X=H).
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Glycerophospholipids
Each derivative is named for the head-group alcohol (X) with
the pre-fix "phosphatidyl-".
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Each
derivative is
named for the
head-group
alcohol (X)
with the prefix
"phosphatidyl".
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In caridiolipin, two phosphatidic acids share a single glycerol,
hence it is also called diphosphatidly-glycerol.
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Glycerophospholipids
2. The fatty acids can vary greatly between organisms,
tissues and cells.
In general, they contain a saturated C16 or C18 fatty acid at
C1 and a C18 to C20 unsaturated fatty acid at C2.
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Glycerophospholipids
3. Common components of cell membranes.
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Structures of some common glycerophospholipids
(lecithin)
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Some phospholipids have ether linked fatty acids
1. Plasmalogens have an ether-linked alkenyl chain where
most glycerophospholipids have an ester-linked fatty acid.
The head group alcohol is choline. ~50% of the heart
phospholipids are plasmalogens.
ETHER
ESTER
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Notes
Plasmalogens:
When the fatty acid at carbon 1 of a glycerophospholipid is
replaced by an unsaturated alkyl group attached by an
ether (rather than by an ester) linkage to the core glycerol
molecule, a plasmalogen is produced.
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2. Platelet-activating factor has a long ether-linked alkyl
chain at C1. Acetic acid is ester-linked at C2, which makes
it more water soluble than most glycerophospholipids.
The head-group alcohol is choline.
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Platelet-activating factor
is a potent molecular signal
released from leucocytes that stimulates platelet aggregation.
It also has a variety of effects on many tissues including roles
in inflammation and the allergic response.
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The principle classes of storage and
membrane lipids
All these lipids have either glycerol or sphingosine as the
backbone. A third class of membrane lipids, the sterols, are
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discussed separately.
SPHINGOLIPIDS
The 3-carbon
backbone is
analogous to the
3-carbons of
glycerol.
At C3 there is the
long chain amino
alcohol sphingosine.
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SPHINGOLIPIDS
At C2 there is a
fatty acid which is
usually saturated
or
monounsaturated,
and can be either
16,18, 22, or 24
carbons long.
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SPHINGOLIPIDS
Ceramide is the
parent compound.
Other polar head
groups can be
attached at
position X.
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SPHINGOLIPIDS
Glycosphingolipids
are a sub-group of
sphingolipids that
contain sachharide
headgroups
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Sphingolipids at cell surfaces are sites of
biological recognition
1.
In humans at least 60 different sphingolipids have been
identified.
2.
Very prominent in neuronal plasma membranes.
3. Carbohydrate moieties of sphingolipids define the human
blood groups.
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Glycosphingolipids as determinants of blood
groups
The human blood groups
(O, A, B) are determined
in part by the
oligosaccharide head
groups of these
glycosphingolipids.
Glc:D-glucose
Gal:D-galactose
GalNAc:N-acetyl-Dgalactosamine
Fuc:fucose
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Phospholipids and sphingolipids are degraded in
lysosomes
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1.
For each hydrolyzable bond in a glycerophospholipid
there is a specific hydrolytic enzyme in the lysosome.
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2.
Phospholipase A1 hydrolyzes the fatty acid at C1.
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3.
Phospholipase A2 hydrolyzes the fatty acid at C2.
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4.
When one fatty acid is removed from either C1 or C2, a
lysophospholipase removes the remaining fatty acid.
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5.
Phospholipases C and D each split one specific
phosphodiester bond in the head group.
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Working with Lipids
• Because lipids are insoluble in water, their extraction and
subsequent fractionation require the use of organic solvents and
some techniques not commonly used in the purification of watersoluble molecules such as proteins and carbohydrates
Complex mixtures of lipids are separated by differences in the
polarity or solubility of the components in nonpolar solvents.
Lipids that contain ester or amide linked fatty acids can be
hydrolyzed by treatment with acid or alkali or with highly
specific hydrolytic enzymes (phospholipases, glycosidases) to
yield their component parts for analysis. Some methods
commonly used in lipid analysis are shown in Figure 10–23
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Lipid Extraction Requires Organic Solvents
Neutral lipids (triacylglycerols, waxes, pigments, and so forth) are
readily extracted from tissues with ethyl ether, chloroform, or
benzene.
Membrane lipids are more effectively extracted by more polar
organic solvents, such as ethanol or methanol.
A commonly used extractant is a mixture of chloroform, methanol, and
water, initially in volume proportions (1:2:0.8) that are miscible,
producing a single phase.
After tissue is homogenized in this solvent to extract all lipids, more
water is added to the resulting extract and the mixture separates
into two phases, methanol/water (top phase) and chloroform (bottom
phase).
The lipids remain in the chloroform layer, and more polar molecules such
as proteins and sugars partition into the methanol/water layer.
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