Transcript BiochemLectch10[1]

CHEM 3303.02
Biochemistry
Chapter 10:
Lipids
Knowledge/ skills
Entire chapter- All except for most of
Lipids as signals, cofactors and
pigments
 Storage lipids
 Structural lipids in membranes
 Vitamins A and D are hormone precursors
 Vitamins E and K and the lipid quinones are
oxidation/reduction cofactors
General information
Insoluble in water
 Principal energy storage ( fats and oils)
 Phospholipids and sterols are structural
elements of biological membranes
 Other as cofactors, electron carriers, light
absorbing pigments, hydrophobic anchors for
proteins
 Chaperones for membrane folding
 Emulsifying agents in digestion
 Many lipids play a passive role for energy
storage or as a barrier, some are active and
play a role as messengers or signalling
molecules such as hormones
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Storage lipids
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Fatty acids Complete oxidation to CO2 and H2O
 Carboxylic acids with hydrocarbon
chains ranging from 4-36 carbons
 Chain can be branched/unbranched
 Chain can be saturated/ unsaturated
 May contain ring structures
naming
Simplified
 Chain length:number of double bonds
 Double bond position given by a Δ followed by
the number of the carbon which is participating
in the double bond. Note that the carboxyl
group carbon is C-1
 E.g. 20:3(Δ3,9,12) denotes a carboxylic acid with
20 carbons and 3 double bonds between C-3
and C4; C-9 and C-10; C-12 and C-13
 Most commonly occurring have even numbers
with unbranched chains- results from the fact
that in biological systems the chains are put
together from two Carbon (acetate) units
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Many double bonds are Δ9, 12,15 in the cis
configuration
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Almost never see alternating single and
double bonds but separated by a methylene
group
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Trans fatty acids produced by fermentation
in diary animals and hydrogenation of fish or
vegetable oils- these increase the levels of
LDL or bad cholesterol and decrease the
levels of HDL or good cholesterol
Physical properties – solubility in water
Largely determined by chain length and degree
of saturation of the hydrocarbon chain
 Longer fatty acyl chain- less soluble
 Fewer double bonds- less soluble
 Compare the solubility of lauric acid (12:0,
Mr200) 0.063mg/ml vs glucose (Mr 180)
1,100 mg/ml
 Carboxylic acid group is polar and ionized at
pH7 and accounts for the slight solubility of the
shorter chain fatty acids in water.
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Physical properties- melting point
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Influenced by length and degree of unsaturation of the
hydrocarbon chain
 At room temp the saturated fatty acids from 12:0 to
24:0 are waxy while the corresponding unsaturated
fatty acids are oily liquids
 Differences due to the degree of packing of the
molecules
 In fully saturated molecules, large degree of free
rotation around the C-C backbone and very flexible
and able to pack tightly into nearly crystalline arrays
with atoms in contact all along their length
 Unsaturated- cis double bond forces a kink in the
hydrocarbon chain- result is that cannot pack as
tightly as the fully saturated form. Overall van der
Waal’s interactions less in unsaturated so less
thermal energy to break bonds hence have lower
melting points
In vertebrates, free fatty acids with a free
carboxylate group are bound to a protein
carrier called serum albumin
 Fatty acid derivatives such as esters or
amides lack the charged carboxylate
group and are less soluble than free fatty
acids.
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Tri acyl glycerols
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Composed
of fatty
acids
linked to
glycerolby what
linkage?
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Ester linkage due
to formation of a
condensation bond
Simple if all acids
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are the same, e.g.
tristerin
Most are mixed
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Polar groups of both glycerol and
fatty acid are linked so
triacylglycerols are nonpolar,
hydrophobic and insoluble
Lower specific gravity- float on
water
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Triacylglycerol function
Energy storage in adipocytes as fat droplets
Oils in plant seeds to provide energy and precursor
molecules for germination
The carbon atoms of fatty acids are more reduced than
carbohydrates so that more than twice the energy
(gram for gram) is yielded
Because the molecule is hydrophobic, do not need
water of hydration associated with polysaccharides (2g
per gram of polysaccharide)
Carbohydrates are a quick energy source and are
readily soluble in water
Fats are better for long term storage
Fats are good insulators against cold temperatures
especially in animals that live in the cold or hibernate
Help with maintaining neutral buoyancy in diving
animals like whales
There are 4 adipocytes in this picture
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Can you tell where they are?
Storage lipids can make up more than 80% of the
adipocyte dry cell mass
The sperm whale
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Sperm whale’s head accounts for over 1/3 of its
body weight. About 90% made up of spermaceti
organ that is filled with spermaceti oil. The oil helps
the animal maintain neutral buoyancy at ocean
depths where it hunts its food and is able to change
density depending on temperature
Saturation of oils
Many vegetable oils such as corn and olive oil
are composed of triacylglycerols with
unsaturated fatty acids- liquid at room temp
 Hydrogenation reduces some double bonds to
single bonds and can give trans double bonds
 Exposure of lipids to air results in oxidative
cleavage of double bonds to produce aldehydes
and shorter chain carboxylic acids that are
more volatile and smell rancid
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Waxes
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Esters of long chain (C14 to C36) saturated and
unsaturated fatty acids with long chain alcohols
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Melting points (60ºC – 100ºC)
Can be used as a metabolic fuel store
Provide water repellent properties- help to waterproof
Lubricate hair and skin
Protect against parasites
E.g. lanolin, beeswax, carnuba wax
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Lipid types
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This is an important overall picture of the
lipids and how they are built
Lipids in membranes
Membranes serve as a barrier to passage of
polar molecules and ions
 The molecules are amphipathic, one end is
hydrophobic and the other is hydrophilic
 The hydrophilic end interact with water on
either side of the membrane- can be a single
OH or more complex as in the phospholipids, or
sugars as in the glycolipids
 The hydrophobic end interact with each other in
a bilipid layer structure
 Membrane lipids can make up 5 to 10% of the
dry mass of cells
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Glycerophospholipid
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Aka phosphoglyceraldehydes
Di acylglycerol
Two fatty acids attached via ester linkage to
C-1 and C-2 of glycerol
 Polar or charged group attached via a
phosphodiester linkage to C-3 with an alcohol
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Head group substituents
Ether lipids
Alkyl ether lipid- saturated
 Plasmalogen- double bond between C-1 and C2
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An example of ester and ether
linkage
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Molecular signal to stimulate platelet
aggregation
Galactolipids
One or two galactose residues connected by
a glycosidic linkage to C-3 of a 1,2
diacylglycerol
 Important in plant thylakoid membranes.
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Sulfolipids
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Sulfonate group bears a negative charge
like a phosphate group
Glycerol dialkyl glycerol tetraether
Found in archaebacteria- live in extreme
conditions of pH and temperature
 Molecule is longer (32C) to make a
membrane with a hydrophobic core and
hydrophilic ends (glycerol)
 Ether bond linkage more stable to hydrolysis
than ester bond
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Sphingolipids
Polar head group but use the long chain amino
alcohol sphingosine instead of glycerol
 Has one long chain fatty acid
 Has a polar head group linked by a glycosidic
linkage or phosphodiester linkage
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Breakdown of membrane lipids
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Membrane lipids
continuously made/broken
Specific enzymes attack
each hydrolyzable bond
Defects in these enzymes
lead to severe
consequences (Tay-Sachs)
Diseases associated with problems in breaking down
sphingolipids
Sterols- e.g cholesterol
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Have 4 fused
Carbon rings, three
6 carbon and one 5
carbon to form the
steroid nucleusrigid and planar
Present in
eukaryotic cells
Cholesterol is
ampiphatic
Are precursors for
steroid hormones
which regulate gene
expression
Examples of sex hormones
Vitamin A, D, E and K
Fat soluble vitamins
 All are isoprenoid compounds synthesized
by condensation of multiple isoprene units
 Vitamin A and D are hormone precursors
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1,25-dihydroxycholecalciferol- a hormone that
regulates calcium uptake in the intestine and
Ca levels in kidney and bone; regulates gene
expression to turn on synthesis of Ca 2+ binding
protein
Vitamin A (retinol), hormone
and visual pigment
Vitamin E- collective name for
tocopherols
Contain a substituted aromatic ring and long
isoprenoid side chain.
 Are hydrophobic and associate with
membranes, lipid deposits and lipoproteins
 Aromatic ring destroys free radicals to protect
fatty acids from being oxidized
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Vitamin K
Undergoes oxidation and reduction during
active prothrombin formation.
 Prothrombin splits the peptide bonds in
fibrinogen to fibrin an insoluble protein that
holds blood clots together
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