Lipids
Chapter 9
L31 07 April 2008
Lipids
From Greek lipos, meaning fat
• Chemically and structurally diverse as are their biological
functions
• Essential components of all living organisms
• Common and defining feature is insolubility in water
• They are hydrophobic (nonpolar) or amphipathic (containing both
nonpolar and polar regions)
• In general, three major biological functions
– Lipid bilayers form biological membranes
– Lipids with hydrocarbon chains serve as energy stores
– Many intra- and intercellular signaling events involve lipid molecules
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Lipids
From Greek lipos, meaning fat
• Structural elements of biological membranes
(phospholipids and sterols)
• Stored forms of energy (fats and oils)
• Intracellular messengers
• Hormones
• Enzyme cofactors
• Electron carriers
• Light-absorbing pigments
• Hydrophobic anchors for proteins
• “Chaperones” to help membrane proteins fold
• Emulsifying agents in the digestive tract
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Lipids
Substances of biological origin that are soluble in organic solvents such
as chloroform and methanol
• Easily separated from other biological materials by
extraction into organic solvents
• Fats, oils, certain vitamins and hormones, and
nonprotein membrane components
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Lipids
Substances of biological origin that are soluble in organic solvents such
as chloroform and methanol
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Fatty acids (FAs)
Carboxylic acids with long-chain hydrocarbon side groups
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Fatty acids (FAs)
Carboxylic acids with long-chain hydrocarbon side groups
•
Not found “free” but are associated with protein carrier or stored as esters or
amides
•
Usually occur in esterified form as major components of various lipids
•
In higher plants and animals, predominant FA residues are those of the C16 and
C18 species: palmitic, linoleic, and stearic acids
•
FAs with <14 or >20 carbon atoms are uncommon
•
Most FAs have an even number of carbon atoms because they are biosynthesized
by concatenation of C2 units
•
Over ½ of FA residues of plant and animal lipids are unsaturated (contain double
bonds) and are often polyunsaturated (contain two or more double bonds)
•
Bacterial FAs are rarely polyunsaturated but are commonly branched,
hydroxylated, or contain cyclopropane rings
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Fatty acids (FAs)
Structural formulae of some C18 fatty acids
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Fatty acids (FAs)
IUPAC nomenclature
• Carboxyl carbon is C-1
• Common nomenclature: α, β, γ, δ, ε, etc. from
C-1 (carbon farthest from carboxyl is ω)
• n indicates “normal” (unbranched hydrocarbon
chain)
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Fatty acids (FAs)
Nomenclature
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Fatty acids (FAs)
Saturated FAs
• Fully reduced or “saturated” with hydrogen
• Flexible, can assume a wide range of
conformations (relatively free rotation
about C-C bonds)
• Lowest-energy conformation is fully
extended, which reduces steric interference
between neighboring methylene (-CH2-)
groups
• Melting points of saturated FAs increase
with their molecular mass
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Fatty acids (FAs)
Unsaturated FAs
•
Unsaturated FAs in biological systems almost always contain
cis double bonds
•
Rigid 30o bend in the hydrocarbon chain reduces packing
efficiency; reduced van der Waals interactions cause their
melting points to decrease with the degree of unsaturation
•
First double bond commonly occurs between C9-C10 (counting
from carboxyl C atom)
•
Called Δ9 or a 9-double bond
•
Double bonds tend to occur every 3rd C atom (e.g., -CH=CHCH2-CH=CH-) and so are not conjugated
•
Two important classes of polyunsaturated FAs are ω-3 or ω-6
FAs (α-linoleic and linoleic acid)
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Fatty acids (FAs)
Degree of unsaturation influences packing
extended, pack into nearly
crystalline arrays, stabilized
by many hydrophobic interactions
kinked, double bonds interfere with
tight packing, less stable aggregates
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mp 13.4
mp 69.6 oC
oC
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mp -11 oC
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Q. At room temperature (25 oC), the saturated
fatty acids from 12:0 to 24:0 have a waxy
consistency, whereas unsaturated fatty acids
of these lengths are oily liquids. Account for
the difference.
A. Difference in melting points is due to
different degrees of packing of the fatty
acids. Degree of packing depends on the
number of hydrophobic contacts.
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Triacylglycerols (TAGs)
Simplest lipids derived from FAs
•
Important metabolic fuels (2-3 times the energy provided by proteins
or carbohydrates)
•
Aka triglycerides, fats, neutral fats
•
Glycerol backbone + 3 FAs linked as esters
•
If 3 FAs are same, TAGs named after FA
•
Most TAGs are mixed
•
Nonpolar, hydrophobic, insoluble in water
•
Stored in cells in anhydrous form (fat droplets)
– 16:0 tripalmitin
– 18:0 tristearin
– 18:1 triolein
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Triacylglycerols (TAGs)
Simplest lipids derived from FAs
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Triacylglycerols (TAGs)
Fat stores in cells
Germinating seeds
Adipocytes
huge fat
droplets
•
•
•
Contain lipases (enzymes that catalyze the hydrolysis of stored TAGs)
Free FAs released for export to sites where they are required as fuel
Advantageous to use FAs for energy (instead of starch or glycogen)
–
–
•
FA are more reduced -> more than twice as much energy gram-for-gram as the
oxidation of carbohydrates
FA are hydrophobic and therefore unhydrated -> no extra weight due to water of
hydration (2g per g of polysaccharide)
Humans have fat tissue composed of adipocytes (under skin, abdominal cavity,
and in mammary glands)
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Triacylglycerols (TAGs)
Fat stores in cells
• Moderately obese people, with 15-20 kg of TAGs
deposited in their adipocytes, could meet their
energy needs for months by drawing on their fat
stores
• In contrast, the human body can store less than a
day’s energy supply in the form of glycogen (polymeric
form of sugar)
• Carbs do offer certain advantages: quick source of
metabolic energy, water solubility
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Triacylglycerols (TAGs)
Fat stores in cells
• TAGs stored under the skin serve as energy stores
and as insulation against low temperatures
• Seals, walruses, penguins, and other warm-blooded
polar animals are amply padded with TAGs
• In hibernating animals (bears, for example), huge fat
reserves accumulated before hibernation serve the
dual purposes of insulation and energy storage
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Spermwhales: fatheads of the deep
•
Sperm whale’s head accounts for over 1/3 of it’s total body weight
•
~90% of the head is made up of the spermaceti organ (blubbery mass that
contains up to 3600 kg of spermaceti oil)
•
Spermaceti oil is a mixture of triacylglycerols and waxes (lots of
unsaturated fatty acids)
•
Mixture is liquid at 37 oC but begins to crystallize at ~31 oC and becomes a
solid within several more degrees
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Spermwhales: fatheads of the deep
Q. Considering that these mammals feed almost
exclusively on squid in very deep waters (>1000 m),
suggest a biological role for spermaceti oil.
A.
For a marine animal to remain at a given depth, it must
have the same density as the surrounding water. The
sperm whale undergoes changes in buoyancy to match
the density of its surroundings by changing the state
of the spermaceti oil. When the temperature of the
oil is lowered by several degrees during a deep dive, it
congeals or crystallizes and becomes denser to match
the density of seawater. During the ascent, the
congealed spermaceti oil warms and melts, decreasing
its density to match that of the surface water.
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Spermwhales: fatheads of the deep
• Unfortunately for the sperm whale population,
spermaceti oil was at one time considered the
finest lamp oil and continues to be
commercially valuable as a lubricant. Several
centuries of intense hunting of these
mammals have driven the sperm whales onto
the endangered species list.
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TAGs in food
solid at r.t.
liquid at r.t.
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TAGs in food
• Natural fats (vegetable oils,
dairy products, and animal fat)
are complex mixtures of simple
and mixed TAGs
• Vegetable oils (corn and olive)
composed largely of TAGs with
unsaturated FAs (liquids at room
temperature)
• Animal fat composed primarily of
saturated FAs are white, greasy
solids at room temperature
– Beef fat, major component is
tristearin (18:0)
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TAGs in food
Hardening of oils
• Vegetable oils converted industrially to solid fats by catalytic
hydrogenation
– Reduces some double bonds to single bonds
– Converts others to trans double bonds
• Margarine and shortening are prepared this way from soybean
oil and safflower oil until they have desired consistency
• Reduction carefully controlled; reducing all of double bonds
would produce a hard fat with the consistency of beef tallow
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TAGs in food
Rancid food
• When lipid-rich foods are exposed too long to O2 in
air, they may spoil and become rancid
– Unpleasant taste and smell
– Oxidative cleavage of double bonds in unsaturated FAs
– Produce aldehydes and carboxylic acids of shorter chain
length (higher volatility)
O
OH
Butyric acid- notably found in rancid butter,
vomit, and foot sweat
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Olestra: fake fats with flavour
•
•
Proctor & Gamble spent 30 years and >$2 billion to develop Olestra
Semisynthetic, does not exist in nature but components do
•
Esterify all of sucrose OH groups with fatty acids from cottonseed oil
and soybean oil
•
•
– Decreases potential toxic effects of new compound
– Component parts are table sugar and vegetable oil
Ester linkages too hindered to be hydrolyzed by digestive enzymes
Tastes like fat, no caloric content because it cannot be digested
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Olestra: fake fats with flavour
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Trans fatty acids
•
•
•
Formed when liquid vegetable oils go through a chemical
process called hydrogenation
Hydrogenated vegetable fat is used by food processors
because it is solid at room temperature and has a longer
shelf life
French fries, doughnuts, cookies high in trans fats
Hydrogenated soybean
oil cloudy, trans fatty
acids
Linolenic soybean oils
clear, no trans-fatty
acids
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Waxes
• Esters of long chain fatty acids (C14-C30) and alcohols
(C16-C30)
• Melting points generally higher than TAGs (60-100 oC)
• Water-repellent properties, firm consistency
– Skin glands of vertebrates secrete waxes to protect hair and
skin and keep it pliable, lubricated, and waterproof
– Waterfowl secrete waxes from their preen glands to keep
feathers water-repellent
– Some plants produce thick layer of waxes to prevent excessive
evaporation of water and to protect against parasites
• Used in manufacture of lotions, ointments, and polishes
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Structural lipids in membranes
•
•
•
•
Membranes are double layers of lipids
Barrier to passage of polar molecules and ions
Amphipathic nature directs packing into sheets called bilayers
5 general types
–
–
–
–
–
Glycerophospholipids
Galactolipids and sulfolipids
Ether lipids
Sphingolipids
Sterols
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Structural lipids
Glycerophospholipids are the major lipid component of membranes
• The most abundant lipids in membranes
• Possess a glycerol backbone
• A phosphate is esterified to both glycerol and another
compound bearing an -OH group
• Phosphatidates are glycerophospholipids with two fatty
acid groups esterified to C-1 and C-2 of glycerol 3phosphate
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Structural lipids
Glycerophospholipids are the major lipid component of membranes
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Structural lipids
Glycerophospholipids
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Structural lipids
Glycerophospholipids
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Structural lipids
Electron micrograph of a liposome
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