Lipids Chapter 9 L31 07 April 2008. S.A. McFarland ©20082 Lipids Chemically and structurally...

LipidsChapter 9L31 07 April 2008

S.A. McFarland ©2008 2

Lipids

• 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

From Greek lipos, meaning fat


Lipids

• 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

From Greek lipos, meaning fat


Lipids

• Easily separated from other biological materials by extraction into organic solvents

• Fats, oils, certain vitamins and hormones, and nonprotein membrane components

Substances of biological origin that are soluble in organic solvents suchas chloroform and methanol


LipidsSubstances of biological origin that are soluble in organic solvents suchas chloroform and methanol


Fatty acids (FAs)Carboxylic acids with long-chain hydrocarbon side groups


Fatty acids (FAs)

• 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

Carboxylic acids with long-chain hydrocarbon side groups


Fatty acids (FAs)Structural formulae of some C18 fatty acids


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)


Fatty acids (FAs)Nomenclature


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


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=CH-CH2-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)


Fatty acids (FAs)

extended, pack into nearlycrystalline arrays, stabilizedby many hydrophobic interactions

kinked, double bonds interfere withtight packing, less stable aggregates

Degree of unsaturation influences packing

S.A. McFarland ©2008 15mp 69.6 oC

mp 13.4 oCmp -11 oC


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.

R. 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.


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 – 16:0 tripalmitin– 18:0 tristearin– 18:1 triolein

• Most TAGs are mixed

• Nonpolar, hydrophobic, insoluble in water

• Stored in cells in anhydrous form (fat droplets)


Triacylglycerols (TAGs)Simplest lipids derived from FAs


Triacylglycerols (TAGs)Fat stores in cells

• 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)

Adipocytes Germinating seeds

huge fatdroplets



• 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



• 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


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



Q. Considering that these mammals feed almost exclusively on squid in very deep waters (>1000 m), suggest a biological role for spermaceti oil.

R. 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.



• 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.


TAGs in food

solid at r.t.

liquid at r.t.


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)


TAGs in food

• 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

Hardening of oils


TAGs in 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

OHButyric acid- notably found in rancid butter, vomit, and foot sweat

Rancid food


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– Decreases potential toxic effects of new compound

• Esterify all of sucrose OH groups with fatty acids from cottonseed oil and soybean oil– 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


Olestra: fake fats with flavour


Trans fatty acids

Hydrogenated soybean oil cloudy, trans fatty acids

Linolenic soybean oilsclear, no 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


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


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


•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 3-phosphate

Structural lipidsGlycerophospholipids are the major lipid component of membranes


Structural lipidsGlycerophospholipids are the major lipid component of membranes


Structural lipidsGlycerophospholipids


Structural lipidsElectron micrograph of a liposome

Lipids Chapter 9 L31 07 April 2008. S.A. McFarland ©20082 Lipids Chemically and structurally...

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Transcript of Lipids Chapter 9 L31 07 April 2008. S.A. McFarland ©20082 Lipids Chemically and structurally...