Chapter NineteenChapter Nineteen

Lipids

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Lipids

• Lipids are:– Biomolecules that contain fatty acids or a steroid

nucleus– Soluble in organic solvents, but not in water– Named for the Greek word lipos, which means “fat.”– Extracted from cells using organic (nonpolar) solvents

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Biological Roles of Lipids

• Energy storage• Membrane/barriers• Emulsification• Messengers (hormones, etc.)• Protective coating (waxes)

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• Lipids are different from other biomolecules because there is no common structural feature– Lipids with fatty acids (long carbon chain carboxylic acids)

Waxes

Fats and oils (trigycerides)

Phospholipids

Sphingolipids– Lipids without fatty acids

Steroids

Types of Lipids

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The structural formulas of these types of lipids illustrate the great structural diversity among lipids.

Lipids cont’d

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• Long-chain carboxylic acids• Insoluble in water• Typically 12-18 carbon atoms (even number)• Some contain double bonds (unsaturated)

corn oil contains 86% unsaturated fatty

acids and 14% saturated fatty acids

Fatty Acids

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The melting point of a fatty acid depends on the length of the carbon chain and on the number of double bonds present in the carbon chain.

Lipids cont’d

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Fatty Acid Formulas

• The formulas for fatty acids are written as condensed and line-bond formulas. For example, caprylic acid with 8 carbon atoms can be written as:

• CH3-(CH2)6-COOH

• CH3-CH2-CH2-CH2-CH2-CH2-CH2-COOH

O

||

• CH3-CH2-CH2-CH2-CH2-CH2-CH2-C-OH

O

OH

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Saturated = C–C bonds

Unsaturated = one or more C=C bonds

natural fatty acid double bonds are cis

Saturated and Unsaturated Fatty Acids

COOH

COOH

Palmitic acid, a saturated acid

Palmitoleic acid, an unsaturated acid

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• Most Contain only single C–C bonds (Saturated)

• Closely packed

• Strong attractions between chains

• High melting points– Melting points increase with chain length

• Solids at room temperature

Properties of Fatty Acids

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• Contain one or more double C=C bonds• Nonlinear chains do not allow molecules to pack

closely• Few interactions between chains• Lower melting points than saturated

– Melting points increase with chain length– And decrease with number of double

bonds

• Liquids at room temperature

Properties of Unsaturated Fatty Acids

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Saturated fatty acids• Fit closely in regular pattern

Unsaturated fatty acids• Cis double bonds

Fatty Acid Structures

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Four 18-carbon fatty acids, which differ in the number of double bonds present.

Lipids cont’d

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Triple esterification reaction between glycerol and three molecules of stearic acid (18:0 acid). Three molecules of

water are a by-product of this reaction.

Simple Triacylglycerol

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Structure of a mixed triacylglycerol in which three different fatty acid residues are present.

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Representative triacylglycerols from (a) a fat and (b) an oil.

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Percentages of saturated, monosaturated, and polyunsaturated fatty acids.

Lipids cont’d

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Lipids cont’d

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Lipids cont’d

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Olestra

• Used in foods as an artificial fat• Sucrose linked by ester bonds to several long-

chain fatty chains• Not broken down in the intestinal tract

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Chemical Properties of Triacylglycerols

• The chemical reactions of triacylglycerols are similar to those of alkenes and esters

• In hydrogenation, double bonds in unsaturated fatty acids react with hydrogen in the presence of a Ni or Pt catalyst

• In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme

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Hydrogenation

Converts double bonds to single bonds

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Hydrogenation converts double bonds in oils to single bonds. The solid products are used to make margarine and other hydrogenated items.

Product of Hydrogenation

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Structural equation for the complete hydrogenation of a triacylglycerol with oleic acid and fatty acid residue.

Lipids cont’d

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Lipids cont’d

During hydrogenation, cisdouble bonds can be convertedto trans double bonds.

Compare the geometry of thetrans 18:2 FA to the 18:0saturated FA. Both havesimilar geometries and caninteract with each other as though the 18:2 was saturated.

Hydrogenated oils may beunhealthy, and spreadablemargarine in tubs may bepreferable to stick form.

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Triglycerides split into glycerol and three fatty acids (H+ or enzyme catalyst)

Hydrolysis

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Oxidation

• Carbon-carbon double bonds in unsaturated fatty acids can be oxidized to form both short chain aldehydes and carboxylic acids– Oils go “rancid”

• They often have bad odors

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• Hydrolysis with a strong base• Triglycerides split into glycerol and the salts of fatty acids • The salts of fatty acids are “soaps”• KOH gives softer soaps

Saponification and Soap

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Saponification

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Lipids cont’d

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Phospholipids

• A lipid that contains one or more fatty acids, a phosphate group, a platform molecule to which the fatty acids and the phosphate group are attached, and an alcohol attached to the phosphate group

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Glycerolphospholipids

• The most abundant lipids in cell membranes• Composed of glycerol, two fatty acids, phosphate,

and an amino alcohol• Has polar and nonpolar regions

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(a) structural formula and (b) molecular model showing the “head and two tails” structure of a phosphatidyl-choline molecule.

Lipids cont’d

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Molecular models for (a) sphingosine and (b) sphingophosopholipid.

Lipids cont’d

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Sphingophospholipids

• Sphingophospholipids are phospholipids that have an amide bond between a fatty acid and sphingosine, an 18-carbon alcohol

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Sphingoglycolipids

• Another type of membrane protein• Glycosphingolipids contain monosaccharides

bonded to the –OH of sphingosine by a glycosidic bond

• Cerebrosides contain only one monosaccharide• Gangliosides are similar to cerebrosides, but

contain two or more monosaccharides

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Lipids cont’d

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Structural formula and model for the cholesterol molecule.

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A severely occluded artery.

Lipids cont’d

Howard Socurek / Medichrome

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Lipoproteins

• Combine lipids with proteins and phospholipids• Are soluble in water because the surface consists

of polar lipids

LDL = low density (more fat than protein)

HDL = high density (more protein, relatively)

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Lipids cont’d

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Cross section of a lipid bilayer.

Lipids cont’d

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Space-filling model of a section of a lipid bilayer.

Lipids cont’d

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The kinks associated with cis double bonds in fatty acid chains prevent tight packing of the lipid molecules in a lipid bilayer.

Lipids cont’d

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Cholesterol molecules fit between fatty acid chains in a lipid bilayer.

Lipids cont’d

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Cell Membranes

• Separate cellular contents from the external environment

• Consist of a lipid bilayer made of two rows of phospholipids

• Have an inner portion made of the nonpolar tails of phospholipids with the polar heads at the outer and inner surfaces

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Fluid Mosaic Model of Cell Membranes

• The lipid bilayer contains proteins, carbohydrates and cholesterol

• The unsaturated fatty acids make cell membranes fluid-like rather than rigid

• Proteins and carbohydrates on the outer surface communicate with hormones and neurotransmitters

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Proteins are important structural components of cell membranes.

Lipids cont’d

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Transport through Cell Membranes

• Diffusion (passive transport)– Moves particlse from a higher to a lower concentration

• Facilitated transport– Uses protein channels to increase the rate of diffusion

• Active Transport– Moves ions against a concentration gradient

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Three Mechanisms

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G. Siegel, B Agranoff, R.W. Albers, and P. Molinoff editiors. In Basic Neurochemistry 4 th Ed., Raven Press, Ltd. 1989, p. 57.

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Bile Salts

• Synthesized from cholesterol and stored in the gall bladder

• Emulsify fats and oils to give a greater surface area for lipid digesting enzymes

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Line-angle structural formulas for cholesterol, cholic acid, and two deoxycholic acids.

Bile Acids and Salts

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A large percentage of gallstones are almost pure crystallized cholesterol.

Lipids cont’d

C. James Webb / Phototake

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Steroid Hormones

• Produced from cholesterol

• Include sex hormones such as androgens (testosterone) in males and estrogens (estradiol) in females– Anabolic steroids are derivatives of testosterone

• Also include aldosterone (controls electrolytes and water balance by kidneys) and cortisone (increases blood glucose levels and stimulates the synthesis of glycogen in the liver)

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Selected sex hormones and synthetic compounds that have similar actions.

Lipids cont’d

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Lipids cont’d

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Relationship of the structures of various eicosanoids to their precursor, arachidonic acid.

Lipids cont’d

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A biological wax has a structure with a small, weakly polar “head” and two long, nonpolar “tails”.

Lipids cont’d

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Plant leaves often have a biological wax coating to prevent excessive loss of water.

Lipids cont’d

© Kevin Schaefer / Peter Arnold, Inc.

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