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18.1 Lipids

18.2 Fatty Acids

Chapter 18 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 solvents.

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Diversity in structure leads to a diversity in biological function Non-polar lipids (fats) are the principal molecules for

energy storage Polar lipids (nitrogen and phosphorus containing) are

components of biological membranes The steroid class of lipids (cholesterol) is found in

biological membranes and used as a precursor for many hormones

Miscellaneous lipids present only in minor quantities in the cell are involved as light-absorbing pigments (β-carotene, retinal), enzyme cofactors (vitamin K), signal molecules (prostaglandins), and electron carries (ubiquinone)

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Classes of Lipids

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Fatty acids are: Long-chain carboxylic acids that have

an even number (usually 12-18) of carbon atoms.

Insoluble in water. Saturated or unsaturated.

Fatty Acids

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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

OH

O

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Ionization General fatty acid formula: R – COOH (R stands for alkyl chain composed of carbon

and hydrogen atoms) The pKa of the fatty acid carboxyl group is about 4.8 Under ordinary conditions the pH of the plasma is 7.4, and

the pH of the intracellular fluid is about 7.0 Almost all (99.9%) of the free fatty acid molecules in body

fluids are ionized (the fatty acid is present as an anion) RCOOH ↔ RCOO- + H+

The ionized group interacts with aqueous media (polar) The hydrocarbon “tail” seeks a non-polar environment

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General nomenclature The carbon atoms of an acid are numbered (or

lettered with Greek lettering system, such as α, β, γ) either from the carboxyl group or from the carbon farthest removed from the carboxyl group (n or omega numbering system)

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH 10 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 10 ω δ γ β α

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Saturated Fatty AcidsSaturated fatty acids have single C–C bonds.

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Systematic namesComponents Common name Systematic name

1 formic methanoic

2:0 acetic ethanoic

3:0 propionic propanoic

4:0 butyric butanoic

5:0 valeric pentanoic

6:0 caproic hexanoic

8:0 caprylic octanoic

10:0 capric decanoic

12:0 lauric dodecanoic

14:0 mystiric tetradecanoic

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Systematic names (continue)Components Common name Systematic name

16:0 palmitic hexadecanoic

16:1 (9) palmitoleic cis-9-hexadecenoic

18:0 stearic octadecanoic

18:1 (9) oleic cis-9-octadecenoic

18:1 (11) vaccenic cis-1-octadecenoic

18:2 (9,12) linoleic all cis-9,12-octadecadienoic

18:3 (9,12,15) linolenic all cis-9,12,15-octadecatrienoic

20:0 arachidic eicosanoic

20:4 (5,8,11,14) arachidonic all cis-5,8,11,14-eicosatetraenoic

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Saturated fatty acids have: Molecules that fit closely

together in a regular pattern. Strong attractions between

fatty acid chains. High melting points that

makes them solids at room temperature.

Saturated Fatty Acids

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Unsaturated Fatty Acids Unsaturated fatty acids typically

contain cis double bonds.

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Unsaturated Fatty AcidsUnsaturated fatty acids have cis C=C bonds.

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Unsaturated fatty acids: Have nonlinear

chains that do not allow molecules to pack closely.

Have low melting points.

Are liquids at room temperature.

Unsaturated Fatty Acids

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Olive Oil

Olive oil contains a high percentage of oleic acid.

Oleic acid is a monounsaturated fatty acid with one cis double bond.

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Melting Points of Fats and OilsVegetable oils contain more unsaturated fatty acids,

which makes their melting points lower.

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Canola oil Canola Oil - Truth and Myths about Canola http://www.canola-council.org/about/

thetruth.html

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Omega-6 and Omega-3 Fatty Acids

The first double bond: In vegetable oils is at carbon 6 (omega-6). In fish oils is at carbon 3 (omega-3).

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Essential fatty acids Mammals cannot synthesize the two main types of polyunsaturated

fatty acids: the linoleic (omega-6) and linolenic (omega-3) Essential fatty acid deficiency is characterized by dermatitis and poor

wound healing Most prostaglandins are synthesized from arachidonic acid (a linoleic

acid derivative) Lack of these prostaglandins causes some aspects of the essential fatty

acid deficiency syndrome A small daily intake of linolenic acid is recommended (this may be

especially important when the nervous system is developing) Docosahexaenoic acid (22:6n-3) – omega-3 fatty acid is needed for

maximum development of the visual and central nervous systems during the neonatal period (it is present in large amounts in the retina and parts of the brain; it enhances the visual response and the function of certain domains in neural membranes)

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ProstaglandinsProstaglandins have: 20 carbon atoms in their fatty acid chains. An OH on carbon 11 and 15. A trans double bond at carbon 13.

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Prostaglandins in the BodyProstaglandins are: Produced by injured tissues. Involved in pain, fever, and

inflammation. Not produced when anti-

inflammatory drugs such as aspirin inhibit their synthesis.

Prostaglandins http://www.elmhurst.edu/

~chm/vchembook/555prostagland.html

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Other eicosanoids When a cell responds to a stimulus, bioactive

mediators (prostaglandins, thromboxanes, leukotrienes, and others) often are formed from the lipids contained in its membranes.

Thromboxanes regulate blood coagulation. Leukotrienes play a role in allergic and

inflammation responses. They were isolated from leukocytes (white blood cells).