CHAPTER 2 The Diversity of Membrane Lipids

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LIPIDS• Lipids serve as energy sources, structural components, coenzymes,

electron carriers, light absorption, protein anchors, signaling• Lipids are fatty acid derivatives• Fatty acids are carboxylic acids with a hydrocarbon chain (C4 to C36)

• Hydrocarbon portion is saturated or partially unsaturated

ω3,6-PUFAs

Sources of Lipids

• Biosynthesis from acetyl-CoA• Dietary intake• Limited ability to synthesize

unsaturated fatty acids (humans)

Acyl Chains

• Vary in length

• Vary in Saturation

-double bond restricts rotation

• Van der Waals forces

-double bond interrupts

Figure 2.1 Structures of three 18C fatty acids.

Structural Aspects• Double bonds in unsaturated FA are in cis configuration• Length and saturation determine melting point• At RT, C12:0 to C24:0 are waxy, whereas unsaturated FA are oily

Figure 2.2 Bond torsion angles in hydrocarbon chains

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Complex Lipids

Classes of lipidsPhospholipids (glycerophospholipids)

Sphingolipids

Sterols and linear isoprenoids

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Glycerophospholipids

• Polar or charged group is attached to the third carbon of glycerol– Basis for

nomenclature

• Most abundant lipid in biological membranes

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Sphingolipids

• One fatty acid joined to sphingosine

• Polar head group

• Sphingomyelin, Glycosphingolipids,Gangliosides

• Sphingomyelin - animal cells, especially myelin

• Some involved in signal transduction & cell surface recognition

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Glycosphingolipids: determinants of the blood groups

Structure of sterols

Steroids

• SteroidsSteroids:: a group of plant and animal lipids that have this tetracyclic ring structure.

Cholesterol• Cholesterol is the major sterol in animal

tissues– It is a component in plasma membranes in all

animal cells.– It is the precursor of all steroid hormones, some

vitamins and bile acids.

Close packing of cholesterol with lipids

• Makes membrane thicker• Decreases translational diffusion

Hop diffusion from tracking single labeled lipid

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Transverse diffusion

Energetically unfavored

Asymmetric distribution of lipids

Figure 2.14 erythrocyte

• Outer membrane is 6-fold higher in sphingolipids

• Interdigitation decreases membrane thickness

• Cholesterol stabilizes acyl chains in most extended form

Polymorphic phases of phospholipids

S = Cross-section area of polar headgroup x lipid length/ lipid volume

•S = 1 for cylindrical, conical S > 1, wedge-shaped S < 1

•Important for fusion, endocytosis, crystallization

Lipid Rafts

• Lipid ordered phase = Lo

• Eukaryotes have 1 million rafts

• Evidence of rafts in intracellular membranes

• May be just outer leaflet

• High in cholesterol, saturated FA, proteins

• Treatment with β-cyclodextrin

• Size range from 10 nm to 200 nm

• Detergent resistant?• Used by the golgi for

protein trafficking• Some contain

caveolae• Important in signaling

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