Lipids

1
Lipids Membranes

• A. Review of Lipid components
• General Concepts
• Fatty Acids
• B. Classes of Lipids
• Triacylglycerides
• Membrane Lipids
• Phospholipids
• Glycolipids
• Cholesterol (a steroid)
• C. Membranes
• Nature of bilayer
• Nature of other components

2
Lipid Functions

• compartmentalization with communication
• membranes
• energy storage
• fats and oils triacylglycerides
• permeability barriers
• suberin, cutin, waxes

3
Biological Lipids - Properties

• highly insoluble in water
• extensive regions of hydrocarbon with only small
  proportion of polar groups
• amphipathic
• regions compatible with water and with
  biological hexane
• formation of micelles, membranes and vesicles
• most lipids contain fatty acids

4
Fatty Acids - rules (review)

• Fatty Acid A long chain carboxylic acid having
  an even number of carbon atoms.
• unbranched
• 12 carbon chain length or longer
• if unsaturated, double bond is cis
• most common have 18 or 16 carbon atoms

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Common Fatty Acids - Review

• Saturated
• 12 lauric CH3(CH2)10COOH
• 14 myristic CH3(CH2)12COOH
• 16 palmitic acid CH3(CH2)14COOH
• 18 stearic acid CH3(CH2)16COOH
• Unsaturated
• 16 palmitoleic CH3(CH2)5CHCH(CH2)7COOH
• 18 oleic CH3(CH2)7CHCH(CH2)7COOH
• 18 linoleic CH3(CH2)4CHCHCH2CHCH(CH2)7COOH
• 18 linolenic CH3CH2CHCHCH2CHCHCH2CHCH(CH2)7
  COOH

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Melting Points of Fatty Acids

• Saturated fatty acids in lipid materials stack
  better than unsaturated fatty acids in these
  materials
• better stacking more van der Waals attractions
  higher total attractions more heat needed to
  break apart attractions higher melting points
• Saturated fatty acids have higher melting points
  than same size unsaturated fatty acids

7
Shapes of Fatty Acids
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Components of Membranes
9
Phospholipids

• Common to all phospholipids
• fatty acid
• phosphate
• polar/ionic alcohol
• Additional components
• Unique to phosphoglycerides
• glycerol
• another fatty acid
• Unique to sphingomyelins
• sphingosine

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Phosphoglycerides (review)

• Phosphoglycerides - Diglycerides that are also
  phosphate esters of glycerol

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Phosphoglyceride domains
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Sphingomyelins
sphingosine
a sphingomyelin
always choline here
fatty acid attached here
phosphate
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Spingomyelin Domains

• Polar head group
• Doubly charged
• Nonpolar tails
• Two hydrocarbon chains
• Same as with phosphoglycerides
• One chain is sphingosine
• Other is fatty acid

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Glycolipids
A glycolipid is like a sphingomyelin except that
the phosphate is replaced by a sugar unit or a
combination of sugar units
sphingosine
sugar unit
fatty acid
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Steroids and Cholesterol
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Properties of membranes

• impermeable to polar molecules and ions
• highly flexible
• highly durable
• contain certain mix of lipids
• contain protein that have a purpose
• in more complex cells, contain carbohydrates
• are asymmetric with respect to chemical
  composition and function

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Membrane Locations

• External membranes
• Cell membrane
• Internal membranes
• Nucleus
• organelles

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Membrane Fluidity

• Tm melting transition temperature
• ordinary melting point is the temperature at
  which the liquid and solid phase are a
  equilibrium
• for lipid bilayers Tm is the temperature at
  which half the component lipids in the bilayer
  are in the "crystalline" state and the other half
  are in the melted state
• ordered state is nonmobile
• disordered state is mobile

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Effect of Lipid Content on Tm

• short chain and/or unsaturated fatty acids in
  lipids produce more "liquidy,oily (fluid)
  materials
• long chain saturated fatty acids give more rigid,
  solid materials
• membrane fluidity is highly influenced by
  proportion of different fatty acids
• effect of cholesterol dampen transition gives
  membrane more of a dynamic range

20
Protein Composition

• integral proteins
• portion of protein is embedded in bilayer
• can be small or large portion
• sections of proteins that are compatible with the
  nonpolar matrix of lipid bilayer nonpolar amino
  acid side chains
• peripheral proteins
• protein that is attached to an integral protein
  not the lipid bilayer

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Domains of Integral Protein

• Portion buried in membrane
• Nonpolar residues
• Residues favoring a helix
• Portion outside membrane
• Polar and charged
• Variable secondary, tertiary structure

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Carbohydrates

• single units or chains can be attached
• important in recognition of cell types
• antigenic responses
• immunity
• intercellular signaling

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Membrane Asymmetry

• lipids
• phosphatidylcholine and sphingomyelin face out
• charge on cholines
• phosphatidylethanolamine and phosphatidylserine
  face in
• neutral and - charge
• carbohydrates
• almost exclusively on outside face
• proteins
• receptors and transport
• characteristics differences based on role

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Fluid mosaic model
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Another membrane model