Membrane Structure

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Chapter 10

• Membrane Structure

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What do we need for cellular life?

• A large number and variety of organic molecules
  and ions.
• Lipid membrane to contain and compartmentalize.
• Proteins as catalysts and structural components.
• Nucleic acids to convey instructions for
  maintenance and reproduction.

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• Membrane processes
• Compartmentalization
• Gradients
• Protein trafficking
• Elimination
• Storage
• Lipid membranes must be
• Fluid
• Insoluble

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• Lipid bilayer

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• Phosphotidylcholine
• amphipathic molecule

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Synthetic membranes

• Liposomes

• Black membranes

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Membrane fluidity

• Lateral diffusion- 2?m per second
• Flip-flop once per month per lipid
• Flippases
• Phospholipid translocators

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• cis-double bonds in hydrocarbon chains affect
  packing
• Saturated membranes thicker
• Changes in degree of saturation occurs seasonally
  in cold-blooded (poikilothermic) animals (eg fish)

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Unsaturation index (UI)
Warm acclimated fish Cold acclimated fish

• Number of double bonds x of occurrence
• High UI results in more fluid membrane
• trout hepatocyte membrane

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Effect of acclimation temperature on goldfish
survival
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• Cholesterol

Fig 10-4
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Cholesterol in the membrane

• Up to 25 of membrane material
• Intercalates between phospholipids
• Stiffens membrane

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500-1000 different lipid varieties
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Glycolipids

• Based on serine in animals (glycerol in bacteria)
• Sugar groups added by Golgi
• Found only in non-cytosolic layer
• Involved in cell protection, adhesion mechanisms,
  ion balance
• Partition into lipid rafts

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Lipid rafts

• Sphingolipids and cholesterol
• Saturated phospholipids
• Glycolipids

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Lipid raft in Golgi network
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Lipids as messengers
Sites of phospholipase cleavage
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Formation of lipid droplets
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Membrane Proteins
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Fig 10-19

• Single pass ?-helix
• Multi pass ?-helix
• ?-barrel
• Amphipathic ?-helix
• Lipid anchor (fatty acid or GPI)
• Non-covalent association with membrane integral
  proteins

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Membrane protein attachment via lipid anchor
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• Hydrophobic side chains in green and yellow
• Polarity of peptide bonds within helix dealt with
  by hydrogen bonding

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Hydropathy plot ( is hydrophobic)

• a b c d e f g h i j
• -1 3 3 2 1 1 -1 -2 -1 0
• 5 8 6 4 1 -2 -4 -3

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Hydropathy plot
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20 of proteins predicted to have transmembrane
region
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Single-pass transmembrane protein

• Hydrophobic region
• Reduced cytosolic cysteine residues
• Disulfide bonds on extracellular surface
• Glycosylation on extracellular surface

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b-barrel proteins
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Membrane proteins can be difficult to study
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Detergent micelles
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An example Purification of Na/K pump
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Membrane proteins can be difficult to study
Freeze-fracture electron microscopy
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Membrane proteins are often very mobile
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Fluorescent recovery after photobleaching
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Fluorescence loss in photobleaching
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However, cells can restrict protein movements in
membrane

• Aggregation (A)
• Lipid rafts
• Tethering (B and C)
• Via interactions with another cell (D)
• Tight Junctions

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Tight junctions
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Red Blood Cells
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• Proteins in red blood cell membranes

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• Spectrin -reinforces membrane
• Ankryin -links spectrin to membrane
• Band 3 -anion transporter (HCO3-)
• Band 4.1 -junctional complex with actin,
• adducin and tropomyosin
• Glycophorin -function unknown (homodimer)

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Corralling of membrane proteins by cortical
cytoskeletal filaments
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Glycocalyx
The cell surface is coated with sugar
residuesLymphocyte