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Chapter 10: Membranes
Know the terminology:Phospholipid, phosphoglyceride, sphingolipid, cholesterol, steroids, phosphotide, polar head group, fatty acid, glycerol, glycoprotein, proteoglycan
bilayer, fluidity, homeoviscous adaptation, integral membrane protein, transmembrane domain, peripheral protein, lipid raft, hydropathy plot
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Membranes allow compartmentation
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Biological membranes are composed of a: (i) Lipid bilayer (ii) Proteins
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Lipid components of the bilayer
Phospholipids• Phosphoglycerides: glycerol, 2 fatty acids and
polar head group• Sphingolipids: sphingosine, 1 fatty acid and polar
head group
Other lipids:• Steroids (cholesterol mainly)• Fatty acids: aliphatic chains with carboxylic acid
group
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Protein components of the bilayer
Simple proteins
Glycoproteins: Proteins with carbohydrate chains
Proteoglycans:Proteins with glycosaminoglycan chains
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PhosphoglyceridesComposed of:(i) A glycerol backbone (with 3 positions)(ii) 2 long chain fatty acids(iii) A polar head group
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Sphingolipids(e.g. sphongomyelin) Composed of:(i) A sphingosine backbone (ii) 1 long chain fatty acid (iii) A polar head group
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Sphingolipids and phosphoglycerides
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Cholesterol
Cholesterol increases “tightness” of membranes but increases fluidity
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Membranes are heterogeneous(1) Inner and outer leaflets are distinct in composition
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Membranes are heterogeneous(2) Regions of membranes can be enriched in specific lipids such as cholesterol (lipid rafts)
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Membrane fluidityMembranes composed of phospholipids are highly mobile.
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Membrane fluidity depends upon lipid composition
Phospholipid movement depends upon:(i) Fatty acid chain length(ii) Saturation(iii) Polar head group(iv) Physical conditions
Membrane fluidity also depends upon presence of other macromolecules:
(i) Cholesterol(ii) Glycolipids
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Homeoviscous adaptationEnvironmental conditions (temperature, salt
concentration) can alter membrane fluidityCells adjust lipid profiles to maintain constant
fluidityReduced temperature “solidifies” membranes
Cell increase fluidity by:• using shorter fatty acids• introducing double bonds into fatty acids• altering polar head groups
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Membrane proteinsMany membranes are primarily protein
(e.g. mitochondrial inner membrane is about~80% protein, 20% lipid)
Proteins can be associated with the membrane many different ways:
(1) Integral proteins are embedded within the membrane
(2) Peripheral proteins are only associated with the membrane (via various connections)
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Topography and membrane proteins
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Topography and membrane proteins
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Transmembrane proteins
Many proteins cross completely through the membrane one or more times
Typically an alpha-helix with hydrophobic amino acids (Figs. 10-19, 10-20)
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Surface hydrophocity of α-helices
Hydrophobic amino acids-greenPolar-blue, Charged-red
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Predicting membrane-spanning domains
Membrane spanning domains can be predicted from primary structures using hydropathy plots.
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Some integral proteins are β-barrels
Many large pores are composed of beta-sheets arranged into a barrel.
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Controlling protein locationIn a naked cell, proteins are free to move within membranes but often cells restrict movement of proteins.
(1) Some proteins interact with each other (self-assembly) (Fig 10-43)
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Controlling protein location
(2) Others can interact directly with the cytoskeleton (or via linkers)
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Controlling protein location
(3) Some interact via external domains (e.g. carbohydrate)
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Controlling protein location
(4) Inter-cellular interactions may prevent movements.