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
Membranes allow compartmentation
Biological membranes are composed of a: (i) Lipid bilayer (ii) Proteins
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
Protein components of the bilayer
Simple proteins
Glycoproteins: Proteins with carbohydrate chains
Proteoglycans:Proteins with glycosaminoglycan chains
PhosphoglyceridesComposed of:(i) A glycerol backbone (with 3 positions)(ii) 2 long chain fatty acids(iii) A polar head group
Lipid bilayer
Sphingolipids(e.g. sphongomyelin) Composed of:(i) A sphingosine backbone (ii) 1 long chain fatty acid (iii) A polar head group
Sphingolipids and phosphoglycerides
Cholesterol
Cholesterol
Cholesterol increases “tightness” of membranes but increases fluidity
Membranes are heterogeneous(1) Inner and outer leaflets are distinct in composition
Membranes are heterogeneous(2) Regions of membranes can be enriched in specific lipids such as cholesterol (lipid rafts)
Membrane fluidityMembranes composed of phospholipids are highly mobile.
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
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
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)
Topography and membrane proteins
Topography and membrane proteins
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)
Surface hydrophocity of α-helices
Hydrophobic amino acids-greenPolar-blue, Charged-red
Predicting membrane-spanning domains
Membrane spanning domains can be predicted from primary structures using hydropathy plots.
Some integral proteins are β-barrels
Many large pores are composed of beta-sheets arranged into a barrel.
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)
Controlling protein location
(2) Others can interact directly with the cytoskeleton (or via linkers)
Controlling protein location
(3) Some interact via external domains (e.g. carbohydrate)
Controlling protein location
(4) Inter-cellular interactions may prevent movements.
