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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 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 Phosphoglycerides Composed 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 fluidity Membranes 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 adaptation Environmental conditions (temperature, salt concentration) can alter membrane fluidity Cells adjust lipid profiles to maintain constant fluidity Reduced temperature “solidifies” membranes Cell increase fluidity by: • using shorter fatty acids • introducing double bonds into fatty acids • altering polar head groups Membrane proteins Many 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, 1020) Surface hydrophocity of a-helices Hydrophobic amino acids-green Polar-blue, Charged-red Predicting membrane-spanning domains Membrane spanning domains can be predicted from primary structures using hydropathy plots. Some integral proteins are b-barrels Many large pores are composed of beta-sheets arranged into a barrel. Controlling protein location In 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 1043) 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.