Membranes Membrane Proteins Transport across the membrane Figures from Moran Principles of Biochemistry 4th 5th edition Lehninger Principles of Biochemistry 4th 5th Ed Voet Voet Biochemistry 3E Voet Voet Pratt Fundamentals of Biochemistry 2E 1 Lipids and Hydrophobic effect Lipids are highly hydrophobic In polar e g water solutions they are under strong influence of the hydrophobic effect Therefore they self organize into noncovalent assemblies Type of self organization depends the shape geometry of a particular lipid 1 Hydrophilic heads are well hydrated 2 Hydrophobic tails barriers for polar molecules Contacting water Contacting other lipids and hydrophobic regions on proteins 2 Lipids form different assemblies based on shape micelle Unstable Stable Membranes Unstable Cylindrical shape Hydrophobic tails Polar solvent water Bilayer forming lipids Polar solvent water Membrane bilayers Polar head Double tailed lipids glycerophospholipids sphingomyelin cerebroside ganglioside Membrane bilayers liposomes Liposomes in Medicine Liposomes can carry different classes hydrophobic and or hydrophilic of trapped molecules They can be targeted to a particular organ region e g tumor Application for drug delivery 5 Membrane bilayers liposomes Liposomes can be prepared via sonication of phospholipids in water solutions of a desired drug 6 Lipid membranes in living cell Biological membranes are non covalent assemblies of lipids and proteins that form boundaries of cells and organelles Lipids are major constituents of all cellular membranes cytoplasmic nuclear ER Golgi endosome lysosome peroxisome mitochondrion autophagosome Lipid and protein composition of these membranes 7 Recall a mitochondria specific lipid discussed earlier differs Properties of Biological membranes 1 Form spontaneously without the input of extra energy 2 Self sealing Small lesions can be healed by lateral diffusion of lipids Larger lesions are healed by a regulated fusion with intracellular vesicles 3 Semi permeable Permeable for nonpolar compounds but virtually impermeable for polar substances 4 Asymmetric composition of membrane leaflets differs substantially 8 Membranes are asymmetric Asymmetric distribution of FA between the leaflets of the cytoplasmic membrane contributes to signaling recognition of intruders bacterial cells and to apoptosis of cells destined to die 1 Which of these lipids are charged 2 What charge do they carry or Distribution of phospholipids in inner and outer leaflets of human erythrocyte Apoptosis programmed cell death 9 membranes Membranes are fluid Lipid can be thought of as a two dimensional fluid Lipid can travel across 1 m of membrane in 1 second average size of human cell is 30 m Hydrophobic bilayer is nearly impermeable to polar groups including the polar or charged head groups of membrane lipids 10 Experimental proof of lateral diffusion Fluorescence recovery after photobleaching FRAP Laser beam 1 Fluorescent membrane marker e g lipid or protein 2 A defined membrane area is bleached uncolored with laser 11 3 Return of fluorescence to the photobleached area indicates lateral diffusion within the membrane 11 Translocation enzymes Leaflet polarity is maintained by specific enzymes Flippase and Floppase Enzymes I Create and maintain an asymmetry between the leaflets II Break this difference when required Ex to initiate phagocytosis of apoptotic cells 12 Temperature and phase transitions Membranes undergo a phase transition at a transition temperature Ttr or melting point Tm Below Ttr gel like solid state highly ordered thicker membrane Below Ttr Above Ttr fluid state less ordered Above Ttr 13 Transition temperature increases with 1 Chain length 2 Degree of saturation Longer lipids with high saturation degree are required to maintain membrane integrity at higher temperatures Unsaturated Lipids maintain membrane fluidity at low temperatures arctic animals Which lipids are relatively more abundant in the membranes and adipose tissues of arctic as compared to tropic fish 1 Arachidate 2 Cholesterol 3 Eicosapentaenoate EPA 4 Leukotriene Lipid Mobility Lipid tails are constantly in motion Viscosity is estimated to be that of light machine oil Terminal methyl groups in yellow Mobility is limited near head groups Cholesterol I Dual role in phase transition 1 Rigid planar structure forces fatty acid tails into extended conformations more rigid structure at high ToC 2 Short length of the rigid ring leads to increased mobility of neighboring fatty acid tails near terminal methyl groups Consequently cholesterol broadens the temperature range of bilayer phase transition by supporting the ordered liquid state in between the solid and fluid membrane phases 17 Cholesterol II Role in Lipid Rafts Cholesterol is enriched in membrane domains called Lipid Rafts organized patches of lipids Lipid rafts are also enriched by glycosphingolipids with large head groups These rafts float in the membrane sea and diffuse as a group instead of as individual lipids Lipid rafts are involved in Cholesterol and glycosphingolipids have complementary shapes endocytosis exocytosis and signal 18 transduction Biological membranes are composed of lipids and proteins Membrane leaflets can be easily separated upon freezing This exposes rough surface proteins Relatively smooth Rough Freeze fracture electron microscopy 19 Membrane structure fluid mosaic model Fluid mosaic proteins diffuse freely unless they are restricted by the cytoskeleton inside or extracellular matrix outside Integral proteins directly interact with membrane lipids via their hydrophobic domains require detergents for isolation Peripheral membrane proteins non covalently interact with lipid head groups or membrane proteins only mild treatment salts to isolate Lipid anchored proteins covalently attached to lipid heads 20 Membrane Proteins Types of membrane proteins Integral membrane proteins can be isolated only upon membrane disruption by detergents Peripheral weakly associated proteins interact with lipid head groups or membrane proteins most commonly via charge interaction can be separated by increasing ionic strength salts Lipid anchored covalent interaction can be extracted after Phospholipase C or D treatment 21 Transmembrane Proteins Transmembrane proteins have hydrophobic regions that span the membrane helix or sheet 22 5 4 3 6 a a 30 Right handed Helix as a Transmembrane Domain Ex 7TM receptors Hydrophobic portion of membrane 30 helices 3 6 residues per turn