PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Chapter 12Biological Membranes & TransportFor chapter 12Focus on the material covered in lecturesWill not be tested on materials covered in Pages 424 - 429Fluid Mosaic Model for Membrane StructureAmphipathic lipid aggregates that form in water or VesicleBilayers are noncovalent, cooperative structuresMonolayer of oil molecules at an air-water interfaceMembrane Phase TransitionsThe "melting" of membrane lipids •Below a certain transition temperature, membrane lipids are rigid and tightly packed •Above the transition temperature, lipids are more flexible and mobile •The transition temperature is characteristic of the lipids in the membraneHigher the proportion of saturated fatty acid, higher is the transition temperature.Sterol content of a membrane has 2 effects on membrane fluidityBelow the transition temperature: Insertion of rigid planar sterol prevents highly ordered packing of fatty acid side chains Membrane fluidityAbove the transition temperature: Rigid planar sterol reduces the freedom of neighboring fatty acid side chains Membrane fluidityCells regulate their lipid composition to achieve a constant membrane fluidity under various growth conditionsMotion of Membrane LipidsLateral DiffusionTransbilayer orflip-flop DiffusionA relatively new discovery! Lipids can be moved from one monolayer to the other by flippase proteins Some flippases operate passively and do not require an energy source Other flippases appear to operate actively and require the energy of hydrolysis of ATP FlippasesDemonstration of lateral diffusion of membrane proteinsMembrane proteins, like membrane lipids, are free to diffuse laterally in the plane of the bilayerRestricted motion of the erythrocyte chloride-bicarbonate exchangerAsymmetric distribution of phospholipids between the inner & outer monolayers of erythrocyte plasma membraneStructure of Membrane ProteinsSinger & Nicolson defined two classes - Integral (intrinsic) proteins - Peripheral (extrinsic) proteins - We'll note a new one – lipid-anchored proteinsPeripheral & Integral ProteinsGlycophorin in the erythrocyteA single-transmembrane-segment proteinOne transmembrane segment with globular domains on either end Transmembrane segment is alpha helical and consists of 19 hydrophobic amino acids Extracellular portion contains oligosaccharides (and these constitute the ABO and MN blood group determinants) Some membrane proteins span the lipid bilayerLipid-linked membrane proteinsCovalently attached lipids anchor membrane proteins to the lipid bilayerGlycosyl phosphatidylinositol (GPI) anchorA relative new class of membrane proteins 4 types have been found: Amide-linked myristoyl anchors Thioester-linked fatty acyl anchors Thioether-linked prenyl anchors Glycosyl phosphatidylinositol anchorsIntegral Membrane ProteinsHeld in the membrane by hydrophobic interactions with lipidsBacteriorhodopsin, a membrane-spanning protein3-D structure of the photosynthetic reaction center of purple bacteriumFirst integral membrane protein to have its structure determined by X-ray diffraction methodsProsthetic group (light-absorbing pigments)Residues that are part of the trans-membrane helicesHydropathy PlotsHydropathy Plots1Porin FhuA, an integral membrane protein with -barrel structureNot all integral membrane proteins are composed of transmembrane helicesPorin allows certain polar solutes to cross the outer membrane of bacteriaPorinsFound both in Gram-negative bacteria and in mitochondrial outer membrane Porins are pore-forming proteins (30-50 kD) Most arrange in membrane as trimers High homology between various porins Porin from Rhodobacter capsulatus has 16-stranded beta barrel that traverses the membrane to form the poreWhy Beta Sheets? for membrane proteins?? - Genetic economy - Alpha helix requires 21-25 residues per transmembrane strand - Beta-strand requires only 9-11 residues per transmembrane strand - Thus, with beta strands , a given amount of genetic material can make a larger number of trans-membrane segments4 examples of integral protein types that function in cell-cell interactionServe as receptors & signal transducersIntegral membrane proteins mediate cell-cell interactions & adhesionEssential part of the blood-clotting processGap JunctionsVital connections for animal cells Provide metabolic connections Provide a means of chemical transfer Provide a means of communication Permit large number of cells to act in synchrony (for example, synchronized contraction of heart muscle is brought about by flow of ions through gap junctions) Hexameric arrays of a single 32 kD protein Subunits are tilted with respect to central axis Pore in center can be opened or closed by the tilting of the subunits, as response to stress Gap JunctionsInduces closure of gap junction central channelMembrane fusion is central to many biological processesMembranes undergo fusion without losing its integrityMembrane fusion during viral entry into a host cellMovements of solutes across a permeable membraneElectrically neutral solutesElectric gradient or membrane potentialEnergy of activationEnergy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membraneFacilitated diffusion or passive transportLikely transmembrane topology of an aquaporin, AQP-1MonomerProposed structure of aquaporin channel(Formed by 4 monomers)Aquaporins form hydrophilic transmembrane channels for the passage of waterWater flows through the channel in single file at the rate of 5 X 108 molecules / secondGlucose transporter of erythrocytes mediates passive transportProposed structure of GluT1Monomer112Shows the distribution of polar& non-polar residues on the surface of a helical segmentA helical wheel diagramSide-by-side association of 5 or 6 amphipathic helicesPolarModel of glucose transport into erythrocytes by GluT1T1 & T2 are 2 different conformationsT1 has glucose binding site on the outer surface of the membraneT2, with the
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