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 28Biological MembranesMonolayer of oil molecules at an air-water interfaceBilayers are noncovalent, cooperative structuresAmphipathic lipid aggregates that form in waterMembrane 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 membrane From LehningerPrinciples of BiochemistryHigher the proportion of saturated fatty acid, higher is the transition temperature.From LehningerPrinciples of BiochemistrySterol 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 conditionsFrom LehningerPrinciples of BiochemistryMotion of Membrane LipidsLateral DiffusionTransbilayer orflip-flop DiffusionFrom LehningerPrinciples of BiochemistryA 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 FlippasesFrom Garrett & GrishamSinger & Nicolson defined two classes o Integral (intrinsic) proteins o Peripheral (extrinsic) proteins o We'll note a new one – lipid-anchored proteinsPeripheral, Integral & Lipid-Linked ProteinsFrom LehningerPrinciples of BiochemistryA. B. C.Lipid-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 anchors From LehningerPrinciples of BiochemistryGlycophorin in the erythrocyteA single-transmembrane-segment protein1. One transmembrane segment with globular domains on either end 2. Transmembrane segment is alpha helical and consists of 19 hydrophobic amino acids 3. Extracellular portion contains oligosaccharides (and these constitute the ABO and MN blood group determinants) Some membrane proteins span the lipid bilayerA plot for glycophorin AHydropathy Plots1From LehningerPrinciples of BiochemistryDemonstration of lateral diffusion of membrane proteinsMembrane proteins, like membrane lipids, are free to diffuse laterally in the plane of the bilayerIntegral Membrane ProteinsHeld in the membrane by hydrophobic interactions with lipidsFrom LehningerPrinciples of BiochemistryErythrocyte membranes contain a variety of proteinsThe human erythrocyte membrane skeletonAsymmetric distribution of phospholipids between the inner & outer monolayers of erythrocyte plasma membraneBacteriorhodopsin, a membrane-spanning proteinFrom LehningerPrinciples of BiochemistryBound retinal3-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 helicesFrom LehningerPrinciples of BiochemistryPorin 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 bacteria From LehningerPrinciples of BiochemistryPorinsFound 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 poreStructure of FhuA, an iron transporter from E.coli22-stranded barrel(seen as a hollow pipe)Cork domain(keeps the channel closed)Binding of ferrichrome-iron on the outer surface causes conformation change & moves the ferrichrome into the barrelPorins are transmembrane channels for small moleculesFrom LehningerPrinciples of BiochemistryGap 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 channelLikely 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 / secondFrom LehningerPrinciples of
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