BMB 462 1st Edition Lecture 4 Outline of Last Lecture I Experimentation with Lipids II Membrane Function III Membrane Structure Composition a Common features b Lipid Composition c Fluid Mosaic Model d Protein Composition IV Introduction to Hydropathy Plots Outline of Current Lecture I Review of Hydropathy Plots II Fluid Mosaic Model a Additional proteins b Membrane dynamics III Membrane Fusion Current Lecture Concepts to remembers from previous courses lectures I Review of Hydropathy Plots a Both the inside and the outside of a membrane are polar the inner portion of the membrane is hydrophobic i To cross the membrane a protein must be partially hydrophobic to fit in the inner membrane and partially hydrophilic to interact with the aquatic exteriors ii If there are 5 peaks in a hydropathy plot you like have a protein with 5 alpha helices 1 the protein crosses the membrane 5 times so the N terminus and the C terminus will be on opposite sides of the membrane iii Beta barrel proteins are approximately 20 transmembrane segments that line a column These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 1 It only takes about 7 9 amino acids to make it across the entire membrane 2 Alternating amino acids are hydrophobic hydrophilic alternately facing the bilayer and lining the cylinder a Because of this hydropathy plots do not show beta barrels iv Polar residues include serine glutamate and lysine Nonpolar residues include phenylalanine and leucine II Fluid Mosaic Model a Additional proteins i Amphitrophic Proteins these are sometimes peripheral sometimes found in the cytosol 1 Signaling determines where these proteins are ultimately found ii Lipid Linked proteins 1 These proteins are bonded to a lipid in the membrane 2 They function while connected to the membrane act like integral proteins a Typically to be removed the membrane has to be disrupted this is not always so though i e when linked to 1 fatty acid there isn t much force needed to separate the protein 3 Lipid linked proteins contain fatty acid linkages isoprenoid linkages bio ether bonds GPI linkages glycosylated phosphatidylinositol a GPI linkages are always outside other bonds are always found inside the membrane Some GPIs are linked to organelle membranes i These do not flip flop in the membrane b Membrane dynamics i Gel vs Liquid state 1 Gel state is highly ordered there is little motion a The acyl chain movement is static b The membrane loses flexibility and runs the risk of breaking apart 2 A fluid state is characterized by a lot of motion a The components are packed tight b A very fluid membrane can let molecules in the cell doesn t want and can fall apart 3 Membrane thickness changes with temperature changes a Temperature and fluidity is a direct relationship as temperature increases membrane fluidity increases and vice versa 4 Lipid composition also impacts fluidity a As unsaturation increases fluidity increases b As the number of short carbon chains increases and the number of long carbon chains decreases fluidity increases c Things that decrease melting point increase fluidity ii Regulation of Membrane fluidity 1 As temperature increases E coli can regulate change the amount of saturated vs unsaturated lipids to decrease or increase as temperatures decrease fluidity 2 At colder temperatures cells need more fatty acids that increase membrane fluidity a i e palmitate is a shorter fatty acid linoleate has more double bonds than oleate etc iii Transverse diffusion energetics 1 For a lipid to move transversely the polar head has to go through the hydrophobic core a This makes the movement thermodynamically unfavourable and slow it takes a day to move to the opposite leaflet and requires help from enzymes 2 Transverse diffusion of proteins does not happen a Anchored proteins do not diffuse much if they are not anchored they will move laterally but much slower than lipids b Cytoskeletal proteins against the membrane cause an energetic barrier to protein movement iv Lateral diffusion energetics 1 In a liquid crystal state there is a lot of lateral movement a Because there are no covalent bonds to hold them steady it doesn t require energy to move so 1 phospholipid can move around a whole E coli in 1 second 2 Protein corrals restrict lateral movement 3 Sphingolipid and cholesterol microdomains rafts a These types of lipids like to cluster into rafts and the clustering hinders lateral movement b Cholesterol is a rigid structure but has broad rings so it likes to pack in tightly with saturated fatty acids 4 Caveolin a These are integral membrane proteins b Function the create invaginations cavities in the membrane called caveolae i Used for endocytosis and signal transduction III Membrane Fusion a This is the process of 2 originally distinct and separate lipid bilayers merging together their hydrophobic cores b It is useful for endocytosis phagocytosis transporting nutrients and waste via vesicles fertilization joining of the egg and sperm c Role of Membrane proteins i SNAREs are main proteins involved in fusion though the exact role is not well known 1 Stands for SNAP Soluble NSF Attachment Protein Receptor 2 They mediate membrane fusion with vesicles ii Proteins are also key in signal transduction and cell cell recognition communication
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