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Biological Membranes

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  1. Biological Membranes

  2. Biological Membranes • Organized assemblies of lipids, proteins and small amounts of carbohydrates • Regulate composition of intracellular medium by controlling flow of nutrients, waste products, ions, etc. in and out of cell • Scaffolding • Oxidative phosphorylation • Photosynthesis • Nerve impulses • Hormone receptors

  3. Types of Membrane Lipids • Glycerophospholipids • Sphingolipids • Cholesterol

  4. Membrane Glycerophospholipids

  5. Sphingolipids(Sphingomyelin)

  6. Cholesterol

  7. Amphiphilicity

  8. Properties of Lipid Aggregates Micelles, Liposomes, and Bilayers Driving Force = Hydrophobic Effect

  9. Van der Waals Envelope(Fatty Acids) Figure 9-13a

  10. Micelle(single-tailed lipids)

  11. Cylindrical Lipids Individual lipids are cylindrical -cross-section of head = tail

  12. Liposomes

  13. Electron Micrograph of Liposome Figure 9-15

  14. Properties/Uses of Liposomes Single Bilayer (inner and outer leaflets) Delivery of Therapeutic Agents Stable — purification Manipulate internal content Delivery — fusion with plasma membrane

  15. Bilayer Formation by Phospholipids Aqueous phase Aqueous phase

  16. Membrane composition

  17. Phase Transition in a Lipid Bilayer(Transition Temperature) Figure 9-18

  18. Transition Temperature=more Rigid; =more fluid • Increases with chain length • Tm = more rigid • Increases with degree of saturation • More saturated = more rigid • Cholesterol decreases membrane fluidity

  19. Membrane composition

  20. Which membrane composition is more rigid?

  21. Asymmetry within Membranes

  22. Lipid Diffusion in Membranes

  23. Transverse Diffusion Figure 9-16a

  24. Flippase/Floppase/Scramblase

  25. Lateral Diffusion Figure 9-16b

  26. Permeability of Lipid Bilayer Semi-permeable Hydrophilic molecules Non-permeable Facilitated diffusion Active transport Hydrophobic molecules Permeable Simple diffusion

  27. Membrane Carbohydrates • Mostly oligosaccharides • Variety of sugars • Glycolipids • Glycoproteins Glycoprotein

  28. Membrane Proteins Peripheral or Extrinsic Proteins Integral or Intrinsic Proteins

  29. Peripheral or Extrinsic Proteins • Easily dissociated • High ionic strength • pH changes • Free of attached lipid • Water-soluble • (e.g. cytochrome c) • Normal amino acid composition

  30. Integral or Intrinsic Proteins • Not easily dissociated or solubilized • Detergents • Chaotropic agents — disrupt water structure • Retain associated lipid • >average hydrophobic amino acds • Significant number hydrophilic amino acds • Asymmetrically oriented amphiphiles • Trans-membrane proteins

  31. Single transmembrane domain Integral Membrane proteins Multple transmembrane domains Lipid Linked

  32. Lipid Linked Proteins

  33. Prenylated Proteins Page 268

  34. Prenylated Proteins Page 268

  35. Glycosylphosphatidylinositol (GPI) Linked Proteins

  36. Core Structure of the GPI Anchors of Proteins Figure 9-24

  37. Composition of Biological Membranes(protein-lipid ratios) • Myelin ~0.23 • Eukaryotic plasma membrane ~1.0 (50% protein and 50% lipid) • Mitochondrial inner membrane ~3.2

  38. Asymmetric Orientation

  39. Detecting Asymmetric Orientation of Membrane Proteins Surface Labeling Proteases

  40. Transmembrane Proteins May contain -Helices (and -Sheets)

  41. Human Erythrocyte Glycophorin A Figure 9-20

  42. Identification of Glycophorin A’s Transmembrane Domain Figure 9-21

  43. Structure of Bacteriorhodopsin Figure 9-22

  44. X-Ray Structure of E. coli OmpF Porin Figure 9-23a

  45. X-Ray Structure of E. coli OmpF Porin Trimer Figure 9-23b

  46. Functions of Membrane Proteins • Catalysis of chemical reactions • Transport of nutrients and waste products • Signaling

  47. Hydrophillic compounds need help

  48. Glucose transporter

  49. Plasma Membrane StructureFluid Mosaic Model Figure 9-25