Chapter 12 Biological Membranes & Transport

1. Chapter 12 Biological Membranes & Transport For chapter 12 Focus on the material covered in lectures Will not be tested on materials covered in Pages 424 - 429

2. Fluid Mosaic Model for Membrane Structure

3. Amphipathic lipid aggregates that form in water or Vesicle Bilayers are noncovalent, cooperative structures

4. Monolayer of oil molecules at an air-water interface

6. Membrane Phase Transitions The "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

7. Higher the proportion of saturated fatty acid, higher is the transition temperature.

8. Sterol content of a membrane has 2 effects on membrane fluidity Below the transition temperature: Insertion of rigid planar sterol prevents highly ordered packing of fatty acid side chains Membrane fluidity Above the transition temperature: Rigid planar sterol reduces the freedom of neighboring fatty acid side chains Membrane fluidity

9. Cells regulate their lipid composition to achieve a constant membrane fluidity under various growth conditions

10. Motion of Membrane Lipids Lateral Diffusion Transbilayer or flip-flop Diffusion

11. Flippases A 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

12. Demonstration of lateral diffusion of membrane proteins Membrane proteins, like membrane lipids, are free to diffuse laterally in the plane of the bilayer

13. Restricted motion of the erythrocyte chloride-bicarbonate exchanger

14. Asymmetric distribution of phospholipids between the inner & outer monolayers of erythrocyte plasma membrane

15. Structure of Membrane Proteins Singer & Nicolson defined two classes - Integral (intrinsic) proteins - Peripheral (extrinsic) proteins - We'll note a new one – lipid-anchored proteins

16. Peripheral & Integral Proteins

17. Some membrane proteins span the lipid bilayer Glycophorin in the erythrocyte A single-transmembrane-segment protein • One 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)

18. Lipid-linked membrane proteins Covalently attached lipids anchor membrane proteins to the lipid bilayer A 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 Glycosyl phosphatidylinositol (GPI) anchor

19. Integral Membrane Proteins Held in the membrane by hydrophobic interactions with lipids

20. Bacteriorhodopsin, a membrane-spanning protein

21. 3-D structure of the photosynthetic reaction center of purple bacterium First integral membrane protein to have its structure determined by X-ray diffraction methods Prosthetic group (light-absorbing pigments) Residues that are part of the trans-membrane helices

22. Hydropathy Plots

23. Hydropathy Plots 1

24. Porin FhuA, an integral membrane protein with b-barrel structure Not all integral membrane proteins are composed of transmembrane a helices Porin allows certain polar solutes to cross the outer membrane of bacteria

25. Porins Found 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 pore

26. Why 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 segments

27. Integral membrane proteins mediate cell-cell interactions & adhesion 4 examples of integral protein types that function in cell-cell interaction Serve as receptors & signal transducers Essential part of the blood-clotting process

28. Gap Junctions Vital 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)

29. Induces closure of gap junction central channel 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

30. Membrane fusion is central to many biological processes Membranes undergo fusion without losing its integrity

31. Membrane fusion during viral entry into a host cell

32. Movements of solutes across a permeable membrane Electrically neutral solutes Electric gradient or membrane potential

33. Energy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membrane Energy of activation Facilitated diffusion or passive transport

34. Aquaporins form hydrophilic transmembrane channels for the passage of water Proposed structure of aquaporin channel (Formed by 4 monomers) Likely transmembrane topology of an aquaporin, AQP-1 Monomer Water flows through the channel in single file at the rate of 5 X 108 molecules / second

36. Glucose transporter of erythrocytes mediates passive transport Monomer 1 12 Proposed structure of GluT1

37. A helical wheel diagram Shows the distribution of polar& non-polar residues on the surface of a helical segment

38. Side-by-side association of 5 or 6 amphipathic helices Polar

39. Model of glucose transport into erythrocytes by GluT1 T1 & T2 are 2 different conformations T1 has glucose binding site on the outer surface of the membrane T2, with the binding site on the inner surface

40. Three general classes of transport system • Differ in # of solutes • transported • & • 2) the direction in which each is transported

41. Summary of transport types X

42. Types of transport Passive: Transported species always moves down its electrochemical gradient and it is not accumulated above the equilibrium point ATP not required Active:Results in accumulation of solute above the equilibrium point ATP is required

43. Three types of ion-transporting ATPase

45. Na+K+ ATPase • In animal cells, this active transport system is responsible for • maintaining intracellular • Na+ and K+ concentrations • for generating transmembrane • electrical potential

46. Postulated mechanism of Na+ andK+ transport by the Na+K+ ATPase

47. Na, K Transport • Hypertension involves apparent inhibition of sodium pump. (Inhibition in cells lining blood) • Studies show this inhibitor to be ouabain!

48. A defective ion channel causes cystic fibrosis CF is the result of one amino acid change in the protein CFTR, a chloride ion channel Topology of cystic fibrosis transmembrane conductance regulator, CFTR

49. Ionophores

50. Gramicidin A classic channel ionophore • Linear 15-residue peptide - alternating D & L • Structure in organic solvents is double helical • Structure in water is end-to-end helical dimer • Unusual helix - 6.3 residues per turn with a central hole - 0.4 nm or 4 A diameter • Ions migrate through the central pore