1. Chapter 10 Membrane Transport Chapter 10 Membrane Transport Biochemistry I Dr. Loren Williams Biochemistry I Dr. Loren Williams Revised 03/11/2013

10. Mammalian Cell inside 12 mM Na + 140 mM K + outside 150 mM Na + 4 mM K +

11. Passive Transport Movement of molecules across membranes driven by electochemical potential Mechanism is either simple diffusion or facilitated (mediated) diffusion Not thermodynamically linked to other processes No ATP hydrolysis Mediation of transport across membranes required for charged, polar or large molecules 1) Ionophores lipid-soluble molecules synthesized by microorganisms facilitate transport of ions across membrane. Valinomycin a potassium-specific passive transporter a dodecadepsipeptide antibiotic produced by several Streptomyces strains highly selective for potassium ions over sodium and other ions Kd for potassium is 10 6 Kd for sodium is 10. facilitates transport of K+ down the electrochemical potential gradient. Deadly

12. Figure 10-1

15. Sodium (Na + ) Oxidation state: +1 Ionic radius: 0.95 Å Preferred ligands: O Number of ligands: 6-7 Preferred geometry: octahedral (not a strong preference) Potassium (K + ) Oxidation state: +1 Ionic radius: 1.33 Å Preferred ligands: O Number of ligands: 4-7 Preferred geometry: variable, octahedral Calcium (Ca 2+ ) Oxidation state: +2 Ionic radius: 0.99 Å Preferred ligands: O Number of ligands: 6-10 Preferred geometry: variable

16. Passive Transport 2) Ion Channels integral membrane proteins found in the membranes of all cells (necessary to keep cells from exploding) highly selective for specific ions (K + vs Na + vs Ca 2+ etc) fast: rate of transport is close to the diffusion limit very tightly regulated gated: flow of ions across the cell membrane is turned off or no in response to stimuli Sensors: pH, ligands, voltage, etc K + channels and anion channels hyperpolarize cells (cause the membrane potential to become more negative), Na + and Ca 2+ channels and non-selective cation channels depolarize cells (cause the membrane potential to become more positive). Ion channels form pores that permit the flux of ions down their electrochemical gradient.

17. Mammalian Cell outside (high Na + ) 150 mM Na + 4 mM K + inside (high K + ) 12 mM Na + 140 mM K + Ion channels form pores that permit the flux of ions down their electrochemical gradient.

19. KcsA K+ Ion Channel 10,000-fold selectivity of K + over Na + homo tetramer four identical protein subunits two transmembrane helices, central pore three parts: a selectivity filter (extracellular side), a dilated water-filled cavity (center), gate (cytoplasmic side, proton-activated, opens at acidic pH)

20. How does ion selectivity work? How is K + distinguished from Na + ? Ion Dehydration: disrupts favorable molecular interactions (  H>0) Ion Coordination: forms favorable molecular interactions (  H<0) To enter the selectivity channel the ions must dehydrate. The enthalpy of ion coordination by the selectivity channel has to offset the unfavorable dehydration enthalpy. Na + is smaller (ionic radius 0.95 Å) than K + (ionic radius 1.33 Å). The coordination geometry in the selectivity channel is bad for Na + : The O-Na + distances are too long. The O-K + distances are just right. Optimum O - Na + distance = 2.4 Å (0.95 + 1.5) Optimum O - K + distance = 2.8 Å (1.33 + 1.5) K + is about the same size as water, Na + is smaller than water. Free energy landscape for K + is featureless throughout the channel.

21. Channels are gated (they open and close) Mechanics (touch, sound, etc): Channels open and close in response to membrane deformation. Ligands (neurotransmitters…): Channels open and close in response to ligand binding Signals (Ca 2+ …): Channels open and close in response to Ca 2+ binding Voltage (changes in membrane potential): Channels open and close in response to Ca 2+ binding

22. Voltage Gating (Kv Channel): Gate # 1 S4 helix (+++++ charged) At the resting potential (-60mV), the gate is closed. Depolarization moves S4 toward the outside (extracelluar side) of the membrane, and opens the channel.

23. Voltage Gating (Kv Channel): Gate # 2 inactivation ball Closes the channel a few msec after the S4 helix opens it. The channel does not reopen until the potential is reset,

24. Figure 10-9 Outside Inside open closed

25. Figure 10-9a Outside

26. Figure 10-9b,c

27. Action Potential o stimulation of a neuron o open Na + channel o depolarize o close Na + channel o open K + channel o hyperpolarize o close K + channel (refractory) o Slow reset to resting potential by ion pumps

28. 10 m/sec

29. Figure 10-3 not covered

30. Figure 10-6b not covered

31. Figure 10-10 not covered

32. Figure 10-11a not covered

33. Figure 10-11b not covered

34. Figure 10-12 not covered

35. Glucose Transporter: has two exclusive conformational states is specific for glucose is driven by chemical potential (concentration) is the basis of selectivity is indicated is similar to systems of amino acid transport, etc

36. Box 10-1a not covered

37. Box 10-1b not covered

38. Box 10-2 not covered

39. Figure 10-14 not covered

40. Page 310 not covered

41. Figure 10-15 not covered

42. Figure 10-16 not covered

43. Page 312 not covered

44. The Na + /K + pump (ATPase) maintains resting potential regulates cell volume, signal transducer/integrator 20% of cell's energy expenditure. 50-70% cell's energy expenditure for neurons Pumps 3 Na + out for every 2 K + in (hydrolyzes 1 ATP in the process).

45. o unphosphorylated pump binds 3 intracellular Na+ and an ATP, o phosphate is transfered from ATP to aspartate of the pump, release of ADP o conformational change exposes the Na + ions to the outside, all Na + are released outside o pump binds 2 extracellular K+ ions. o dephosphorylation of the pump, o both K+ are released on the inside o cycle completed start

46. Box 10-3 not covered

47. Figure 10-18 not covered

48. Figure 10-19 not covered

49. Figure 10-20 not covered

50. Figure 10-21 not covered

51. Figure 10-21a not covered

52. Figure 10-21b not covered

53. Figure 10-22 not covered

54. Figure 10-23 not covered

55. Figure 10-23a not covered

56. Figure 10-23b not covered