Chapter 10 Membrane Transport

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

2. Thermodynamics of Transport

10. Mammalian Cell inside 12 mM Na+ 140 mM K+ outside 150 mM Na+ 4mM 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 106 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 (Ca2+) 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 Ca2+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 Ca2+ 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.

18. KcsAK+ channel

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 (DH>0) Ion Coordination: forms favorable molecular interactions (DH<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 (Ca2+…): Channels open and close in response to Ca2+ binding Voltage (changes in membrane potential): Channels open and close in response to Ca2+ 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. Outside open closed Inside Figure 10-9

25. Outside Figure 10-9a

26. Figure 10-9b,c

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

28. 10 m/sec

29. not covered Figure 10-3

30. not covered Figure 10-6b

31. not covered Figure 10-10

32. not covered Figure 10-11a

33. not covered Figure 10-11b

34. not covered Figure 10-12

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. not covered Box 10-1a

37. not covered Box 10-1b

38. not covered Box 10-2

39. not covered Figure 10-14

40. not covered Page 310

41. not covered Figure 10-15

42. not covered Figure 10-16

43. not covered Page 312

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

46. not covered Box 10-3

47. not covered Figure 10-18

48. not covered Figure 10-19

49. not covered Figure 10-20

50. not covered Figure 10-21