Chapter 2

1. Chapter 2 Transport of ions and small molecules across membranes By Stephan E. Lehnart & Andrew R. Marks

2. 2.1 Introduction • Cell membranes define compartments of different compositions. • The lipid bilayer of biological membranes has a very low permeability for most biological molecules and ions.

3. 2.1 Introduction • Most solutes cross cell membranes through transport proteins. • The transport of ions and other solutes across cellular membranes controls: • electrical functions • metabolic functions

4. 2.2 Channels and carriers are the main types of membrane transport proteins • There are two principal types of membrane transport proteins: • Channels • Carriers

5. 2.2 Channels and carriers are the main types of membrane transport proteins • Ion channels catalyze the rapid and selective transport of ions down their electrochemical gradients. • Transporters and pumps are carrier proteins. • They use energy to transport solutes against their electrochemical gradients. • In a given cell, several different membrane transport proteins work as an integrated system.

6. 2.3 Hydration of ions influences their flux through transmembrane pores • Salts dissolved in water form hydrated ions. • The hydrophobicity of lipid bilayers is a barrier to movement of hydrated ions across cell membranes.

7. 2.3 Hydration of ions influences their flux through transmembrane pores • By catalyzing the partial dehydration of ions, ion channels allow for the rapid and selective transport of ions across membranes. • Dehydration of ions costs energy, whereas hydration of ions frees energy.

8. 2.4 Electrochemical gradients across the cell membrane generate the membrane potential • The membrane potential across a cell membrane is due to: • an electrochemical gradient across a membrane • a membrane that is selectively permeable to ions

9. 2.4 Electrochemical gradients across the cell membrane generate the membrane potential • The Nernst equation is used to calculate the membrane potential as a function of ion concentrations. • E: equilibrium potential (volts) • R: the gas constant (2 cal mol–1 K–1) • T: absolute temperature (K; 37°C = 307.5 °K) • z: the ion’s valence (electric charge) • F: Faraday’s constant (2.3 104 cal volt–1 mol–1) • [X]A: concentration of free ion X in compartment A • [X]B: concentration of free ion X in compartment B

10. 2.4 Electrochemical gradients across the cell membrane generate the membrane potential • Cells maintain a negative resting membrane potential with the inside of the cell slightly more negative than the outside. • The membrane potential is a prerequisite for electrical signals and for directed ion movement across cellular membranes.

11. 2.5 K+ channels catalyze selective and rapid ion permeation • K+ channels function as water-filled pores that catalyze the selective and rapid transport of K+ ions. • A K+ channel is a complex of four identical subunits, each of which contributes to the pore.

12. 2.5 K+ channels catalyze selective and rapid ion permeation • The selectivity filter of K+ channels is an evolutionarily conserved structure. • The K+ channel selectivity filter catalyzes dehydration of ions, which: • confers specificity • speeds up ion permeation

13. 2.6 Different K+ channels use a similar gate coupled to different activating or inactivating mechanisms • Gating is an essential property of ion channels. • Different gating mechanisms define functional classes of K+ channels.

14. 2.6 Different K+ channels use a similar gate coupled to different activating or inactivating mechanisms. • The K+ channel gate is distinct from the selectivity filter. • K+ channels are regulated by the membrane potential.

15. 2.7 Voltage-dependent Na+ channels are activated by membrane depolarization and translate electrical signals • The inwardly directed Na+ gradient maintained by the Na+/K+-ATPase is required for the function of Na+ channels.

16. 2.7 Voltage-dependent Na+ channels are activated by membrane depolarization and translate electrical signals • Electrical signals at the cell membrane activate voltage-dependent Na+ channels. • The pore of voltage-dependent Na+ channels is formed by one subunit, but its overall architecture is similar to that of 6TM/1P K+ channels. • Voltage-dependent Na+ channels are inactivated by specific hydrophobic residues that block the pore.

17. 2.8 Epithelial Na+ channels regulate Na+ homeostasis • The epithelial Na+ channel/degenerin family of ion channels is diverse. • The epithelial Na+ channels and Na+/K+-ATPase function together to direct Na+ transport through epithelial cell layers. • The ENaC selectivity filter is similar to the K+ channel selectivity filter.

18. 2.9 Plasma membrane Ca2+ channels activate intracellular functions • Cell surface Ca2+ channels translate membrane signals into intracellular Ca2+ signals.

19. 2.9 Plasma membrane Ca2+ channels activate intracellular functions • Voltage-dependent Ca2+ channels are asymmetric protein complexes of five different subunits. • The α1 subunit of voltage-dependent Ca2+channels forms the pore and contains pore loop structures similar to K+ channels.

20. 2.9 Plasma membrane Ca2+ channels activate intracellular functions • The Ca2+channel selectivity filter forms an electrostatic trap. • Ca2+channels are stabilized in the closed state by channel blockers.

21. 2.10 Cl– channels serve diverse biological functions • Cl– channels are anion channels that serve a variety of physiological functions. • Cl– channels use an antiparallel subunit architecture to establish selectivity.

22. 2.10 Cl– channels serve diverse biological functions • Selective conduction and gating are structurally coupled in Cl– channels. • K+ channels and Cl– channels use different mechanisms of gating and selectivity.

23. 2.11 Selective water transport occurs through aquaporin channels • Aquaporins allow rapid and selective water transport across cell membranes. • Aquaporins are tetramers of four identical subunits, with each subunit forming a pore.

24. 2.11 Selective water transport occurs through aquaporin channels • The aquaporin selectivity filter has three major features that confer a high degree of selectivity for water: • size restriction • electrostatic repulsion • water dipole orientation

25. 2.12 Action potentials are electrical signals that depend on several types of ion channels • Action potentials enable rapid communication between cells. • Na+, K+, and Ca2+currents are key elements of action potentials. • Membrane depolarization is mediated by the flow of Na+ ions into cells through voltage-dependent Na+ channels.

26. 2.12 Action potentials are electrical signals that depend on several types of ion channels • Repolarization is shaped by transport of K+ ions through several different types of K+ channels. • The electrical activity of organs can be measured as the sum of action potential vectors. • Alterations of the action potential can predispose for arrhythmias or epilepsy.

27. 2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling • The process of excitation-contraction coupling, which is initiated by membrane depolarization, controls muscle contraction. • Ryanodine receptors and inositol 1,4,5-trisphosphate receptors are Ca2+ channels. • Ca2+ ions are released from intracellular stores into the cytosol through them.

28. 2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling • Intracellular Ca2+ release through ryanodine receptors in the sarcoplasmic reticulum membrane stimulates contraction of the myofilaments. • Several different types of Ca2+ transport proteins, including the Na+/Ca2+-exchanger and Ca2+-ATPase are important for • decreasing the cytosolic Ca2+ concentration • controlling muscle relaxation

29. 2.14 Some glucose transporters are uniporters • To cross the blood-brain barrier, glucose is transported across endothelial cells of small blood vessels into astrocytes.

30. 2.14 Some glucose transporters are uniporters • Glucose transporters are uniporters that transport glucose down its concentration gradient. • Glucose transporters undergo conformational changes that result in a reorientation of their substrate binding sites across membranes.

31. 2.15 Symporters and antiporters mediate coupled transport • Bacterial lactose permease functions as a symporter. • It couples lactose and proton transport across the cytoplasmic membrane. • Lactose permease uses the electrochemical H+ gradient to drive lactose accumulation inside cells. • Lactose permease can also use lactose gradients to create proton gradients across the cytoplasmic membrane.

32. 2.15 Symporters and antiporters mediate coupled transport • The mechanism of transport by lactose permease likely involves inward and outward configurations. • They allow substrates to: • bind on one side of the membrane and to • be released on the other side • The bacterial glycerol-3-phosphate transporter is an antiporter that is structurally related to lactose permease.

33. 2.16 The transmembrane Na+ gradient is essential for the function of many transporters • The plasma membrane Na+ gradient is maintained by the action of the Na+/K+-ATPase. • The energy released by movement of Na+ down its electrochemical gradient is coupled to the transport of a variety of substrates. • The Na+/Ca2+-exchanger is the major transport mechanism for removal of Ca2+ from the cytosol of excitable cells.

34. 2.16 The transmembrane Na+ gradient is essential for the function of many transporters • The gastrointestinal tract absorbs sugar through the Na+/glucose transporter. • The Na+/K+/Cl–-cotransporter regulates intracellular Cl– concentrations. • Na+/Mg2+-exchangers transport Mg2+ out of cells.

35. 2.17 Some Na+ transporters regulate cytosolic or extracellular pH • Na+/H+ exchange controls intracellular acid and cell volume homeostasis. • The net effect of Na+/HCO3–-cotransporters is to remove acid by directed transport of HCO3–.

36. 2.18 The Ca2+-ATPase pumps Ca2+ into intracellular storage compartments • Ca2+-ATPases undergo a reaction cycle involving two major conformations, similar to that of Na+/K+-ATPases. • Phosphorylation of Ca2+-ATPase subunits drives: • conformational changes • translocation of Ca2+ ions across the membrane

37. 2.19 The Na+/K+-ATPase maintains the plasma membrane Na+ and K+ gradients • The Na+/K+-ATPase is a P-type ATPase that is similar to the Ca2+-ATPase and the H+-ATPase. • The Na+/K+-ATPase maintains the Na+ and K+ gradients across the plasma membrane. • The plasma membrane Na+/K+-ATPase is electrogenic: • it transports three Na+ ions out of the cell for every two K+ ions it transports into the cell.

38. 2.19 The Na+/K+-ATPase maintains the plasma membrane Na+ and K+ gradients • The reaction cycle for Na+/K+-ATPase is described by the Post-Albers scheme. • It proposes that the enzyme cycles between two fundamental conformations.

39. 2.20 The F1Fo-ATP synthase couples H+ movement to ATP synthesis or hydrolysis • The F1Fo-ATP synthase is a key enzyme in oxidative phosphorylation. • The F1Fo-ATP synthase is a multisubunit molecular motor. • It couples the energy released by movement of protons down their electrochemical gradient to ATP synthesis.

40. 2.21 H+-ATPases transport protons out of the cytosol • Proton concentrations affect many cellular functions. • Intracellular compartments are acidified by the action of V-ATPases. • V-ATPases are proton pumps that consist of multiple subunits, with a structure similar to F1Fo-ATP synthases.

41. 2.21 H+-ATPases transport protons out of the cytosol • V-ATPases in the plasma membrane serve specialized functions in: • acidification of extracellular fluids • regulation of cytosolic pH

42. Supplement: Most K+ channels undergo rectification • Inward rectification occurs through voltage-dependent blocking of the pore.

43. Supplement: Mutations in an anion channel cause cystic fibrosis • Cystic fibrosis is caused by mutations in the gene encoding the CFTR channel. • CFTR is an anion channel that can transport either Cl– or HCO3–. • Defective secretory function in cystic fibrosis affects numerous organs.