Chapter 2

Transport of ions and small molecules across membranes

ByStephan E. Lehnart & Andrew R. Marks

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.

• Most solutes cross cell membranes through transport proteins.

• The transport of ions and other solutes across cellular membranes controls:– electrical functions – metabolic functions

2.1 Introduction

2.2 Channels and carriers are the main types of membrane transport proteins

• There are two principal types of membrane transport proteins: – Channels– Carriers

• 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.

2.2 Channels and carriers are the main types of membrane transport proteins

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.

• 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.

2.3 Hydration of ions influences their flux through transmembrane pores

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

• 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

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.

2.4 Electrochemical gradients across the cell membrane generate the membrane potential

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.

• 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

2.5 K+ channels catalyze selective and rapid ion permeation

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.

• The K+ channel gate is distinct from the selectivity filter.

• K+ channels are regulated by the membrane potential.

2.6 Different K+ channels use a similar gate coupled to different activating or inactivating mechanisms.

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.

• 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.

2.7 Voltage-dependent Na+ channels are activated by membrane depolarization and translate electrical signals

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.

2.9 Plasma membrane Ca2+ channels activate intracellular functions

• Cell surface Ca2+ channels translate membrane signals into intracellular Ca2+ signals.

• 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.

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.

2.9 Plasma membrane Ca2+ channels activate intracellular functions

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.

• Selective conduction and gating are structurally coupled in Cl– channels.

• K+ channels and Cl– channels use different mechanisms of gating and selectivity.

2.10 Cl– channels serve diverse biological functions

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.

• The aquaporin selectivity filter has three major features that confer a high degree of selectivity for water:– size restriction– electrostatic repulsion– water dipole orientation

2.11 Selective water transport occurs through aquaporin channels

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.

• 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.

2.12 Action potentials are electrical signals that depend on several types of ion channels

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.

• 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

2.13 Cardiac and skeletal muscles are activated by excitation-contraction coupling

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.

• 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.

2.14 Some glucose transporters are uniporters

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.

• 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.

2.15 Symporters and antiporters mediate coupled transport

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.

• 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.

2.16 The transmembrane Na+ gradient is essential for the function of many transporters

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–.

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

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.

• The reaction cycle for Na+/K+-ATPase is described by the Post-Albers scheme.– It proposes that the enzyme cycles between two

fundamental conformations.

2.19 The Na+/K+-ATPase maintains the plasma membrane Na+ and K+ gradients

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.

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.

• V-ATPases in the plasma membrane serve specialized functions in:– acidification of extracellular fluids– regulation of cytosolic pH

2.21 H+-ATPases transport protons out of the cytosol

Supplement: Most K+ channels undergo rectification

• Inward rectification occurs through voltage-dependent blocking of the pore.

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.