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Membrane Channels and Pumps

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Two factors determine whether a molecule will cross a membrane: ... Large residues may occlude channel by forming tight hydrophobic ring. Open State ... – PowerPoint PPT presentation

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Title: Membrane Channels and Pumps


1
Chapter 13
  • Membrane Channels and Pumps

2
Membrane Channels and Pumps
  • Lipid bilayer impermeable to ions and polar
    molecules

3
Transport of Molecules
  • Two factors determine whether a molecule will
    cross a membrane
  • (1) permeability of molecule in lipid bilayer
  • Lipophilic molecules
  • Simple diffusion High to Low
  • Increase in entropy powers transport across the
    membrane
  • (2) availability of an energy source
  • Charged or polar molecules

4
Membrane Proteins
  • Membrane proteins confer permeability
  • Protein Transporters to cross membrane
  • Channels
  • Pumps

5
Types of Transport
  • Active Transport
  • Pumps
  • Thermodynamically uphill transport of ions or
    molecules
  • Requires an input of energy (ATP or light energy)
  • Passive Transport (Facilitated Diffusion)
  • Channels
  • Thermodynamically downhill transport of ions

6
Types of Transport
  • Active Transport
  • ATP-driven pumps
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter
  • Passive Transport (Facilitated Diffusion)
  • Ion Channels
  • Gap Junctions

7
Expression of Transporters
  • Each cell type expresses a specific set of
    transporters in its plasma membrane
  • Transporters determines
  • Ionic composition inside cell
  • Substrates taken-up from cells environment
  • Types of biochemical reactions within a cell

8
Free Energy Stored in Concentration Gradients
  • Z - electrical charge of transported species
  • ?V potential across membrane
  • F Faraday Constant (96.5 kJ/V-mol) or (23.1
    kcal/V)

9
Free Energy Stored in Concentration Gradients
  • Transport of an uncharged molecule from
  • c1 1 x 10-3 M to
  • c2 1 x 10-1 M
  • at 25oC (298 K)

10
Free Energy Stored in Concentration Gradients

11
Free Energy Stored in Concentration Gradients

12
Free Energy Stored in Concentration Gradients

13
Types of Transport
  • Active Transport
  • ATP-driven pumps
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter
  • Passive Transport (Facilitated Diffusion)
  • Ion Channels
  • Gap Junctions

14
Active Transport
  • ATP-Driven Pump
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter

15
Active Transport
  • ATP-Driven Pump
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter

16
P-type ATPase
  • Form key phosphorylated intermediate
  • ATP hydrolysis
  • Phosphoryl group linked to a specific aspartate
    residue in ATPase
  • Aspartate residue conserved
  • ATP hydrolysis coupled to orientation change and
    affinity change in ion-binding site

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P-type ATPases
  • Na-K ATPase
  • H-K ATPase
  • Gastric
  • Pumps H into stomach to lower pH below 1.0
  • Ca2 ATPase
  • Pumps Ca2 from cytoplasm into sarcoplasmic
    reticulum of muscle cells

20
Ca2 ATPase
  • Sarcoplasmic reticulum of muscle cells
  • Integral Membrane Protein
  • 80 of sarcoplasmic reticulum membrane protein
  • Role in muscle contraction
  • Removal of Ca2 from cytosol
  • 0.1 ?M in cytosol 1.5 mM SR

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P-type ATPases
  • Forms key phosphorylated intermediate
  • Phosphoryl group linked to a specific aspartate
    residue in ATPase
  • Aspartate residue conserved
  • Conformation change
  • ATP hydrolysis
  • E1 and E2
  • Six conformational states E1, E1-(Ca2)2, E1-
    (Ca2)2 (ATP), E1- (Ca2)2 (ADP), E2-P, and E2

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Na-K Pump
  • Same mechanism
  • Three Na ions transported out of cells as result
    of phosphorylation and transition to E2 state
  • E2 binds two K ions and transport into the cells
    by hydrolysis of phosphoaspartate.

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Ion Gradient
  • Important energy storage forms
  • Animal cells intracelllular concentrations
  • High concentration of K
  • Low concentration of Na
  • Na-K ATPase / Na-K Pump
  • Specific transport system
  • Enzyme
  • Energy provided from ATP hydrolysis

31
Na-K Gradient
  • Controls cell volume
  • Renders neurons and muscle cells electrically
    excitable
  • Drives the active transport of sugars and amino
    acids

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Active Transport
  • ATP-Driven Pump
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter

34
ATP-Driven Pumps
  • ATP-Binding Cassette
  • Conformation change upon ATP binding and
    hydrolysis
  • Bound ion transported across membrane

35
ATP Binding Cassette
  • Undergo conformational changes on ATP binding and
    hydrolysis
  • Conformational change coupled with each dimeric
    transporter unit
  • Drive uptake or efflux of specific compounds or
    act as gates for open membrane channels

36
Multidrug Resistance
  • MDR or P-glycoprotein
  • Membrane protein 170 kd
  • ATP-dependent pump
  • Extrude wide range of small molecules from cells
    that express it
  • Cystic Fibrosis
  • CFTR-cystic fibrosis transmembrane conductance
    regulator
  • ATP-regulated chloride channel in plasma
    membranes of epithelial cells

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Active Transport
  • ATP-Driven Pump
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter

44
Secondary Transporters
  • Energy from concentration gradient downhill
    flow of one gradient power formation of another
    gradient
  • Antiporters
  • Drive uphill flow in opposite direction
  • Symporters
  • Drive uphill flow in same direction

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Types of Transport
  • Active Transport
  • ATP-driven pumps
  • P-type ATPase
  • ATP-Binding Cassette
  • Secondary Transporters
  • E. coli lactose transporter
  • Passive Transport (Facilitated Diffusion)
  • Ion Channels
  • Gap Junctions

52
Passive Transport Facilitated Diffusion
  • Ion Channels
  • Gap Junctions

53
Ion Channels
  • Highly selective for particular ions
  • Open and Closed States
  • Regulated transitions between open and closed
    states
  • Spontaneous conversion of open state to an
    inactivate state

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Ion Channels
  • Ligand-Gated Channel
  • Open and close in response to binding of specific
    chemicals
  • Voltage-Gated Channel
  • Open and close in response to electrical
    potential across membrane

58
Ion Channels Propagation of Nerve Impulses
  • Ligand-Gated Channel
  • Acetylcholine Receptor Channel
  • communicates nerve impulse between certain
    neurons
  • Voltage-Gated Channels
  • Na Channel
  • K Channel
  • Conduct the nerve impulse down the axon of a
    neuron

59
Axon
  • The part of a nerve cell or neuron that transfers
    a nerve impulse from the nerve cell body to a
    synapse with another cell.

60
Nerve Cell
61
Nerve Cell
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Synaptic Cleft
  • The minute space between the cell membrane
  • of an axon terminal and that of the target cell
  • with which it synapses.
  • 50 nm

64
Ion Channels Propagation of Nerve Impulses
  • Ligand-Gated Channel
  • Acetylcholine Receptor Channel
  • communicates nerve impulse between certain
    neurons
  • Voltage-Gated Channels
  • Na Channel
  • K Channel
  • Conduct the nerve impulse down the axon of a
    neuron

65
Ion Channels Propagation of Nerve Impulses
  • Ligand-Gated Channel
  • Acetylcholine Receptor Channel
  • communicates nerve impulse between certain
    neurons
  • Voltage-Gated Channels
  • Na Channel
  • K Channel
  • Conduct the nerve impulse down the axon of a
    neuron

66
Ligand-Gated ChannelAcetylcholine Receptor
  • Integral membrane protein that responds to the
    binding of the neurotransmitter acetylcholine
  • Communication of nerve impulse
  • Neurotransmitter acetylcholine
  • Binding of acetylcholine to receptor changes the
    ionic permeability
  • Large influx of Na and smaller outflux of K

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Acetylcholine Receptor
  • Purification
  • Nonionic detergent to postsynaptic membrane
    prepartion
  • Affinity chromatography using immobilized
    cobratoxin
  • Structure
  • 268 kd
  • Pentamer ?2, ?, ?, ?
  • Arranged in a ring that creates a pore through
    the membrane
  • Acetylcholine binds at the ??-? and ?-? interfaces

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Mechanism of Channel Opening???
  • Alternating ridges of small polar or neutral
    residues and large nonpolar residues
  • Closed State
  • Large residues may occlude channel by forming
    tight hydrophobic ring
  • Open State
  • Acetylcholine binding could allow allosteric
    rotation of membrane-spanning helices to allow
    small polar residues to line the pore and allow
    passage of Na and K

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Ion Channels Propagation of Nerve Impulses
  • Ligand-Gated Channel
  • Acetylcholine Receptor Channel
  • communicates nerve impulse between certain
    neurons
  • Votage-Gated Channels
  • Na Channel
  • K Channel
  • Conduct the nerve impulse down the axon of a
    neuron

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Nerve Cell
81
Voltage-Gated Channel
  • Na Channel
  • K Channel
  • Propagation of Nerve Impulses

82
Membrane Polarization
  • A typical neuron
  • -60 mV at rest (polarized)
  • 30 mV at peak of action potential (depolarized)
  • Amplified depolarization propagated along nerve
    terminal
  • Polarized
  • Membrane of resting neuron
  • Inside of membrane negatively charged relatively
    to the outside of the membrane

83
Membrane Polarization
  • Depolarized
  • Nerve impulse causes membrane to undergo a large
    and short-lived increase in ion permeability
    across the membrane
  • First Na influx is so great that for a msec
    the inside becomes positively charged relative to
    the outside.
  • Second Permeability of Na returns to normal
    and the permeability of K outflux increases and
    restore the inside of the cell to its original
    negative state within few msec.
  • Efficient signaling over large distances only
    1/one million of sodium and potassium ions in
    nerve cells flow across plasma membrane during
    action potential

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Potassium Channel
  • Pore forming segment S5 and S6
  • Tetramer of identical subunits that form pore in
    shape of a cone than runs through the center of
    the structure
  • Selectivity Filter
  • Thr-Val-Gly-Tyr-Gly (TVGYG)

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Rapid Rates of Transport
  • Tight Binding
  • How can rapid flow of ions be achieved with tight
    binding of K ions to the carbonyl oxygen
    (channel)??????

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Ca2 Channel Selectivity
  • Glutamate residue in region between segments S5
    and S6 plays a major role in determining ion
    selectivity

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Action Potential
  • A momentary change in electrical potential on
    the surface of a nerve or muscle cell that takes
    place when it is stimulated, especially by the
    transmission of a nerve impulse
  • Stimulating a nerve fiber causes an action
    potential to spread across the nerve cell, making
    it contract.

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Gap Junctions
  • Cell - to - cell channels
  • Connect interiors of contiguous groups of cells
  • Polar molecules with mass of less than 1 kd can
    pass through gap junctions
  • Important for intercellular communication

110
Gap Junctions
  • Traverse two membranes
  • Connect cytosol to cytosol
  • Connexons forming channel synthesized by
    different cells
  • Form readily when cells brought together
  • Open for seconds minutes
  • Closed by Ca2 and H
  • Also controlled by membrane potential and
    hormone-induced phosphorylation

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Connexins
  • Several inherited disorders of humans such as
    certain congenital heart defects and certain
    cases of congenital deafness have been found to
    be caused by mutant genes encoding connexins.

114
Aquaporins
  • Specific water channels
  • 24 kd
  • Six membrane-spanning alpha helices
  • Two loops containing hydrophilic residues line
    channel
  • Flow rate 106 water molecules per sec
  • Do not conduct protons positive residues in
    center of channel

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