The Potassium Ion Channel

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The Potassium Ion Channel

• Presentation by Mark Ams

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The K ion channel

• K channels (and others) are proteins that switch
  between closed and open conformations in response
  to an external stimulus called gating.
• These protein conformational changes account for
  the ion channels ability to select out only K
  ions allowing many fundamental biological
  processes to occur.
• So, how does the K channel work?

A simplified ion channel cartoon.
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What is the structure of the K ion channel?

• The x-ray structure of Streptomyces lividanss K
  ion channel was solved by R. MacKinnon and
  coworkers. Their discoveries are as follows
• K channel is a tetramer with 4-fold symmetry
  about a central pore.
• The four subunits around the central pore contain
  2 transmembrane a-helices
• inner helix faces into the pore
• outer helix faces out to the lipid bilayer.

A
B
Views of the tetramer. A) View
from extracellular side. B) View perpendic- ular
to A.
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A pore that blossoms...

• The pore subunits open like a blooming flower
  facing the outer cell.
• Within the petals 4 inner helices arranged like
  poles of a teepee,4 pore helices, and a
  selectivity filter tuned to select only K
  cations near the extracellular surface.

Elements of the K Channel. The selec- tivity
filter is orange and the central cavity is marked
by the red asterisk. The helical segments are
the outer helix (M1), pore helix (P), and inner
helix (M2). The gate is formed by the inner
helix bundle.
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The ion conduction pore
A.

• From inside the cell, the pore widens from a
  tunnel to a 10 Å cavity.
• A K ion can move thru the internal pore and
  cavity as a hydrated species.
• But, the selectivity filter separating the cavity
  from extracellular solution is so narrow that K
  must dehydrate to enter.
• The internal pore lining and cavity is
  hydrophobic.
• The selectivity filter, conversely, is lined only
  with polar main chain atoms of the signature
  sequence amino acids.

B.
A) The solvent-accessible surface of the K
channel. B) The entire internal pore.
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The Cavity and Internal Pore

• An ion traveling through the pore must overcome
  an energy barrier, which is highest at the
  membrane center.
• The cation, having an electric field, has a
  higher energy than its surroundings in the
  bilayer center.
• The cavity overcomes this destabilization by
  surrounding the ion with water.
• The four pore helices (pointed directly at the
  cavitys center) impose a negative electrostatic
  potential, aiding to stabilize the ion.
• The result A low resistance pathway from the
  cytoplasm to selectivity filter creating a high
  throughput.

A representation of K ion channel as an
integral protein.
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The Selectivity Filter
A.

• Two essential features of the filter
• The main chain atoms create a stack of oxygen
  rings, making numerous closely spaced sites for
  coordinating a dehydrated K ion.
• K thus has only a small distance to diffuse from
  one site to the next within the selectivity
  filter.
• The selectivity filter is basically the protein
  packing around it. The amino acid side chains
  (Val, Tyr) point away from the pore, positioned
  like a cuff around the filter.
• The result This structure acts as a layer of
  springs stretched outward to hold the pore open
  at its proper diameter.

B.
Views of the selectivity filter. A) View of the
electron density (green) in the select- ivity
filter. B) The selectivity filter with the chain
closest to the viewer removed.
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How does the selectivity filter conduct ions?
A. B. C.

• In 150mM K, the filter contains two K ions.
• The structure implies a single K ion would be
  held tightly, but presence of two K ions results
  in mutual repulsion explaining their locations
  near opposite ends of the selectivity filter.
• When a second ion enters, the attractive force
  between a K ion and the selectivity filter is
  balanced by the repulsive force between ions
  allowing conduction to occur.
• This explanation accounts for both a strong
  interaction between K ions and the selectivity
  filter and high throughput caused by
  electrostatic repulsion.

Outer Ion
Inner Ion
Cavity Ion
Ion positions in the pore. A) Rb, B) Cs
Fourier maps of two selectivity filter ions and
one cavity ion. C) Electron density map
showing density at the cavity ion position.
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How does the K channel select ions so well?

• When an ion enters the selectivity filter, it
  dehydrates.
• To overcome the energetic cost of dehydration,
  the carbonyl oxygen atoms replace of the water
  oxygen atoms of water. They come in close
  contact with the ion, and act like a water
  substitute.
• The selectivity filter is held open in such a way
  to stop Na ions (smaller radius) from entering.
• MacKinnon and coworkers propose a K ion fits in
  the filter precisely so that the energetic costs
  and gains are balanced. The selectivity filter,
  having molecular springs holding it open, stops
  the carbonyl oxygen atoms from coming close
  enough to compensate for the cost of dehydration
  of a Na ion.
• The result Na ions are not equally stabilized
  by the oxygen, allowing for the selectivity.

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Summary

• The following outlines the structure and function
  of K channels
• The pore is structured as an inverted teepee,
  with the selectivity filter held at its wide end.
• The selectivity filter is narrow, while the rest
  of the pore is wider and has an inert hydrophobic
  lining. These structural properties favor a high
  K throughput by minimizing the distance that K
  interacts with the channel.
• A large water-filled cavity aids to overcome the
  high electrostatic energy barrier facing a cation
  in the low dielectric membrane center.
• The K selectivity filter is lined by carbonyl
  oxygen atoms, providing multiple closely spaced
  sites. The filter is fixed in an optimal
  geometry so that a dehydrated K ion fits with
  proper coordination but the Na ion is too small.
• Two K ions at close proximity in the selectivity
  filter repel each other. The repulsion overcomes
  the strong interactions between the ion and
  protein, allowing rapid conduction and high
  selectivity.

Ions traveling down Abbey Channel!
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References

• Doyle, D.A. et al. The structure of the potassium
  channel molecular basis of K conduction and
  selectivity. Science 280, 69-77 (1998).
• Jiang, Y. et at. The open pore conformation of
  potassium channels. Nature 417, 523-526 (2002)
• Image 1. used from
• http//opal.msu.montana.edu/cftr/IonChannelPrimers
  /beginners.htmHow Ion Channels Work