Lecture 14 Ion transport

Question 1

Why are proteins necessary for molecules to cross membranes?

Answer 1

• Cells and cellular organelles are bounded by a membrane
• Impermeable to water soluble and charged molecules
• Therefore, specific proteins exist to enable such molecules to be transported across membranes
• These specific proteins can enable crossing of the membrane in one of TWO ways:
• As CHANNELS or as PUMPS (later)

Question 2

How do channel proteins operate?

Answer 2

• Channel proteins create holes in the membrane through which molecules can pass
• The size of the hole controls the maximum size of
  molecules which may pass

Question 3

How does the potassium channel operate?

Answer 3

• The potassium channel has a relatively short polypeptide chain as one subunit - They form a tetramer of subunits
• They form a channel by restricting the size of the hole
  down the middle
  -This is the selectivity filter
• Made from five conserved residues
• Peptide bond carbonyls perform this role
• Potassium channels only allow bare ions through
• Water of solvation is removed from around most of the ion when it is passed through the channel
• The only role for water is to act as a buffer to keep separate K+ ions away from each other in the channel
• Some proteins form channels which open ONLY in response to a specific activator
  +Example: The acetylcholine receptor channel

Question 4

How does an acetylcholine receptor channel operate?

Answer 4

• Consists of five polypeptide subunits:
  + α (x 2), β, γ, and δ
• These form a channel with approx. 5-fold symmetry
  (pseudo five-fold)
• The channel allows both sodium ions (Na+) and potassium ions (K+) to pass through it
• Both these ions are positively charged
• Three rings of residues with negatively charged side chains in the channel allow Na+ and K+ through but repel negatively charged ions
• Act as selectivity filter for the channel
• This channel must be ACTIVATED by the binding of a
  molecule of the neurotransmitter

Question 5

What kind of channel protein is acetylcholine?

Answer 5

• Acetylcholine is used to conduct nerve impulses across synapses in the nervous system
• Binding of acetylcholine (to a site on each of the α subunits) moves all the subunits leading to opening of the channel
• This is an allosteric effect
• The binding of acetylcholine causes the helix that lines the channel from each of the subunits to rotate
• Moves LARGE residue out of the channel
• Moves SMALL residue into the channel
• Leaves a HOLE in the centre as a result
• A channel protein like this that opens in response to the binding of a molecule is called: A LIGAND GATED CHANNEL
• Some proteins ONLY open as channels in response to the voltage created across the membrane by the presence of charged molecules (ions)
  + Example: Voltage-gated sodium channel

Question 6

What is voltage-voltage sensitivity?

Answer 6

• Further nerve impulse transmission in the nervous system occurs through “waves of depolarization”
• Sodium and potassium ions flow into and out of
  “nerve-cell” cells (neurons) in response to changes
  in the voltage across the neuron membrane
• Such a system requires that there be an ion transport
  system that can sense the VOLTAGE across that neuron membrane
• A voltage across a membrane means:
• More positive charges than negative charges on one side
• More negative charges than positive charges on the other side
• Therefore, what is required is a system that is sensitive to positive (or negative) charges

Question 7

Describe how a voltage gated sodium channel facilitates movement of molecules.

Answer 7

• This protein forms a channel through the membrane which allows sodium ions (Na+) through, but ONLY when there is a certain voltage across the neuron membrane
• This is therefore a VOLTAGE GATED CHANNEL
• The sodium channel is one long polypeptide, separated into four sections which sit in the membrane
• Each section has SIX α HELICES (S1-S6)
• Since like charges repel and unlike charges attract, any channel protein that includes some charges might be sensitive to opposing charges
• The voltage gated sodium channel contains a feature
  sensitive to the voltages across membranes called:
• An S4 HELIX as part of an S1-S4 helix group
• This is an α helix that contains charged amino acid residues and is free to move in response to surrounding charges on ions on either or both sides of the membrane
• When the S4 helix moves it triggers a protein
  conformation change that opens the channel in the protein
  -The selectivity of the sodium channel arises from two principle features:
• Ion size = Tailored towards Na+
• Charge = Similar sized molecules to Na+ cannot get through if they are not positively charged

Question 8

How do voltage gated potassium channels operate?

Answer 8

• Voltage gated potassium channels help restore neurons to their resting potential after the voltage gated sodium channels have opened
• Again, these channels have a smaller polypeptide chain than the sodium channels
• Again, about one quarter the size
• They form a tetramer of subunits however again, each subunit has an S4 helix
• The architecture of these channels is similar therefore to that of the sodium channel
• The S1-S4 group is part of a “PADDLE” that is proposed to move when sensing voltage changes
• Voltage gated potassium channels are highly selective to K+ = ~ 100 fold more permeable to K+ than Na+