Lecture 12: Ion Channels Flashcards
How fast do ions get transported through channels?
Fast 10^8
What generates differences in ion concentration across the PM and using what?
pumps and ATP.
What do passive channels do in response to Na/K channels?
Leak some K out the cell. Na/K channel does 3 Na out, 2K in.
Which ions move inwards and which outwards?
Na, Ca, cl inwards, K outwards.
How many putative ion channels are there?
over 400
What are the two types of ion channels in humans?
voltage and ligand gated
What do they play key roles in?
The transmission of noxious stimuli from the env to the brain.
What is the most diverse channel family?
The K channel
What can K channels respond to?
Voltage, ligand or heat
What are the four types of K channel?
- 2 TM domains with a P-loop between
- 6 TM domains. S4 has a role in voltage sensing in the voltage gated K channels and lots of Arg here (charged) for action potential transmission, and S1-4 is the gating mechanism
- 8TM and 2 P loops
- 4TM and 2 P loops. 2 repeats of type 1. Are leakage channels and targets of anaesthetics.
What is the basic structure of a K+ channel?
4 identical subunits
What does each subunit of the Kcsa (normal) channel have?
2 TM domains s5,6 and a p (pore) segment
What forms the selectivity filter and how long is it?
The 4 P segments from each subunit which partly penetrate the lipid bilayer from the exoplasmic surface. 12 A long.
What happens in the bacterial K+ channel, compare to AQP?
8TM domains form an inverted cone generating a water filled cavity called the vestibule which allows ions to go fairly deep before selectivity like AQP. The ions are channeled 1 at a time like AQP.
How was crystallisation of Kcsa achieved?
Removed flexible c terminal by cleavage with chymotrypsin to 3.2 A and then improved to 2A by binding with monoclonal Fab fragment.
Explain the structure of the K + pore
Formed by P loops forming a vestibule in the middle of the membrane. The K+ remain hydrated and 4 CO groups are spaced at exact distances to act as transient binding sites for the dehydrated K+.
Why can’t Na+ pass through?
the carbonyl oxygens are two far away for the smaller Na+ to compensate for the energy expense associated with dehydration required for entry. Not good interactions
How many ions are located in the selectivity filter and where is the other one and why?
2 and the third is in the centre of the vestibule where is is stabilised by the more negatively charged end of the pore helix.
What enhances the cation recruitment?
More negatively charged AAs at the cytosolic side
What is the selectivity of K+ over Na+?
10,000:1
Why can’t ca2+ pass through?
Bind water more tightly so harder to dehydrate
Describe the bacterial Na/K channel and how it differs to Kcsa
is non-selective and allows Na and Ca to pass through. Similar structure to Kcsa but lacks the S1 and S2 and has a wider vestibule. The Na remains hydrated in the selectivity filter which differs to K+ in Kcsa.
What is highly conserved across species? What do mutations here cause?
The P-segment. Inability to channel K+. Replacing the AA sequence of P-segment of bacterial K+channel with mammalian makes no difference.
What is the knock-on mechanism?
The filter is packed with K+ ions in direct contact and repulsion between them is key to efficient movement
