Ion channels Flashcards
What are ion channels?
- Catalyse the movement of inorganic ions down their electrochemical gradient
- Passive movement, however simple systems responsible for complex functions
What are the 3 basic properties of ion channels?
- High conduction rates (near maximum diffusion rate possible)
- Ion selectivity
- Gating (stimulus driven)
What is the typical structure of an ion channel? How does the K ion channel differ?
- 4 subunits with 6 transmembrane domains
- K channels have 4 monomer units which are linked
What are the different transmembrane regions of the K channel and what are their functions?
S5-S6 - pore lining subunits
P loop - Located between the above, important for selectivity
S4 - ‘voltage sensor’ senses changes in membrane vpltage to open or close the channel
S1-S3 - structural/regulatory
What are the intracellular components of the K channel?
- Intracellular N and C termini
- N terminus has T1 domain
What is the structure of the Kir channel?
Only 2 transmembrane domains
S5 and S6 (referred to as M1 and M2) as well as P loop
However need linking 4 subunits to form functional channel
What is the use of water in the ion channel cavity?
Ion is allowed to be surrounded by water, oxygen cage stabilises ion allowing it to pass through lipid membrane without substantial energy barriers
How are negative ions repelled from a K channel?
Helix dipoles are aligned on the channel with negative end towards towards entrance to selectivity region, functions to repel ions of a similar charge
What is the selectivity region?
Highly conserved in K channels (first observed in bacteria) of TTVGYG, orders carbonyl groups towards centre of the pore
Why is Na too small to travel through a K channel?
Customised oxygen cages, Na can only interact with two COO- groups from selectivity region (K binds with 4) therefore of high energy and cannot travel through membrane
What is the function of multiple ion occupancy?
Allows ion transfer to be rapid, ions push eachother through. Capacity for 4 however tend to have 2K and 2 water as more energetically favourable
How is the K+ channel gated?
s6 gate - Conserved intracellular glycine hinge, hinges form strong bond with helices to block off fluid filled cavity. Undergoes structural rearrangement to open
Ball and chain - insertion of inhibitory peptide into cavity can quickly block channel
Selectivity filter - can undergo structural rearrangement however this is slower
How si voltage sensed in the K channel?
- S4 helix has a range of +ve amino acids
- Pushed to intracullar side when closed
- Pushed to the intracellular side when open, unwinding helices so ions may transit
