Lecture 3 Flashcards
What is selectivity
Determined by the reversal potential and is a measure of how easily the ion enters the pore
What is permeability
Determined from the magnitude of the current and is a measure of how easily the ion moves through the pore.
What is the Vrev of an ion channel determined by
The relative selectivity of that channel
What will the Vrev of a selective ion channel be close to
The Nernst potential of that ion, hints at the dominant channel within that cell.
How is the relative selectivity for an ion channel worked out
Ratio of selectivity between ions is worked out. Done by taking two different experimental conditions with different ion compositions intra/extracellularly. The different values can then be put into the Goldman equation
Vrev (B) - Vrev (A) = Shift in Vrev
Shift rev = Goldman (B) - Goldman (A)
What is the highly conserved signature sequence of amino acids lining the pore region of K channels
TXXTXGYG
Essential for a common property of K channels i.e. K selectivity.
How was the importance of the signature region of K channels for their relative selectivity experimentally investigated
Kv1.X shaker channels had their amino terminus cut off, removing the ball and chain fast inactivation. Mutations were made in positions 1-8 along the signature sequence and an IV curve was plotted for each.
What was the result of mutations in the signature sequence of the Kv1.X shaker channels
Four of the mutants showed a loss of selectivity. amino acids in position 1,5,6 and 8 were found to be important. In positions 6 and 8 mutations had ratios of PNa:PK of 1 meaning selectivity was completely abolished by the mutation of a single amino acid in the signature sequence.
In what scenario would an inwardly rectifying K channel see an efflux of K ions
Inward rectification is merely a property of the channel, whether or not K ions move in or out of the cell is dependent on the membrane potential. A highly positive membrane potential will cause K ions to leave the cell. i.e. cardiac repolarisation
Does each pore region in Na share a highly conserved amino acid sequence like K channels
No, Each pore region is entirely different.
How were pore regions of Nav channels mutated experimentally to investigate selectivity
Single amino acids in each pore region were mutated to a cysteine and the effects of a single mutation were observed. D1 = position D400 D2 = E755 D3 = K1237 D4 = A1529
What was the result of mutating a single amino acid in the D1/D2 pore regions
The size of the current went down but the channel was still selective for Na, this mutation therefore had an impact on the permeability of the channel.
What was the effect of the single mutation in the D3 and D4 pore regions
Single mutation was capable of changing the selectivity of the ion channel from K for Ca - thought that Ca branched from Na channels in the past so have similarity.
