Question

The three-dimensional structure of the potassium channel provided a rationale for the selectivity of potassium ions and the rejection of sodium ions. Which of the following is the basis for the ability of the potassium channel to discriminate between these two ions?

The sodium ion is too small to pass through the channel.

The opening of the channel is structurally arranged such that potassium ions can bind, but is too small for sodium ions.

A restriction in the channel allows for potassium ions to be resolvated by protein ligands but is too large to effectively resolvate sodium ions.

The restriction of the channel desolvates sodium ions but not potassium ions.

Answer 1

1. The sodium ion is too small to pass through the channel.

Incorrect: Being smaller, Na⁺ ions shall theoretically pass through a larger pore size of K⁺ channel.

2. The opening of the channel is structurally arranged such that potassium ions can bind, but is too small for sodium ions.

Incorrect: Na⁺ can also bind to K⁺ channel opening.

3. A restriction in the channel allows for potassium ions to be resolvated by protein ligands but is too large to effectively resolvate sodium ions.

Correct: The carboxyl group O-atom of eight acidic amino acid residues forms a “selectivity filter”- assume eight O-atoms arranged at the periphery of a circular gate. Positively charged K⁺ ions interact with all the O-atoms of this filter and triggers a conformal change causing their passage through the ion channel. A Na⁺ ion, being smaller than K⁺ ions, can’t optimally interact with all the O-atoms of the selectivity filter, so can’t trigger its passage through the ion channel, too. channel     

4. The restriction of the channel desolvates sodium ions but not potassium ions.

Incorrect: Na⁺ is not desolvated in K⁺ ion channel, thus it can’t also pass through it.