Question

Sodium and potassium ion channels have several negatively charged residues at the entry to the channel. On what basis do K+ channels specifically select for K+ ions? (In other words, why dont K+ channels enable Na+ ions to cross the membrane?)
A. ionic radius, Na+ is to smallB. energy cost, it is to energetically costly to dehydrate Na+C. charge, Na is slightly more electronegative than K and does not interact with the channel carbonyl groupsD. charge, Na is slightly less electronegative than K and does not interact with the channel carbonyl groupsE. ionic radius, Na+ is too big

Answer 1

Answer:

A. Ionic radius, Na⁺ is to small.

Explanation:

The channel pore contains a narrow selective filter that is delimited by carbonyl oxygens (C = O) of the polypeptide skeleton. When a K⁺ ion enters the selection filter it interacts with these carbonyl oxygens, and the water molecules to which the K⁺ was attached move, allowing the dehydrated K⁺ to pass through the pore. However, a dehydrated Na⁺ is too small to interact with these carbonyl oxygens in the selective filter, which is kept open. As a consequence, this hydrated sodium ion is too large to pass through the canal.