Sodium and potassium ion channels have several negatively charged residues at the entry to the channel. Identify the bases on which K channels specifically select for K ions, in other words, why do K channels not enable Na ions to cross the membrane

Mechanism in attachment.

Explanation:

K⁺ channels have narrow pores, which prevents the passage of larger ions. However, since Na⁺ has an ionic radius smaller than K⁺, this does not explain the selective permeability of these channels towards K⁺.

In this case, the channel pore has a narrow selective filter which is delimited by the backbone carbonyl oxygens (C=O). When a K⁺ ion enters in the selection filter it interacts with these oxygens, and the molecules of water bound to K⁺ are displaced, which allows the passage of the dehydrated ion K⁺ through the pore. However, a dehydrated Na⁺ is too small to interact with these carbonyl oxygens in the selective filter, which stays open. As a consequence, the Na⁺ ion remains bonded to the water molecules in a hydrated complex that is too big to pass through the channel.

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