Question

Explain the ion selectivity of K+ channel. Why K+ can pass this channel, while Na+ cannot in spite of its smaller size?

Answer 1

Potassium channel has a selectivity filter which is characterized by this sequence TVGYG. This sequence is present in a very constricted part of the channel. The degree of constriction is so high that ion to be passed has to remove its water molecules. When the ion comes in the channel water molecules are replaced by a polar oxygen atom. All these qualities are present in K+ ions hence it easily passes through K+ channels by passing through the selectivity filter. On the other hand, Na+ ions though smaller in size in comparison to K+ ions cannot pass through this channel as it has high hydration energy. This means it is not easier to remove its water molecules from the most constricted part of the channel. Also, Na+ binding is thermodynamically unfavourable making it unable to pass through the K+ channel.