Question

The K+-channel is very selective for K+ and efficient in the transporting speed. What structural feature most successfully explains these two functional characteristics of the channel?

Select one:

a. The selectivity channel is just wide enough for K+ ion so that no other ions can go in.

b. The selectivity channel has a structure in which the main chain oxygen atoms from the constituent subunits are arranged to form hydrogen bonds to K+ ions as if K+ is hydrated in water.

c. The channel has a wide hydrophobic vacuum whole through which K+ is shot out using the K+ concentration gradient energy.

d. The selectivity channel is made of a β-barrel, whose diameter is geometrically perfect for K+.

Answer 1

Answer:

Option b. - The selectivity channel has a structure in which the main chain oxygen atoms from the constituent subunits are arranged to form hydrogen bonds to K⁺ions as if K⁺ is hydrated in water.

The K⁺ channels are designed to allow only K⁺ ions across the membrane and block all other ions, especially Na⁺ even though Na⁺ ions are smaller than K⁺. This is accomplished by selectivity filter. Normally K⁺ ions float around on a cushion of 8 water molecules. The special proteins lining the channel have 8 molecules of oxygen from amino acid constituents oriented to their centre channel. The K⁺ molecule entering the channel has to shed all the water molecules and is instead cushioned by these oxygen molecules and is moved along the to the inside of the cell, where again it is cushioned by water molecules! Na⁺ molecules being smaller in size do not get the support of these oxygen molecules on the amino acid and hence the filter does not allow them to move in.