The aquaporin water channel

a. Cartoon rendering of human aquaporin-5 tetramer looking along the four water channels. The two short helices containing the NPA sequence are shown in blue.

b. A cutaway view of the water channel in one of the monomers. The narrowest part of the channel is where the two short helices meet. Note the location of Asn76 and Asn192 at this restriction. The two helical macrodipoles and Arg195 provide an electrostatic barrier to H3O+ passage.

c. Schematic view of the aquaporin channel, showing the electrostatic repulsion of H3O+ and the reorientation of the water molecules as they pass through the central restriction.