Aquaporins

Water molecules are polar in nature, hence, they cannot diffuse rapidly across the non-polar lipid bilayer.

Therefore, cell membranes have special channel proteins called aquaporins that selectively transport water molecules according to the osmotic gradient.

These proteins function as tetrameric clusters, with each monomer acting as an independent water channel.

A single aquaporin channel comprises six transmembrane alpha-helices connected by five loops, of which two hydrophobic loops contain a conserved amino acid motif – Asparagine-Proline-Alanine or NPA.

The NPA residues form a constricted site that confines the water molecules into a single file as they pass through the channel.

Additionally, repulsion from the hydrophobic walls of the channel accelerates the movement of the water molecules.

While passing through the constricted site, the oxygen atoms interact with the asparagine residues of the NPA motif, interrupting the hydrogen-bonded bridges between the water molecules that would have otherwise allowed protons to hop in and pass through the channel.

Disruption of the hydrogen-bonded network prevents proton movement, making the aquaporin selectively permeable to water.