Aquaporins

Water molecules are polar and cannot diffuse rapidly across the non-polar lipid bilayer. Aquaporins are specialized channel proteins that selectively transport water molecules according to the osmotic gradient, enabling efficient water movement across membranes where passive diffusion would be too slow.

Aquaporins function as tetrameric clusters, with each monomer acting as an independent water channel. Each channel comprises six transmembrane alpha-helices connected by five loops, two of which contain a conserved NPA (Asparagine-Proline-Alanine) motif that forms a constricted site confining water molecules into single-file passage.

As water molecules pass through the NPA constricted site, oxygen atoms interact with asparagine residues, interrupting hydrogen-bonded bridges between water molecules. This disruption of the hydrogen-bonded network prevents protons from hopping through the channel, making aquaporins selectively permeable to water only.

Water-selective aquaporins are widely expressed in water-transporting organs and tissues including the kidneys, exocrine glands, and central nervous system. They are also found in the lungs, skeletal muscles, and gastrointestinal organs, where they facilitate cell migration, neuroexcitation, and epithelial fluid transport.

Aquaporin mutations or deficiency cause renal dysfunction, epilepsy, skin disease, cancer, neurological disorders, and cardiac ailments. For example, mutations in AQP1 or AQP2 impair urine concentration, while AQP2 loss-of-function mutations cause non-X-linked nephrotic diabetes insipidus, characterized by unusually high urine output.

Aquaglyceroporins transport both water and glycerol, playing roles in adipocyte metabolism, skin hydration, and cell proliferation. Some aquaporins also function as gas channels, transporting volatile substances like carbon dioxide and ammonia, and are highly expressed in cells involved in gaseous exchange such as red blood cells and pulmonary capillaries.

Some plant aquaporins have evolved gated mechanisms that close in response to harsh environmental conditions such as drought, stress, or flooding. This gating prevents water exchange when it would be harmful to the organism, demonstrating adaptive regulation of water transport across membranes.