Aquaporin 1 water channel
Bos taurus (bovine)
The membranes of red blood cell or renal cell are permeated by water channels. Large amounts of water, about a billion water molecules per second, travel through these channels in response to osmotic gradients. Rapid transport and water selectivity is important for the function of water channels. For example, the kidney reclaims water from the urine though these channels. If the channels were shut down we would pass 200 liters of water per day. These channels are believed to be involved in many physiological processes, and exist in various organisms. The 2003 Nobel Prize in chemistry honors their discoverer Peter Agre. Mammalian water channels are referred to as aquaporins (AQP), and classified into 10 families, AQP0 to AQP9. The AQP family can be divided into two major groups: those permeable to water but not to small organic and inorganic molecules and those permeable to glycerol or urea as well as water.
This example shows the structure of AQP1 from bovine red blood cells belonging to the first group. The functional unit of AQP1 is a tetramer with each monomer providing an independent water pore. An extracellular and a cytoplasmic vestibule of each monomer are connected by a narrow pore. Since the diameter of this small pore, about 3 angstrom is only slightly larger than that of water, 2.8 angstrom the pore function as a selective filter preventing the permeation of other molecules such as hydrated ions. This is one of the structural explanations why AQP1 has such extremely high water selectivity. Within the small pore, four waters are localized along three hydrophilic nodes, which punctuate an otherwise hydrophobic pore segment. This combination facilitates rapid water transport. Residues of the constriction region, in particular His 182, which is conserved among all known water-specific channels, play a critical role in water specificity. Waters are positioned in such a way that they do not to create an intermolecular pore penetrating hydrogen network. Thus transport of protons through these channels is also unfavourable.
Protein Data Bank (PDB)
Sui, H. Han, B.G. Lee, J.K. Walian, P. Jap, B.K.; "Structural basis of water-specific transport through the AQP1 water channel."; Nature; (2001) 414:872-878 PubMed:11780053.
author: Tomoki Matsuda