A MIRACLE THAT CAN CHANGE THE WORLD

An essential building block in the water membrane technology of Aquaporin A/S is the aquaporin molecule. The aquaporin molecule is described in the following.

Living cells are enclosed by a lipid bilayer membrane, separating the cells from other cells and their extra cellular medium. Lipid bilayer membranes are essentially impermeable to water, ions, and other polar molecules; yet, in many instances, such entities need to be rapidly and selectively transported across a membrane, often in response to an extra- or intracellular signal. The water-transporting task is accomplished by aquaporin water channel proteins (Preston et al., 1992). 


Academic version of the scientific animation movie on the aquaporin membrane technology 
     Aquaporins are crucial for life in any form and they are found in all organisms, from bacteria via plants to man. Aquaporins facilitate rapid, highly selective water transport, thus allowing the cell to regulate its volume and internal osmotic pressure according to hydrostatic and/or osmotic pressure differences across the cell membrane. The physiological importance of the aquaporin in human is perhaps most conspicuous in the kidney, where ~150-200 litres of water need to be reabsorbed from the primary urine each day, that is, aquaporin facilitated water transport is invoked when water rapidly must be retrieved from a body fluid.

In kidneys, this is made possible mainly by two aquaporins denoted AQP1 and AQP2 (11 different aquaporins are known in humans). In plants, aquaporins are also critical for water absorption in the root and for maintaining the water balance throughout the plant (Agre et al., 1998, Borgnia et al., 1999).  

Studies of water transport in various organisms and tissues suggested that aquaporins have a narrow pore preventing any large molecule, ions (salts) and even proton (H
3O+) and hydroxyl ion (OH-) flow while maintaining an extremely high water permeation rate; ~ 109molecules H2O per channel per second (Agre et al., 1998, Borgnia et al., 1999).

Until 2000 and 2001 where the first high-resolution 3D structure of AQP1 and that of the related glycerol-conducting bacterial channel protein aquaglyceroporin GlpF were reported (Fu et al., 2000; Murata et al., 2000; Ren et al., 2001; Sui et al., 2001) little was known about the origin of water selectivity.

However, based on the experimental structures, detailed computer models were put forward explaining not only the high permeation rate and the strict water selectivity but also the ability of aquaporins to prevent proton leakage (de Groot and Grubmüller, 2001; Tajkhorshid et al., 2002, Jensen et al., 2003, Zhu et al. 2003, de Groot et al., 2003,
Burykin and Warshel 2003, Ilan et al., 2004, Chakrabarti at al., 2004). In essence, the architecture of the aquaporin channel allows water molecules to pass only in single file while electrostatic tuning of the channel interior controls aquaporin selectivity against any charged species, that is, transport of any salt (ion) as well as protons and hydroxyl ions is abrogated (de Groot and Grubmüller, 2001; Tajkhorshid et al., 2002, Jensen et al., 2003, Zhu et al. 2003, de Groot et al., 2003, Burykin and Warshel 2003, Ilan et al., 2004, Chakrabarti at al., 2004). In short, this implies that only water molecules pass through the aquaporin water pore, nothing else.

In the short span of just over ten years from the discovery of aquaporins in 1992 (Preston et al., 1992) to now an almost complete atomic-level understanding of aquaporin water channel function has been reached as recently underscored by awarding the Nobel Prize in chemistry to Professor Peter Agre in 2003 for his discovery of the aquaporin water channel.

The physiological roles of water channels in both eukaryotic and prokaryotic organisms have been elucidated and their roles in living cells are becoming increasingly well documented. The understanding of aquaporins and their role in life has opened the possibility of using aquaporins in an industrial context. Aquaporin's goal is to use aquaporins as cornerstones in water filtering devices to be employed in industrial and household water filtration and purification. 

References:
 

  • Agre, P., M. Bonhivers, and M. J. Borgnia. (1998).The aquaporins, blueprints for cellular plumbing systems. Journal of Biolgical Chemistry, 273, 14659–14662
  • Burykin and A. Warshel (2003). What really prevents proton transport through aquaporin? Charge self-energy vs. proton wire proposals, Biophysical Journal 85, 3696-3706
  • Chakrabarti, N., Tajkhorshid, E., Roux, B. and Pommes, R. (2004). Molecular basis of proton blockage in aquaporins, Structure 12, 65-74
  • de Groot, B. L., and Grubmüller, H. (2001). Water permeation across biological membranes: mechanism and dynamics of aquaporin-1 and GlpF, Science 294, 2353-2357
  • de Groot, B. L., Frigato, T., Helms, V. and Grubmüller, H. (2003). The mechanism of proton exclusion in the aquaporin-1 channel, Journal of Molecular Biology 333, 279-293
  • Fu, D., Libson, A., Miercke, L. J., Weitzman, C., Nollert, P., Krucinski, J., and Stroud, R. M. (2000). Structure of a glycerol-conducting channel and the basis for its selectivity, Science 290, 481-6
  • Ilan, B., Tajkhorshid, E., Schulten, K. and Voth, G. (2004). The mechanism of proton exclusion in aquaporin water channels. PROTEINS: Structure, Function, and Bioinformatics, 55, 223-228
  • Jensen, M. O., Tajkhorshid, E., and Schulten, K. (2003). Electrostatic tuning of permeation and selectivity in aquaporin water channels, Biophysical Journal 85, 2884-2899
  • Murata, K., Mitsuoka, K., Hirai, T., Walz, T., Agre, P., Heymann, J. B., Engel, A., and Fujiyoshi, Y. (2000). Structural determinants of water permeation through aquaporin-1, Nature 407, 599-605
  • Preston, G. M., P. Piazza-Carroll, W. B. Guggino, and P. Agre. (1992). Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 water channel. Science, 256, 385–387
  • Ren, G., Reddy, V. S., Cheng, A., Melnyk, P., and Mitra, A. K. (2001).Visualization of a water-selective pore by electron crystallography in vitreous ice, Proc Natl Acad Sci U S A 98, 1398-1403
  • Sui, H., Han, B. G., Lee, J. K., Walian, P., and Jap, B. K. (2001). Structural basis of water-specific transport through the AQP1 water channel, Nature 414, 872-8
  • Tajkhorshid, E., Nollert, P., Jensen, M. O., Miercke, L. J., O'Connell, J., Stroud, R. M., and Schulten, K. (2002). Control of the selectivity of the aquaporin water channel family by global orientational tuning, Science 296, 525-530
  • Zhu, F., Tajkhorshid, E. and Schulten, K. (2003). Theory and simulation of water permeation in aquaporin-1. Biophysical Journal, 86, 50-57
  • Borgnia, M., S. Nielsen, A. Engel, and P. Agre. (1999). Cellular and molecular biology of the aquaporin water channels. Annual Review of Biochemistry, 68, 425–458