Not Awkwa-porin… Aquaporin!

This amazing little protein is one of the most important proteins your body has!

Perhaps you’ve recently heard that Google is going to give Kansas City ultra-high-speed internet service. They are helping internet service providers offer internet to everyday users at one giga-bit per second. This speed is ridiculous! Well, for technology, this is impressive. But, for the functions happening in our body, this really isn’t unreasonable. In fact, the protein Aquaporin has always handled speeds this high. It is capable of channeling 10,000,000,000 molecules of water each second! 10,000,000,000 molecules of water each second!!!

And this is an important thing to accomplish. As you might remember from basic chemistry, the diffusion of water across the plasma membrane was important enough to give it a unique name: osmosis. But, unlike plasma membranes, which allow other molecules like oxygen and carbon dioxide through, aquaporins are very uniquely designed for water molecules. To put things into context, a lipid plasma membrane is able to hold water at a concentration of 1 ppm. This is really, quite a low figure!

Aquaporins are crucial to our bodies for functions other than the mere ‘transportation’ of water. Aquaporins allow cells in our lungs and throat to excrete water vapor, keeping our respiratory system from drying out. Aquaporins in plant roots and kidneys allow cells to absorb water at elevated rates. Some aquaporins even keep your eyes from drying out.

Aquaporin actually makes an appearance in the plasma membrane of every single one of our cells! Being a membrane protein, it was rather difficult to develop a crystal structure, but since that was just recently accomplished, the future is bright. Manipulation of Aquaporin has implications for water purification, drug delivery, nutrition, and even fuel cell applications. So, a vote for Aquaporin is a vote for the future!

Aquaporins; The Cellular Highway for Water Molecules

1. 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 - Although, we know water can travel through cell membranes, specialized proteins are an important part of facilitating such a constant movement. These 'water channels' now called aquaporins exist across such cellular membranes, or lipid bilayer membranes, and allow the rapid and selective transport of water into or out of the cell. Furthermore, there seems to be a sort of regulation of these water channels to increase or decrease water permeability.
   
   
2. Agre, P., M. Bonhivers, and M. J. Borgnia. (1998).The aquaporins, blueprints for cellular plumbing systems. Journal of Biolgical Chemistry, 273, 14659–14662 - Since the original discovery of Aquaporins in 1992, several units, including the kidneys and plant roots, which require a higher than average flux of water into and out of cells have been researched. The requirement for a high flow of water is met by the specific function of aquaporins. Even the lungs in mammals have special arrangements of aquaporins to ensure humidification of the airways. Much is still being learned about the physiological role and structure of aquaporins.
   
   
3. 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 - It has been found that many aquaporin variations exist. Those common in humans are AQP1, AQP2, and nine others. The permeability of water through many of these aquaporin water channels has been estimated to be an amazing 10^9 molecules of water per second! Furthermore, some variations of aquaporins, called aquaglyceroporins, act as a channel for glycerol and small solutes in addition to water. And, though much progress has been made in characterizing the structure of aquaporins, much is still unclear about these variations. This paper goes on to review the aquaporins found in plants, amphibians, insects, and some bacteria.
   
   
4. Kumar, M., M. Grzelakowski, J. Zilles, M. Clark, and W. Meier. (2007). Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z. Proceedings of the National Academy of Sciences of the United States of America, 104:52, 20719-20724 - Some exciting work is being done in an effort to assess how aquaporins might have application in drug delivery or water purification. By placing AqpZ (an aquaporin found from E. coli) in a synthetic polymer membrane, scientists found that water permeability was observed at up to 800 times that of the pure polymer.

Aquaporin Images

RCSB Protein Data Bank - Structure for 3NE2 - Archaeoglobus fulgidus aquaporin