Aquaporins create a channel for water molecules to cross through cell membranes
Four Channels in One
Exploring the Structure
Aquaporin water channel (PDB entry 1ymg)
The water channel through aquaporin could cause a serious problem: strings of water are normally able to transfer protons quickly from molecule to molecule by the "Grotthuss mechanism." This would be a disaster, since it would quickly deplete the electrochemical gradient that powers many of the pumps at the cell surface. Aquaporin prevents this by forcing the water molecules to sit in a specific orientation that won't work as a proton wire. There are two characteristic asparagine amino acids that hold a central water molecule (or two water molecules in PDB entry 3zoj ) in a specific orientation. This water, along with other amino acids lining the channel, then interacts with the flanking waters in the queue, forcing the ones at the top to point one direction, and the ones at the bottom to point the opposite direction. Click on the image to view an interactive JSmol that shows the waters and the aquaporin amino acids that orient them. This image was created with PDB entry 1ymg , and the hydrogen atoms, which are not seen in the crystallographic structure, were added manually.
Topics for Further Discussion
- Structures of several different types of aquaporins are available in the PDB--you can use the "Compare Structures" tool to explore their similarities.
- Many of the structures of aquaporin are closed, with amino acids moved to block the water channel. Scientists are still studying whether this is part of an active gating process.
May 2014, David Goodselldoi:10.2210/rcsb_pdb/mom_2014_5