Biomedical Engineering Reference
In-Depth Information
file as they are passing through the channel. The restriction of water most likely occurs
due to an electric field created by the charges on the protein structure, inducing the major-
ity of the channel's core to be hydrophobic. This electric field also dictates the direction of
the water molecules as the flow through the channel. As water molecules enter the chan-
nel, they typically are oriented with the oxygen atom facing the entrance of the channel.
As the molecules enter the NPA motif, the water molecules flip, so that the oxygen atom
is facing toward the channel's exit. It is believed that the orientation of oxygen changes
due to a hydrogen-bonding event with the two asparagine molecules within the NPA
motif. Therefore, because each water molecule must be re-oriented to pass through the
aquaporin channel and it can only be re-oriented by interacting with the two asparagine
molecules within the NPA motif, only one water molecule can flow through the channel at
a time. Through these two restrictions it has been observed that the permeability of aqua-
porin channels toward water molecules is on the order of 6
E
2
14 cm
3
/s.
There is a second constriction of the aquaporin channel, usually toward the extracellular
side of the cell membrane, which acts to restrict the movement of other molecules through
the channel. This selectivity filter is termed the aromatic/arginine selectivity filter in aqua-
porin channels. The selectivity filter is a grouping of amino acids that interact with only
water molecules and helps them through the narrowing created by this filter. Other mole-
cules that do not interact with the selectivity filter cannot pass through this narrowing.
The aromatic ring weakens the hydrogen bonds between water molecules and then the
partial negative charge on the oxygen atom interacts with the positive charge on the argi-
nine. The interaction between the oxygen and arginine allows water through the channels
and prevents the passage of other molecules, especially protons.
Four aquaporin channels associate with each other in the membrane, so that in one loca-
tion there are four possible passageways for water to move through the cell membrane.
Each aquaporin channel can have a slightly different protein structure. There are at least
four different aquaporins in mammals and upward of 10 aquaporin channels found within
plants. The different structures between the aquaporin molecules may allow for the move-
ment of a small quantity of ions through the channels, although most aquaporins restrict
ion movement. Also, there are some aquaporin channels that respond to stimuli from
external hormones or other paracrine molecules. Upon stimulation, the rate of water
movement (and possibly the direction of movement) can be altered.
It is important to remember that aquaporins do not actively transport water across the
cell membrane; instead they facilitate the diffusion of water across the cell membrane. Due
to the slow diffusion of water across the lipid bi-layer, aquaporins effectively increase the
overall rate of water movement across the cell membrane.
10.4 FLOW OF AQUEOUS HUMOR
Immediately after the formation of aqueous humor (which uses aquaporin channels)
within the intercellular space of the ciliary processes, aqueous humor flows between the
ciliary muscle ligaments that attach to the lens (
Figure 10.2
). From here, the aqueous
humor flows through the pupil into the anterior chamber of the eye. The fluid flows
throughout
the anterior chamber and passes through a trabeculae meshwork. The
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