发信人: doggiebear (doggiebear), 信区: Biology
标 题: SBIG: Channels - 7
发信站: Unknown Space - 未名空间 (Fri Oct 17 00:32:08 2003) WWW-POST






water channel has the similar configuration as chloride channel. That means,
each subunit is an independant channel. But it forms tetramer. So one tetramer
has four channels.

question about water selectivity. No charge this time.

But the problem remains same. That is we need something to "bind" the water
but not too tightly. the channel is designed in such way that most of it is
hydrophobic but has hydrophilic patches on it, which solved both problems.

Another thing need to be addressed is the selection against ions.

First, consider non-proton ions, like k or cl. These ions are
hydrated most of the time. We already know that they are dehyrated before
entering the selective filter. If not dehydrated, they are too big and cannot
move in. For water channels, it does not have sufficient numbr of partially
charged groups (like carbonyl in K channel wall) that can substitue water.
Still with some water molecules on it, the hydrated ions are
too big to go through.

For protons, it is a different story. Normally, when water flows, it forms
hydrogen network. Properly orientated, this network can transport protons
along it with great effiecency. But the location of many charged residues in
the selective filter disrupt this kind of orientation, which effectively
blocks the transportation of protons. (see figure below)









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※ 修改:·doggiebear 於 Oct 17 00:32:08 修改本文·[FROM: 208.180.]
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