发信人: doggiebear (doggiebear), 信区: Biology
标 题: SBIG: Aquaporin, the water channel - 3
发信站: Unknown Space - 未名空间 (Fri Oct 17 23:34:00 2003) WWW-POST



The structure of aquaporin -1 has been solved. it is a tetramer. But more like
chloride channel, each of its four subunit has an independant pore. So the
tetramer actually is composed by four channels. (see figure below)





The selection problem is solved by arranging three hydrophlic patches along
the
hydrophobic channel wall. This hydrophilic patches contribute four water
binding sites and form the selective filter. Look at the following figure.
Four water molecules (green sphere) are shown here,each bound to one site.
Why there are only four water binding sites? because there is a balance
between recognition and transportation. four sites is enough for select water
but does not bind water molecule too strong to let it go.



Water channel are selective against ions. That means, if you put salty water
at one side of the channel, only water, not Na or Cl ion, can go through. Why?

From our discussion on potassium channel, we know that the dehydration of an
ion is a critical step for its recognition. Without proper dehydration, the
ion will be too large to move into any selective filter.

This is the case here. Unlike in the potassium channel, where we have lots of
backbone carbonyl groups facing the channel wall to strip water off from the
ion, here in aquaporin, we do not have much such groups. Four water binding
sites are enough for forming hydrogen bond with water molecules but not enough
to completely dehyrate (calculation shows they can dehydrate less than half)
an ion. This is why ions cannot go through.

For protons, the story is a bit different. Normally, when water flows, it
forms a hydrogen bond network. Water molecules are well orientated and protons
can be transported along this linear network with super effiecency. Basically
instant shuttle. kinds of superconductivity. But the water binding sites break
the hydrogen bond network, which effectively blocks the transportation
of protons. (see figure below)





Please go to reference:

Nature vol 414 page 872

Nature vol 407 page 599







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I will vote a democrat even he is a dog.
Liberalism is something to be proud of!
※ 修改:·doggiebear 於 Oct 17 23:34:00 修改本文·[FROM: 208.180.]
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