发信人: doggiebear (doggiebear), 信区: Biology
标 题: Re: SBIG: Channels
发信站: Unknown Space - 未名空间 (Thu Oct 16 21:30:32 2003) WWW-POST

The directinality of the channel is a good question.
I do not know. (though I can search for answer)
I say it is imporant becasue it verify the role of the helix term and the
cavity. Based on this principle, I will say the channel should have
directionality. That means, ion can only come out from the cell, not the
opposite, or the stabilization effect will not be there.
I bet ppl has done experiment on it. This is kind of classical
electriphysiology research.

I think dehydration is the the critical step for ion selection. The selective
filter is small enough to for only dehydrated ion. Therefore, when an hydrated
ion comes in, it is recognized by charged groups on the selective filter and
tranfered from water molecules to the interation partners of the filter and
start the transfering process.





【 在 space (排骨教主) 的大作中提到: 】
: 我这几天脖子给拧了,没好好看文章. 今天在医院把K+ channel匆匆看了一下,很不得
: 要领,和我缺乏生物知识有关系.首先我就没搞清楚K+ channel是不是双向的, 其次
: 我也没搞清楚hydration和dehydration在这里面的角色是什么, 最后我也不清楚
: 这channel尺寸(宽度和高度)怎么样. 我先看看你们的讨论学习学习吧.
:
: 【 在 doggiebear (doggiebear) 的大作中提到: 】
: : Now comes the problem how an ion actually pass through the channel.
: : We know the membrane is a barrier against ions. Let's quote space's idea:
: : "consider a plane interface formed by two semi-infinite mediums with
: : different dielectrics(e1, e2). A charge "q" in medium "e1" at position
: : z=z1(let's define z-direction as the normal direction toward medium "e1"
from
: : the
: : intereface) induces a charge distribution on the interface and it feels a
: : force by the field
: : generated by the charges on the intereface. The field generated by the
charges
: : on the intereface in medium "e1" can be proven to be equivalent to a
field
: : generated by a virtual image charge "q'" located at z=-r in medium "e2".
The
: : image charge is:
: : q'=-q*(e2-e1)/(e2+e1)
: : For e2<e1, sign(q')=sign(q), thus q feels a repulsive force due to
: : Coulomb interaction. "
: : So there is a repulsion force that prevent an ion approaches a membrane.
: : So what can it do.
: : Let's see the following figure
: : http://biop.ox.ac.uk/www/lj2001/doyle/doyle_1.jpg
: : First, potassium channel is smart enough to introduce a water filled
cavity.
: : Calculation shows, to transfer an ion from water into membrane is very
energy
: : costly. But to transfer it into a water filled cavity embedded in the
membrane
: : require much less energy. (still delta G positive, but much smaller)
: : Second, the channel is cleverly designed. It has four helix with their
C-term
: : (which is negatively charged in alpha helix) pointed precisely to the
cavity.
: : This greatly stabilize the ion. The stablization effect is same over the
axial
: : length of the cavity (so ion can be ejected, not stuck somewhere). But the
: : stablization effect decreases dramatically if a lateral displacement
happens.
: : So the ion is "focused" to the center of the channel. Full simulation
shows,
: : combined with these two features, to transport an ion through the channel
is
: : actually energetic favorable (delta G minus). Finally!
: : So basically the potassium channel is an ion focus and eject device. Good
job,
: : nature.
:
:

--
I will vote a democrat even he is a dog.
Liberalism is something to be proud of!
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