发信人: doggiebear (doggiebear), 信区: Biology
标 题: SBIG: Channels - 3
发信站: Unknown Space - 未名空间 (Fri Oct 17 00:10:44 2003) WWW-POST

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.Basically an energy barrier (see figure below)





So what can it do.

Let's see the following figure






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!
※ 修改:·doggiebear 於 Oct 17 00:10:44 修改本文·[FROM: 208.180.]
※ 来源:．Unknown Space - 未名空间 mitbbs.com．[FROM: 208.180.]