发信人: doggiebear (doggiebear), 信区: Biology
标 题: SBIG: Channels - 5
发信站: Unknown Space - 未名空间 (Fri Oct 17 00:27:11 2003) WWW-POST

http://www.hhmi.org/news/images/mackinnon5.gif


Now comes the chloride channel.

What is the differnce.

Potassium channel transports potassium, which is a cation, with positive
charge on it.

chlordie channel transports chloride, which is an anion, with negative charge
on it.

This is the difference.

Look at its structure, you will find each monomer of the chloride channel is
an independent channel. The native form is dimer. So you have two channels
within the dimer. This is not the case for potassium channels. It forms one
channel using four subunit. So a potassium channel tetramer has only one
channel.

Remember the potassium channel? It uses four C-term of the alpha helix to
stablize potassium ion. (see figure below left) What happens here? yes. the
helix comes back. but use the postively charged N-term to stabilize the ion.
(see figure below right. note in the chloride channel, the arrangement of
helices are different from that in potassium channel.)


* * *
* * *
* * +
- - Cl

K +
*
*



So one lesson is:

the terminus of helix is important for channel function.

this actully happens in other proteins.
For example, the protein strutre I solved is a kinase. It has an N-termus of a
helix pointing to the phosphate binding site (of ATP). So it can stablize the
negative charges accumulated on the phosphorus when a nucleaphilic attack
takes place, basically stabilizing the transition state.

So, remeber the helix.






--
I will vote a democrat even he is a dog.
Liberalism is something to be proud of!
※ 修改:·doggiebear 於 Oct 17 00:27:11 修改本文·[FROM: 208.180.]
※ 来源:．Unknown Space - 未名空间 mitbbs.com．[FROM: 208.180.]