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Introduction
Ion channels are transmembrane proteins which allow ions to pass through biological membranes. Some of these channels are very selective, others have a low level of selectivity. The NaK channel is a non-selective one : It conduits cations more than anions but it let pass several cations : Na+, K+, Rb+, and Ca2+ [1.1].
Understanding how these channels work is important because in the organism a lot of messages are transmitted through electric currents (which are ionic currents across the membrane) : nerves impulse, photoreceptors, etc. Thus, these not very selective NaK channels are very interesting for the inhibition of intercellular messages for instance.
Structure
Active Site & Ions Passing
There are
4 ions binding sites in the NaK channel [1]. This diversity allows by different mechanisms to conduit several cations. They have similar chemical environments but they have
different ion selectivity. Two of them (sites S3 and S4) are conserved, that is to say they are the same than in the high selective K+ channel while S1 and S2 become a vestibular structure where K+ and Na+ ions can diffuse [2].
We will see for every binding site how his structure allows the passage of one or several ions.
External Site
We may notice the presence of a glycine (Gly67) which brings four carbonyl oxygen atoms, more inward oriented, able to bind with water molecules. This create an environement which can chelate K+ and Rb+ ions, but avoid the binding of Na+.
Moreover, thanks to a space intercation between Asp 66 and Gly67, the external site has a higher affinity for divalent cations such as Ca2+ and Ba2+ rather than monovalent such as K+ and Rb+[3].
The nature of the ligands is carbonyl-water.
Vestibule
In the case of the vestibule, there are too four carbonyl oxygen atom which brings by a valine (Val64). For instance, Na+ is neared to the ligand by this way: distance Na+-ligand=2,9 Ä. Moreover, ions are partially hydrated by four water molecules( they are along with the carboxyl oxygene atoms) : distance ions-H2O=4 Ä. The presence of water allows a greater flexibility in the ion binding so the vestibule may adapt to monovalent cations such as Na+, K+ and Rb+. However, this structure has a greater selectivity for K+ than Na+ : water molecules help to create a selectivity filter thanks to ligand geometry: octahedral arrangement which is impossible with Na+ because of a smaller radius and a hydratation by 5-6 molecules of water [4,5].
The nature of the ligands is carbonyl-water.
Site 3
He is the most non selective ion binding site which let pass mono and divalent cations, so a contamination can occur : presence of unkonwn species of ion at this site.
Moreover we may underscore a higher affinity for K+ than Na+ because of several reason :
- First, we can find 4 backbone carbonyl oxygen from Val64 which participate in K+ and Rb+ ions chelation because of the formation of an octahedral ligand: an octahedral arrangement oxygen ligands in the channel pore is more favorable for K+ than Na+.
- The lack of selectivity is due to the fact that the NaK channel have an almost identical structure when it is in complex with Na+, K+ or Rb+ : there is no big rearrangement in the structure of the protein depending on the bound ion. So the structure is stable with any ions, so it is non selective. Moreover, it could have a heavy atom contamination but it happens in a smaller extent with K+ than with Na+.
The amino-acids of the site 3 participate a lot in the transfert of Na+. In this case, Na+ binds because of an H-bonding interactions between Asp66 and the backbone amide of Asn68 which stabilize the structure. Furthermore, Val64 and Thr65 form a ion binding cage where Na+ ions tend bind at upper or lower ends (not in the center).
The nature of the ligands are carbonyl-carbonyl.
Site 4
Regulation
Structural highlights
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