Sodium and Potassium e Channels and Pumps - Biological Inorganic Chemistry - page 183

Detailed structure of the K þ -selectivity filter (two subunits). Oxygen atoms (red) coordinate K þ ions (green spheres) at positions FIGURE 9.5

1 4 from the extracellular side. Single letter amino acid code identifies select amino acids of the signature sequence (yellow: carbon, blue:

nitrogen, and red: oxygen). Green and gray dashed lines show O

e

K þ and hydrogen-bonding interactions, respectively. ,,, (From MacKinnon,

2004 . Reproduced with permission from John Wiley & Sons.)

to disruption of the ability of the channel to distinguish between K þ ions and Na þ ions. From the relative electron
density of the sites, it is clear that each site is occupied by K þ only half of the time; in other words, that at any
given time only two of the binding sites are occupied, with water molecules in the intermediate sites. Thus, two K þ
ions permanently occupy the sites 1 and 3 or 2 and 4, with a water molecule sandwiched between them

FIGURE 9.6 Two K þ ions in the selectivity filter are hypothesised to exist predominantly in the two specific configurations 1,3 and 2,4.
(From MacKinnon, 2004 . Reproduced with permission from John Wiley & Sons.)

( Figure 9.6 ) . The X-ray structures also give support to a previously suggested 'knock-on' mechanism whereby K þ
ions can traverse the channel. Additional K þ ions coordinated by eight water molecules are observed at the
extracellular mouth of the channel and in the central cavity. When one of these ions enters either end of the filter, it
displaces the equilibrium of the two K þ ions already resident, with the consequence that the column of K þ ions
moves along until one of the ions is ejected, and the new K þ ion takes its place in the filter.