Energetic optimization of ion conduction rate by the K+ selectivity filter.
The K+ selectivity filter catalyses the dehydration, transfer and rehydration of a K+ ion in about ten nanoseconds. This physical process is central to the production of electrical signals in biology. Here we show how nearly diffusion-limited rates are achieved, by analysing ion conduction and the corresponding crystallographic ion distribution in the selectivity filter of the KcsA K+ channel. Measurements with K+ and its slightly larger analogue, Rb+, lead us to conclude that the selectivity filter usually contains two K+ ions separated by one water molecule. The two ions move in a concerted fashion between two configurations, K+-water-K+-water (1,3 configuration) and water-K+-water-K+ (2,4 configuration), until a third ion enters, displacing the ion on the opposite side of the queue. For K+, the energy difference between the 1,3 and 2,4 configurations is close to zero, the condition of maximum conduction rate. The energetic balance between these configurations is a clear example of evolutionary optimization of protein function.
Morais-Cabral
J H
JH
Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06520, USA.
Zhou
Y
Y
MacKinnon
R
R
D001426:Bacterial Proteins; D001665:Binding Sites; D018360:Crystallography, X-Ray; D004058:Diffusion; D004563:Electrochemistry; D017136:Ion Transport; D007700:Kinetics; D010316:Particle Size; D011188:Potassium / Q000737:chemistry / Q000378:metabolism*; D015221:Potassium Channels / Q000737:chemistry / Q000378:metabolism*; D012413:Rubidium / Q000737:chemistry / Q000378:metabolism; D014867:Water / Q000737:chemistry / Q000378:metabolism
D001426:Bacterial Proteins; D015221:Potassium Channels; C107790:prokaryotic potassium channel; D014867:Water; D012413:Rubidium; D011188:Potassium