Energetic optimization of ion conduction rate by the K+ selectivity filter

The K+ selectivity filter catalyses the dehydration, transfer and rehydrati on of a K+ ion in about ten nanoseconds. This physical process is central t o the production of electrical signals in biology. Here we show how nearly diffusion-limited rates are achieved, by analysing ion conduction and the c orresponding 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 conrguration) an d water-K+-water-K+ (2,4 conrguration), until a third ion enters, displacin g 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 m aximum conduction rate. The energetic balance between these configurations is a clear example of evolutionary optimization of protein function.