Energetics of ion conduction through the K+ channel.
K+ channels are transmembrane proteins that are essential for the transmission of nerve impulses. The ability of these proteins to conduct K+ ions at levels near the limit of diffusion is traditionally described in terms of concerted mechanisms in which ion-channel attraction and ion-ion repulsion have compensating effects, as several ions are moving simultaneously in single file through the narrow pore. The efficiency of such a mechanism, however, relies on a delicate energy balance-the strong ion-channel attraction must be perfectly counterbalanced by the electrostatic ion-ion repulsion. To elucidate the mechanism of ion conduction at the atomic level, we performed molecular dynamics free energy simulations on the basis of the X-ray structure of the KcsA K+ channel. Here we find that ion conduction involves transitions between two main states, with two and three K+ ions occupying the selectivity filter, respectively; this process is reminiscent of the 'knock-on' mechanism proposed by Hodgkin and Keynes in 1955. The largest free energy barrier is on the order of 2-3 kcal mol-1, implying that the process of ion conduction is limited by diffusion. Ion-ion repulsion, although essential for rapid conduction, is shown to act only at very short distances. The calculations show also that the rapidly conducting pore is selective.
Bernèche
S
S
Department of Biochemistry, Weill Medical College of Cornell University, New York, New York 10021, USA.
Roux
B
B
D001426:Bacterial Proteins; D018360:Crystallography, X-Ray; D004058:Diffusion; D004734:Energy Metabolism; D017136:Ion Transport; D008958:Models, Molecular; D011188:Potassium / Q000737:chemistry / Q000378:metabolism*; D015221:Potassium Channels / Q000737:chemistry / Q000378:metabolism*; D011487:Protein Conformation; D013816:Thermodynamics
D001426:Bacterial Proteins; D015221:Potassium Channels; C107790:prokaryotic potassium channel; D011188:Potassium