In living cells, ion channels passively allow for ions to flow through as the concentration gradient relaxes to thermal equilibrium. Most ion channels are selective, only allowing one type of ion to go through while blocking another. One salient example is KcsA, which allows for larger K<sup>+</sup> ions through but blocks the smaller Na<sup>+</sup> ions. This counter-intuitive selectivity has been explained by two distinct theories that both focus on equilibrium properties: particle-channel affinity and particle-solvent affinity. However, ion channels operate far from equilibrium. By constructing minimal kinetic models of channels, we discover a ubiquitous kinetic ratchet effect as a non-equilibrium mechanism to explain such selectivity. We find that a multi-site channel kinetically couples the competing flows of two types of particles, where one particle's flow could suppress or even invert the flow of another type. At the inversion point (transition between the ratchet and dud modes), the channel achieves infinite selectivity. We have applied our theory to obtain general design principles of artificial selective channels.

Kinetic ratchet effect as a non-equilibrium design principle for selective channels

ACS Fall 2021

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