Elucidating ion configurations and water dynamics in a K+ ion channel using two-dimensional infrared spectroscopy

Potassium (K⁺) ion channels are transmembrane proteins that regulate the passage of K⁺ ions through cell membranes. The selectivity filter is the narrowest part of the pathway of ions through the channel.
It plays a determining role in the remarkably high ion selectivity and transport rates. Despite decades of work the precise mechanistic details of a transport through ion channels are still elusive. According to the so-called soft-knock mechanism water molecules alternate between K⁺ ions in the selectivity filter and co-transport with the ions.
In contrast, the hard-knock mechanism assumes that water is absent from the selectivity filter during ion conduction.
Two-dimensional infrared spectroscopy (2D IR) is an ultrafast technique that measures molecular vibrations. Carbonyl stretching vibrations of a protein backbone are sensitive probes of the local chemical environment and can be used to discriminate between water and K⁺ ions in the selectivity filter. I will present our recent line shape simulations and experiments, performed by our collaborators, on a prokaryotic K⁺ channel KcsA.
Our results are clearly consistent with all the previous 2D IR experiments and undoubtedly illustrate the prevalence of the soft-knock ion configurations in the closed conductive state of the KcsA channel. Additionally, I will discuss our most recent simulations and 2D IR experiments with various waiting times reporting on ion, water, and protein dynamics inside the selectivity filter of the KcsA channel.