Ion-protein interactions of a potassium ion channel studied by attenuated total reflection Fourier transform infrared...

An understanding of ion-protein interactions is key to a better understanding of the molecular mechanisms of proteins, such as enzymes, ion channels, and ion pumps. A potassium ion channel, KcsA, has been extensively studied in terms of ion selectivity. Alkali metal cations in the selectivity filter were visualized by X-ray crystallography. Infrared spectroscopy has an intrinsically higher structural sensitivity due to frequency changes in molecular vibrations interacting with different ions. In this review article, I attempt to summarize ion-exchange-induced differences in Fourier transform infrared spectroscopy, as applied to KcsA, to explain how this method can be utilized to study ion-protein interactions in the KcsA selectivity filter. A band at 1680 cm^-1 in the amide I region would be a marker band for the ion occupancy of K⁺, Rb⁺, and Cs⁺ in the filter. The band at 1627 cm^-1 observed in both Na⁺ and Li⁺ conditions suggests that the selectivity filter similarly interacts with these ions. In addition to the structural information, the results show that the titration of K⁺ ions provides quantitative information on the ion affinity of the selectivity filter.