Ion–protein interactions of a potassium ion channel studied by attenuated total reflection Fourier transform infrared spectroscopy
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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.
KeywordsInfrared spectroscopy Membrane proteins Ion–protein interactions Ion channel
This review article was written based on two original papers (Furutani et al. 2012, 2015) which are collaborative works with Profs. Hideki Kandori and Shigetoshi Oiki. I would like to thank them and collaborators in their laboratories. The author would also like to thank Enago (www.enago.jp) for the English language review. The article was partly supported by JSPS KAKENHI Grant Numbers JP26640047 and JP26708002.
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Conflict of interest
Yuji Furutani declares that he has no conflicts of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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