Ion-Protein Interaction in Channel and Pump Proteins Studied by FTIR Spectroscopy

Abstract

Membrane proteins play crucial roles for receiving external stimuli, transporting and permeating protons and ions to generate biological energy and signal. The molecular mechanisms of membrane proteins have been deepened with the progress in structural biology. Especially, the structural changes connecting the inactive and active states of protein are nowadays investigated by many physicochemical methods. Fourier-transform infrared (FTIR) spectroscopy is one of powerful method for investigating molecular structures in the level of chemical bonds. Perfusion-induced difference infrared spectroscopy has been applied to extract structural changes of protein in response to various stimuli. The selectivity of ion is important for understanding the molecular mechanisms of the ion channels and transporters. I have been studying a sodium ion pump, V-type ATPase [1], and a potassium ion channel, KcsA [2] by using FTIR spectroscopy with attenuated total reflection (ATR) configuration. KcsA is a well-known potassium ion channel whose precise X-ray structures in various conditions have been reported. I have applied ATR-FTIR spectroscopy on KcsA and succeeded to assign the C=O stretching vibrations of the selectivity filter in the amide I region [2]. Recently, I proposed a novel rapid buffer-exchange system for time-resolved ATR-FTIR spectroscopy to monitor ion-binding reaction of protein. The anion-binding reaction of a membrane protein, Natronomonaspharaonis halorhodopsin was confirmed by an increase of the retinal absorption band at 1528 cm−1 in accordance with the buffer-exchange reaction finished within ∼25 ms, which was monitored by the infrared absorption change of a nitrate band at 1350 cm−1 [3].[1] Y. Furutani et al. J. Am. Chem. Soc.133 (9), 2860-3, 2011.[2] Y. Furutani et al. J. Phys. Chem. Lett.3, 3806-10, 2012.[3] Y. Furutani et al. BIOPHYSICS9, 123-129, 2013.