Unique structure of the retinal chromophore enabling sodium ion transport in the sodium ion-pumping protein KR2

Abstract

Active ion transport across membranes is vital to maintaining the electrochemical gradients of ions in cells and is mediated by transmembrane proteins. Photoexcitation of some microbial rhodopsins leads to ion transport across membranes. They contain all-trans-retinal as a chromophore, which is covalently bound to a lysine residue through a protonated Schiff base linkage and surrounded by seven transmembrane α helices. Absorption of a photon results in the chromophore isomerization and leads to a cyclic reaction. To reveal mechanism in ion pumping, it is essential to elucidate sequential changes in the chromophore structure in a photocycle. Resonance Raman spectroscopy enables us to examine the evolution of the structural changes of the retinal chromophore and the protein moiety.[1-7]