Biophysical Society Conference | Tahoe 2023

Proton Reactions: From Basic Science to Biomedical Applications

Monday Speaker Abstracts

THE ROLE OF PROTON TRANSFER IN ION-TRANSPORTING MICROBIAL RHODOPSINS Keiichi Inoue 1 ; 1 The University of Tokyo, The Institute for Solid State Physics, Kashiwa, Japan Microbial rhodopsins are a superfamily of photoreceptive membrane proteins consisting of a seven transmembrane-helical architecture and a retinal chromophore. While microbial rhodopsins exhibit diverse biological functions in a light-dependent manner, ion channel and ion pump rhodopsins are widely used in optogenetics to regulate neural activity by light. Despite their importance in optogenetic application, the mechanistic elements essential for pump and channel functions are not completely understood. Here we studied the dynamics of proton transfer and the conformation change of the retinal chromophore by laser flash photolysis, time resolved Raman spectroscopy, and laser electrophysiology. Microbial rhodopsins exhibit the photocyclic reaction including several photo-intermediates. In contrast to the best characterized outward proton pumping rhodopsin (bacteriorhodopsin), most elementary processes between the respective photo-intermediates of inward proton pumping rhodopsins (xenorhodopsin and schizorhodopsin) were slowed down in D 2 O solvent compared with in H 2 O solvent. These kinetic isotope effects (KIE) indicate that most elementary processes of inward proton pumping rhodopsins are rate-limited by the proton transfer in the protein [1,2]. In contrast, whereas the channel opening of cation (C1C2 and ChRmine) and anion (GtACR1) channelrhodopsins do not show significant KIE, the channel closing rate became slower in D 2 O solvent than in H 2 O solvent [3,4]. Interestingly, the channel opening of C1C2 was induced by the unique twisting of the retinal polyene chain. Our results indicate that the conformation changes critical for the functions of ion-transporting rhodopsins are tightly coupled with intramolecular proton transfer events and artificial modification of their rates would be useful for the development of next generation optogenetic tools. 1. Tahara, S. et al.: J. Phys. Chem. Lett., 6, 4481-4486 (2015).2. Kawasaki, Y. et al.: Biochim. Biophys. Acta Biomembr., 1864, 184016 (2022).3. Shibata, K. et al.: J. Am. Chem. Soc., published on the web, doi:10.1021/jacs.3c01879 (2023).4. Shibata, K. et al.: Manuscript in preparation.

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