Configurational Changes of Retinal Schiff Base during Membrane Na + Transport by a Sodium Pumping Rhodopsin.

Microbial rhodopsins are photoreceptors containing the retinal Schiff base chromophore and are ubiquitous among microorganisms. The Schiff base configuration of the chromophore, 15- anti (C═N trans ) or 15- syn (C═N cis ), is structurally important for their functions, such as membrane ion transport, because this configuration dictates the orientation of the positively charged NH group that interacts with substrate ions. The 15- anti / syn configuration is thus essential for elucidating the ion-transport mechanisms in microbial rhodopsins. Here, we identified the Schiff base configuration during the photoreaction of a sodium pumping rhodopsin from Indibacter alkaliphilus using Raman spectroscopy. We found that the unique configurational change from the 13- cis , 15- anti to all - trans , 15- syn form occurs between the photointermediates termed O1 and O2, which accomplish the Na ⁺ uptake and release, respectively. This isomerization is considered to give rise to the highly irreversible O1 → O2 step that is crucial for unidirectional Na ⁺ transport.