Proteorhodopsin Hydration and Dynamics is Governed by Surrounding Environment

Abstract

Proteorhodopsin (PR) is a light-activated retinal protein with the ability to pump protons from the cytoplasmic to the extracellular side of the cell [1]. The proton pumping process is highly efficient and can be reversed depending on environmental pH. These properties make PR a promising candidate in design of applications involving harvesting of solar energy. Although the photointermediates in the photocycle are well-defined spectroscopically, the role of dynamics and internal hydration of PR on the process of the proton pumping is poorly understood. In this study, we have used molecular dynamics (MD) simulations to determine the role of hydration, protonation state of the proton donor E108, and membrane environment on the dynamics of PR during the early stages of its photocycle. Our findings show that hydration of the protein interior directly affects the dynamics of the transmembrane helices. Specially, helices F and G and the EF loop are highly dynamic, in agreement with experimental observations for PR and bacteriorhodopsin [2]. Residues that play an important role in the photocycle are generally more hydrated. In addition, hydration of E108 in the deprotonated state is considerably larger than in the protonated state. Finally, solubilization with detergent micelles remarkably alters the stability and mobility of different components of PR compared to a bilayer environment. The complex of PR with dodecylphosphocholine (DPC) detergent micelle is highly dynamic, such that the helices are displaced from their position in the initial structure and the EF loop is partially unfolded, while PR complexes with n-dodecyl-β-D-maltoside (DDM) are more stable.[1] Bamann, C. et. al., 2014 Biochemica et Biophysica Acta 1837:614.[2] Andersson, C. et. al., 2009 Structure 17:1265.