Tuning Function of the Light-Driven Proteorhodopsin Proton Pump by Formation of Oligomeric and Surfactant-Based Synthetic Complexes

Abstract

The Proteorhodopsin (PR) membrane protein equips marine bacteria worldwide with the capability of converting light energy into chemical energy. The proton pump absorbs light via an intrinsic retinal molecule, followed by subsequent conformational changes that store energy in the form of a proton gradient across the bacterial cell membrane. Because PR exists in the complex membrane environment, a multitude of factors could contribute to its light absorption and energy-transducing properties, including protein-protein, protein-lipid, and electrostatic interactions. Here we demonstrate by steady-state and time-resolved optical absorption spectroscopy that there are profound functional effects (e.g. altered activity and photocycle kinetics) modulated by the association of PR with other PR molecules to form oligomeric complexes, and by reconstitution in membrane-mimetic surfactant environments with varying chemical and physical characteristics.We study the molecular-level properties accompanying these functional consequences of environment with magnetic resonance techniques, specifically amino acid side-chain dynamics probed by Electron Paramagnetic Resonance (EPR) and local hydration dynamics by Overhauser Dynamic Nuclear Polarization (ODNP). Such information lends insight into the mechanisms tuning PR function and can be applied towards the design of function-enhancing synthetic environments. For example, surface water diffusion is faster in a model lipid bilayer than in surfactant micelles, concurrent with faster conformational motion, and possibly efficiency (Hussain, et.al, Angew. Chem. 52, 2013). We further have developed a synthetic platform for potential device applications by the incorporation of active PR into surfactant-directed mesostructured silica films, in both its hexameric and monomeric forms, and using different surfactants. This provides a unique opportunity to apply results from the biophysical characterization of PR to the design of an optimized solar energy-harvesting biomaterial, tuning molecular interactions to match the desired functional properties.