Directional Proton Transfer in Membrane Proteins Achieved through Protonated Protein-Bound Water Molecules: A Proton Diode

摘要

The key function of energy-transducing membrane proteins is the creation of a proton gradient by directional proton transfer. The role of protein-bound water molecules herein is not fully understood, as X-ray diffraction analysis has resolved the positions of oxygen, but not of hydrogen atoms in such protein-water complexes. Here we show, now time-resolved at atomic resolution, how a membrane protein achieves directional proton transfer via protein-bound water molecules in contrast to random proton transfer in liquid water. A combination of X-ray structure analysis, time-resolved FTIR spectroscopy, and molecular dynamics (MD) simulations elucidates how directionality is achieved. Using the proton-pump bacteriorhodopsin as the paradigm, we show how controlled conformational changes of few amino acid residues rearrange preordered water molecules and induce directional proton transfer. This mechanism is analogous to an electronic diode: a "proton diode".