Proton Transfer Pathways in Bacteriorhodopsin at 2.3 Angstrom Resolution

Photoisomerization of the retinal of bacteriorhodopsin initiates a cyclic reaction in which a proton is translocated across the membrane. Studies of this protein promise a better understanding of how ion pumps function. Together with a large amount of spectroscopic and mutational data, the atomic structure of bacteriorhodopsin, determined in the last decade at increasing resolutions, has suggested plausible but often contradictory mechanisms. X-ray diffraction of bacteriorhodopsin crystals grown in cubic lipid phase revealed unexpected two-fold symmetries that indicate merohedral twinning along the crystallographic c axis. The structure, refined to 2.3 angstroms taking this twinning into account, is different from earlier models, including that most recently reported. One of the carboxyl oxygen atoms of the proton acceptor Asp85 is connected to the proton donor, the retinal Schiff base, through a hydrogen-bonded water and forms a second hydrogen bond with another water. The other carboxyl oxygen atom of Asp85 accepts a hydrogen bond from Thr89. This structure forms the active site. The nearby Arg82 is the center of a network of numerous hydrogen-bonded residues and an ordered water molecule. This network defines the pathway of the proton from the buried Schiff base to the extracellular surface.