PROTON-TRANSFER PATHWAYS IN BACTERIORHODOPSIN AT 2.3 ANGSTROM RESOLUTION

Photoisomerization of the retinal of bacteriorhodopsin initiates a cyc lic reaction in which a proton is translocated across the membrane. St udies 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 la st decade at increasing resolutions, has suggested plausible but often contradictory mechanisms. X-ray diffraction of bacteriorhodopsin crys tals grown in cubic lipid phase revealed unexpected two-fold symmetrie s that indicate merohedral twinning along the crystallographic c axis. The structure, refined to 2.3 angstroms taking this twinning into acc ount, is different from earlier models, including that most recently r eported. One of the carboxyl oxygen atoms of the proton acceptor Asp(8 5) 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 Asp(85) accepts a hydrogen bo nd from Thr(89). This structure forms the active site. The nearby Arg( 82) is the center of a network of numerous hydrogen-bonded residues an d an ordered water molecule. This network defines the pathway of the p roton from the buried Schiff base to the extracellular surface.