Proton Transfer Pathways in Bacteriorhodopsin at 2.3 Angstrom Resolution
Hartmut Luecke,
*
Hans-Thomas Richter,
Janos K. Lanyi
*
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.
H. Luecke, Department of Molecular Biology and Biochemistry and
Department of Physiology and Biophysics, University of California,
Irvine, CA 92697, USA.
H.-T. Richter and J. K. Lanyi, Department of Physiology and
Biophysics, University of California, Irvine, CA 92697, USA.
*
To whom correspondence should be addressed. E-mail:
HUDEL@UCI.EDU or JLANYI@ORION.OAC.UCI.EDU
