Structural Changes in Bacteriorhodopsin During Ion Transport at 2 Angstrom Resolution
Hartmut Luecke,
123*
Brigitte Schobert,
23
Hans-Thomas Richter,
23
Jean-Philippe Cartailler,
13
Janos
K. Lanyi
23*
Crystal structures of the Asp96 to Asn mutant of the
light-driven proton pump bacteriorhodopsin and its M photointermediate produced by illumination at ambient temperature have been determined to
1.8 and 2.0 angstroms resolution, respectively. The trapped photoproduct corresponds to the late M state in the transport cycle--that is, after proton transfer to Asp85 and release
of a proton to the extracellular membrane surface, but before
reprotonation of the deprotonated retinal Schiff base. Its density map
describes displacements of side chains near the retinal induced by its
photoisomerization to 13-cis,15-anti and an
extensive rearrangement of the three-dimensional network of hydrogen-bonded residues and bound water that accounts for the changed
pKa values (where Ka is
the acid constant) of the Schiff base and Asp85. The
structural changes detected suggest the means for conserving energy at
the active site and for ensuring the directionality of proton
translocation.
1 Department of Molecular Biology and
Biochemistry,
2 Department of Physiology and
Biophysics,
3 UCI Program in Macromolecular
Structure, University of California, Irvine, CA 92697, USA.
*
To whom correspondence should be addressed. E-mail:
hudel{at}uci.edu or jlanyi{at}orion.oac.uci.edu
