Water as a cofactor in the unidirectional light-driven proton transfer steps in bacteriorhodopsin.
Recent evidence for involvement of internal water molecules in the mechanism of bacteriorhodopsin is reviewed. Water O-H stretching vibration bands in the Fourier transform IR difference spectra of the L, M and N intermediates of bacteriorhodopsin were analyzed by photoreactions at cryogenic temperatures. A broad vibrational band in L was shown to be due to formation of a structure of water molecules connecting the Schiff base to the Thr46-Asp96 region. This structure disappears in the M intermediate, suggesting that it is involved in transient stabilization of the L intermediate prior to proton transfer from the Schiff base to Asp85. The interaction of the Schiff base with a water molecule is restored in the N intermediate. We propose that water is a critical mobile component of bacteriorhodopsin, forming organized structures in the transient intermediates during the photocycle and, to a large extent, determining the chemical behavior of these transient states.
Maeda
Akio
A
Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA. amaeda@uiuc.edu
Morgan
Joel E
JE
Gennis
Robert B
RB
Ebrey
Thomas G
TG
D001436:Bacteriorhodopsins / Q000737:chemistry / Q000378:metabolism*; D008027:Light; D008958:Models, Molecular; D010777:Photochemistry; D011487:Protein Conformation; D011522:Protons; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D014732:Vibration; D014867:Water / Q000032:analysis
D011522:Protons; D014867:Water; D001436:Bacteriorhodopsins