The effect of protein conformation change from alpha(II) to alpha(I) on the bacteriorhodopsin photocycle.
The bacteriorhodopsin (bR) photocycle was followed by use of time-resolved Fourier-transform infrared (FTIR) spectroscopy as a function of temperature (15-85 degrees C) as the alpha(II) --> alpha(I) conformational transition occurs. The photocycle rate increases with increasing temperature, but its efficiency is found to be drastically reduced as the transition takes place. A large shift is observed in the all-trans left arrow over right arrow 13-cis equilibrium due to the increased stability of the 13-cis isomer in alpha(I) form. This, together with the increase in the rate of dark adaptation as the temperature increases, leads to a large increase in the 13-cis isomer concentration in bR in the alpha(I) form. The fact that 13-cis retinal has a much-reduced absorption cross-section and its inability to pump protons leads to an observed large reduction in the concentration of the observed photocycle intermediates, as well as the proton gradient at a given light intensity. These results suggest that nature might have selected the alpha(II) rather than the alpha(I) form as the helical conformation in bR to stabilize the all-trans retinal isomer that is a better light absorber and is capable of pumping protons.
Wang
J
J
Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
El-Sayed
M A
MA
D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D055592:Biophysical Phenomena; D001703:Biophysics; D004355:Drug Stability; D019617:Halobacterium salinarum / Q000378:metabolism / Q000528:radiation effects; D010777:Photochemistry; D011487:Protein Conformation; D011522:Protons; D017550:Spectroscopy, Fourier Transform Infrared; D013237:Stereoisomerism; D013696:Temperature
D011522:Protons; D001436:Bacteriorhodopsins