All-trans to 13-cis retinal isomerization in light-adapted bacteriorhodopsin at acidic pH.
The flash photolysis kinetic spectra of the intermediate M(412) of bacteriorhodopsin were monitored during the process of acid titration. In the light-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin decreased (pK(a)=3.40+/-0.05) as the pH decreased from 7.3 to 1.9. In the dark-adapted state, the maximum peak amplitude of M(412) absorbance of bacteriorhodopsin increased as the pH decreased from 6.9 to 4.1, and then decreased (pK(a)=2.85+/-0.05) as the pH dropped to 2.1. These different trends in the change in the maximum peak amplitude suggested that not only the transition of purple membrane to blue membrane had taken place in both light and dark-adapted states, but also the fraction of all-trans-bR had changed during the acid titration. The pH-dependent absorption changes at 640 nm of bacteriorhodopsin in both light- and dark-adapted states were also observed. The pK(a)-values of the purple-to-blue transition were 3.80+/-0.05 in light-adapted state and 3.40+/-0.05 in dark-adapted state, respectively. According to Balashov's method, the fraction of all-trans-bR was assayed as the pH decreased. All these results indicated that the purple-to-blue transition of light-adapted bacteriorhodopsin was accompanied by an all-trans to 13-cis retinal isomerization at acidic pH.
Chen
De-liang
DL
Institute of Biophysics, Academia Sinica, 15 Datun Rd., Beijing 100101, PR China.
Wang
Guang-yu
GY
Xu
Bing
B
Hu
Kun-sheng
KS
D001436:Bacteriorhodopsins / Q000378:metabolism*; D004224:Diterpenes; D019617:Halobacterium salinarum / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008027:Light; D010782:Photolysis; D012172:Retinaldehyde / Q000378:metabolism*; D013053:Spectrophotometry
D004224:Diterpenes; C031338:13-cis-retinal; D001436:Bacteriorhodopsins; D012172:Retinaldehyde