Binding of a single divalent cation directly correlates with the blue-to-purple transition in bacteriorhodopsin
- Univ. of Illinois, Urbana (United States)
The authors have characterized a unique divalent cation binding site on bacteriorhodopsin which controls the blue-to-purple transition in the purple membrane of Halobacterium halobiu. To identify this site they first showed the correlation between the binding of one Ca{sup 2+} per bacteriorhodopsin and the amount of blue membrane converted to purple membrane. When the free Ca{sup 2+} was reduced below 1 {mu}M, and the pH was set below 5.0 with 0.5 mM citrate, only binding to this high-affinity site was observed, and we could separate its effect from the effect of other divalent cations binding to the membrane under other conditions. Second, the titration of purple membrane showed that protons are taken up in two distinct steps, about 13 with a pK{sub a} of 4-5 and an additional 2 protons with a pK{sub a} for the purple-to-blue transition in 5mM MgSO{sub 4}. Taken together, these observations strongly suggest a direct role for cations in the regulation of the bacteriorhodopsin color under normal conditions. They also found that the intrinsic pK{sub a} for the purple-to-blue transition is about 2.05, suggesting this is the pK{sub a} of the group or groups that, when protonated, lead to the blue membrane. Previously published data can now be interpreted to suggest that the cation regulates an active site near the retinal chromophore. A binding site for the divalent cation that includes Asp-212 and interactions with the protonated Schiff base, Asp-85, Tyr-57, Tyr-185, and Arg-82 is proposed.
- OSTI ID:
- 5109481
- Journal Information:
- Proceedings of the National Academy of Sciences of the United States of America; (United States), Journal Name: Proceedings of the National Academy of Sciences of the United States of America; (United States) Vol. 88:1; ISSN 0027-8424; ISSN PNASA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
140505* -- Solar Energy Conversion-- Photochemical
Photobiological
& Thermochemical Conversion-- (1980-)
ABSORPTION SPECTRA
ALKALINE EARTH METAL COMPOUNDS
CALCIUM COMPOUNDS
CATIONS
CHARGED PARTICLES
CHEMICAL REACTIONS
CROSS-LINKING
IONIZATION
IONS
MEMBRANES
ORGANIC COMPOUNDS
PHOTOSYNTHETIC BACTERIA
PHOTOSYNTHETIC MEMBRANES
PIGMENTS
POLYMERIZATION
PROTEINS
RHODOPSIN
SPECTRA
TITRATION