Red shift of the purple membrane absorption band and the deprotonation of tyrosine residues at high pH : origin of the parallel photocycles of trans-bacteriorhodopsin

At high pH (> 8) the 570 nm absorption band of all-trans bacteriorhodopsin (bR) in purple membrane undergoes a small (1.5 nm) shift to longer wavelengths, which causes a maximal increase in absorption at 615 nm. The pK of the shift is 9.0 in the presence of 167 mM KCl, and its intrinsic pK is sim...

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Bibliographic Details
Published in:Biophysical journal 1991-08, Vol.60 (2), p.475-490
Main Authors: BALASHOV, S. P, GOVINDJEE, R, EBREY, T. G
Format: Article
Language:English
Subjects:
bacteriorhodopsin
Biological and medical sciences
effects on
Fundamental and applied biological sciences. Psychology
Halobacterium
Molecular biophysics
purple membranes
residues
tyrosine
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Summary:At high pH (> 8) the 570 nm absorption band of all-trans bacteriorhodopsin (bR) in purple membrane undergoes a small (1.5 nm) shift to longer wavelengths, which causes a maximal increase in absorption at 615 nm. The pK of the shift is 9.0 in the presence of 167 mM KCl, and its intrinsic pK is similar to 8.3. The red shift of the trans-bR absorption spectrum correlates with the appearance of the fast component in the light-induced L to M transition, and absorption increases at 238 and 297 nm which are apparently caused by the deprotonation of a tyrosine residue and red shift of the absorption of tryptophan residues. The pH and salt dependent equilibrium between the two forms of bR, "neutral" and "alkaline," bR bR sub(a), results in two parallel photocycles of trans-bR at high pH, differing in the rate of the L to M transition. In the pH range 10-11.8 deprotonation of two more tyrosine residues in observed with pK's similar to 10.3 and 11.3 (in 167 mM KCL). It is suggested that the shifts of the absorption bands at high pH are due to the appearance of a negatively charged group inside the protein (tyrosinate) which causes electrochromic shifts of the chromophore and protein absorption bands.
ISSN:0006-3495
1542-0086
DOI:10.1016/S0006-3495(91)82074-4