Evidence for a carboxyl group in the vicinity of the retinal chromophore of bacteriorhodopsin
Herz, J.M.; Hrabeta, E.; Packer, L.
Biochemical and Biophysical Research Communications 114(2): 872-881
1983
ISSN/ISBN: 0006-291X PMID: 6882459 Document Number: 199725
Carboxyl groups of bacteriorhodopsin in [Halobacterium halobium] purple membranes were activated using a hydrophobic reagent and then covalently labeled with a pH-sensitive reporter group, nitrotyrosine methyl ester. The membrane-bound reporter group had different spectral properties and a pK 3 U higher than in solution. In purple membranes, an isosbestic point between the 428-nm absorption peak of nitrotyrosine methyl ester and the bacteriorhodopsin 570-nm chromophore seen in alkaline titration, indicated interactions between the reporter group and retinal. Modification of white membranes revealed similar, unusual spectral and ionization properties. The hydrophobic environment, not retinal interactions per se, are responsible for the ionization behavior of the reporter group. A carboxyl group may be near the retinal chromophore of bacteriorhodopsin.