Premium
Titration kinetics of Asp‐85 in bacteriorhodopsin: exclusion of the retinal pocket as the color‐controlling cation binding site
Author(s) -
Fu X,
Bressler S,
Ottolenghi M,
Eliash T,
Friedman N,
Sheves M
Publication year - 1997
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/s0014-5793(97)01194-0
Subject(s) - bacteriorhodopsin , titration , chemistry , divalent , protonation , halobacteriaceae , titration curve , chromophore , binding site , kinetics , inorganic chemistry , crystallography , ion , membrane , photochemistry , organic chemistry , biochemistry , physics , halobacterium salinarum , quantum mechanics
The spectrum (the purple↔blue transition) and function of the light‐driven proton pump bacteriorhodopsin are determined by the state of protonation of the Asp‐85 residue located in the vicinity of the retinal chromophore. The titration of Asp‐85 is controlled by the binding/unbinding of one or two divalent metal cations (Ca 2+ or Mg 2+ ). The location of such metal binding site(s) is approached by studying the kinetics of the cation‐induced titration of Asp‐85 using metal ions and large molecular cations, such as quaternary ammonium ions, R 4 N + (R=Et, Pr, a divalent ‘bolaform ion’ [Et 3 N + ‐(CH 2 ) 4 ‐N + Et 3 ] and the 1:3 molecular complex formed between Fe 2+ and 1,10‐phenanthroline (OP). The basic multi‐component kinetic features of the titration, extending from 10 −2 to 10 4 s, are unaffected by the charge and size of the cation. This indicates that cation binding to bR triggers the blue→purple titration in a fast step, which is not rate‐determining. In view of the size of the cations involved, these observations indicate that the cation binding site is in an exposed location on, or close to, the membrane surface. This excludes previous models, which placed the color‐controlling Ca 2+ ion in the retinal binding pocket.