FLASH-INDUCED CHANGES IN BUFFERING CAPACITY OF REACTION CENTERS FROM PHOTOSYNTHETIC BACTERIA REVEAL COMPLEX INTERACTION BETWEEN QUINONE POCKETS

Citation
L. Kalman et al., FLASH-INDUCED CHANGES IN BUFFERING CAPACITY OF REACTION CENTERS FROM PHOTOSYNTHETIC BACTERIA REVEAL COMPLEX INTERACTION BETWEEN QUINONE POCKETS, Biochimica et biophysica acta. Bioenergetics, 1365(3), 1998, pp. 513-521
Citations number
32
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Biology,Biophysics
ISSN journal
0005-2728
Volume
1365
Issue
3
Year of publication
1998
Pages
513 - 521
Database
ISI
SICI code
0005-2728(1998)1365:3<513:FCIBCO>2.0.ZU;2-T
Abstract
A novel method was applied to determine light-induced protonation in r eaction centers from photosynthetic purple bacteria. Changes in buffer ing capacities upon flash excitation were detected in (0.03% Triton X- 100) detergent solution of reaction centers from Rhodobacter (Rb.) sph aeroides and Rb, capsulatus wild type and mutant strains with empty or occupied secondary quinone (Q(B)) binding sites in the presence of an external electron donor. The light-induced differences in buffering c apacities between pH 4 and 11 were analyzed in terms of protonatable r esidues, Due to its differential nature, this method is more sensitive to the position and shift of pK(a) of the individual groups than the direct method based on proton uptake measurements, Out of the four dif ferent ionizable residues which were used to fit the curves, the two g roups with apparent (dark) pK(a) values between 8.4-8.8 and 9.5-10.0 ( depending on the species and conditions) disappeared when the native u biquinone(10) was replaced by menadione at the primary quinone (Q(A)) binding site of Rb. sphaeroides or when the key protonatable residues (L212Clu and L213Asp) were replaced by non-protonatable alanines in th e Q(B) binding site of the AA+M43D mutant from Rb. capsulatus, The exp erimentally observed acidic and neutral residues remained unchanged, T hese results obtained from modifications in both quinone sites reveal the origin of the alkaline pH groups: they reflect the interaction of Q(A)(-) and the cluster of ionizable residues around L212Glu in the Q( B) binding pocket. The involvement of two residues with close pK(a) va lues reflects the complex titration of the cluster. The interaction be tween the quinone pockets is best described qualitatively as a network of ionizable residues extending from the Q(A) Site to the Q(B) site. (C) 1998 Elsevier Science B.V. All rights reserved.