TEMPERATURE-DEPENDENCE OF THE ELECTROGENIC REACTION IN THE Q(B) SITE OF THE RHODOBACTER-SPHAEROIDES PHOTOSYNTHETIC REACTION-CENTER - THE Q(A)(-)Q(B)-]Q(A)Q(B)(-) TRANSITION

The temperature dependencies for the kinetics and relative amplitudes of electrogenic reaction(s) coupled with the first reduction of the se condary quinone acceptor Q(B) were measured with dark-adapted chromato phores of Rhodobacter sphaeroides. The kinetics, while acceptable fitt ed by a single exponent at room temperature, clearly split into two co mponents below 15 degrees C (rise times, 25 mu s and 300 mu s at pH 7. 0 and 10 degrees C) with the slow phase ousting the fast one at pH>9.0 , The activation energies of the fast and slow phases Here estimated a t pH 7.0 as <10 kJ/mol and 60-70 kJ/mol, respectively, To explain the kinetic heterogeneity of the Q(B)-->Q(B)(-)transition, we suggest two possible conformations for the neutral oxidized ubiquinone at the QB s ite: one with a hydrogen bond between the side chain carboxyl of Glu-L 212 and the methoxy oxygen at C-3 of the Q(B) ring (Q(B)-H-Glu centers ) and the other one, without this bond (Q(B):Glu(-)centers), The fast phase is attributed to Q(A)(-)Q(B)-H-Glu-->Q(A)Q(B)-H-Glu transition, whereas the slow one to the (B):Glu(-)-->Q(A)(-)Q(B)-H-Glu-->Q(A)Q(B)( -)-H-Glu transition. (C) 1997 Federation of European Biochemical Socie ties.