Investigation on the detergent role in the function of secondary quinone in bacterial reaction centers

In this paper are reported studies on the detergent role in isolated reacti on centers (RC) from Rhodobacter sphaeroides, over a large range of lauryld imethylamino-N-oxide (LDAO) concentrations, in influencing the thermodynami cs of the quinone exchange reaction as well as the protein aggregation. The occurrence of the quinone exchange reaction between the Q(B)-binding site (where Q(B) is the second quinone molecule of two in the RC) and the ubiqui none 0 dissolved in the different environments (water, LDAO micelles and de tergent phase of the protein-detergent complex) has also been analyzed. Mea surements carried out in Q(B)-depleted RC to which exogenous quinone has be en added show that the relative amplitudes of the slow and fast phase of th e recombination reaction depend on this parameter. The overall amount of th e restored Q(B)-functionality is affected by the concentration of the LDAO in solution. Interpolation of the titration curves with a quadratic functio n obtained by simple considerations allowed the binding constant of UQ(0) t o the Q(B)-binding site to be calculated. From the fitting procedure, the d istribution of the quinone in the different environments present in solutio n was evaluated, indicating that the exchange reaction can take place only between the Q(B)-site and the detergent phase. The dependence of the quinon e pool size upon the volume of the phase in which the interacting quinone i s solubilized is also discussed. The increasing difficulty in saturating th e Q(B)-pocket above the LDAO critical micellar concentration is finally rel ated to the association of protein-detergent complexes to form large protei n clusters.