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

Abstract

In this paper are reported studies on the detergent role in isolated reaction centers (RC) from Rhodobacter sphaeroides, over a large range of lauryldimethylamino-N-oxide (LDAO) concentrations, in influencing the thermodynamics 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 ubiquinone 0 dissolved in the different environments (water, LDAO micelles and detergent phase of the protein-detergent complex) has also been analyzed. Measurements carried out in Q(B)-depleted RC to which exogenous quinone has been added show that the relative amplitudes of the slow and fast phase of the recombination reaction depend on this parameter. The overall amount of the restored Q(B)-functionality is affected by the concentration of the LDAO in solution. Interpolation of the titration curves with a quadratic function obtained by simple considerations allowed the binding constant of UQ(O) to the Q(B)-binding site to be calculated. From the fitting procedure, the distribution of the quinone in the different environments present in solution was evaluated, indicating that the exchange reaction can take place only between the Q(B)-site and the detergent phase. The dependence of the quinone pool size upon the volume of the phase in which the interacting quinone is solubilized is also discussed. The increasing difficulty in saturating the Q(B)-pocket above the LDAO critical micellar concentration is finally related to the association of protein-detergent complexes to form large protein clusters.