Influence of Cardiolipin on the Functionality of the Q_ASite of the Photosynthetic Bacterial Reaction Center

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Abstract

The effect of cardiolipin on the functionality of the Q(A) site of a photosynthetic reaction center (RC) was studied in RCs from the purple non-sulfur bacterium Rhodobacter sphaeroides by means of time-resolved absorbance measurements. The binding of the ubiquinone-10 to the Q(A) site of the RC embedded in cardiolipin or lecithin liposomes has been followed at different temperatures and phospholipid loading. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. The presence of cardiolipin does not affect the affinity of the Q(A) site for ubiquinone but has a marked influence on the rate of P+QA(-) --> PQA electron transfer. The P+QA(-) charge recombination kinetics has been examined in liposomes made of cardiolipin/lecithin mixtures and in detergent (DDAO) micelles doped with cardiolipin. The electron-transfer rate constant increases upon cardiolipin loading. It appears that the main effect of cardiolipin on the electron transfer can be ascribed to a destabilization of the charge-separated state. Results obtained in micelles and vesicles follow the same titration curve when cardiolipin concentration evaluated with respect to the apolar phase is used as a relevant variable. The dependence of the P+QA(-) recombination rate on cardiolipin loading suggests two classes of binding sites. In addition to a high-affinity site (compatible with previous crystallographic studies), a cooperative binding, involving about four cardiolipin molecules, takes place at high cardiolipin loading.

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