Stigmatellin and Other Electron Transfer Inhibitors as Probes for the QB Binding Site in the Reaction Center of Photosynthetic Bacteria

Abstract

Quinones are vital for energy transduction both in photosynthetic and respiratory electron transfer chains. in the cyclic electron transfer system of photosynthetic bacteria there are four unique ubiquinone (UQ10) binding sites (1). The primary electron acceptor, QA, in the reaction center protein complex (RC) is a tightly bound quinone which is capable of accepting only single electrons from the light-activated (BChl)2- The secondary electron acceptor in the RC, QB, acts as a two electron gate between QA (n=l) quinol oxidation of the RC and the n=2 redox components of the Qpool and ultimately , the ubiquinol oxidoreductase (bc 1 complex) . in the bc 1 complex, the Qz site is a discrete n=2 site, and the Qc site appears to be a two electron gate analogous to QB. in this paper we present a method for probing the properties of the QB site in isolated photosynthetic RCs from Rhodopseudomonas sphaeroides. A comparison of these results with known properities of other Q binding sites in both photosynthetic and respiratory electron transport systems will allow us to determine common structural elements of Q binding sites.