Properties of photosynthetic reaction centers isolated from chromatophores of Chromatium minutissimum

Abstract

Functional characterisitcs of photosynthetic reaction centers from the sulphur purple bacterium Chromatium minutissimum were investigated. In the absence of the secondary quinone acceptor (QB), rereduction of P+ following a flash occurs by recombination with the reduced primary quinone (QA) in approx. 40 ms, independent of pH within the range 6–10. The function of the secondary quinone acceptor is reconstituted by adding ubiquinone (Q−6), as indicated by the pronounced slowing of P+ dark recovery and by flash-induced binary oscillations of semiquinone QB formation. The time of the QA−QB → QAQB− and QA−QB− → QAQB2− transitions is approx. 20 μs at pH 6.3. The spectrum of the reduced primary (QA−) and secondary (QB−) acceptor suggests that the primary quinone acceptor is menaquinone and secondary quinone acceptor is ubiquinone. It is found that the association constant for the secondary quinone acceptor in the RC protein, measured through the ratio of slow and fast components of pigment dark relaxation, is smaller in alkaline conditions than in acidic ones. The kinetics of flash-induced oxidation of high-potential heme, measured at 556 and 420 nm, were well approximated by a single exponential with τ ≅ 1.5 μs. Analysis of the kinetics of cytochrome dark reduction by the reduced quinone acceptors (via pigment P) permits us to estimate the equilibrium constant of electron transfer between high-potential heme and P as 130, indicating Em ≅ 360 mV for C+/C couple. The equilibrium constant of electron transfer between QA−QB and QAQB− states of RC, estimated from kinetics of dark recovery of photooxidized pigment, is characterized by strong pH-dependence and is ≅ 22 at pH 10 and more than 300 at pH 6 with pK ≅ 7.6. The P+ QB− dark relaxation rate in Chromatium is pH independent at low pH, but slows down drastically in Rs. rubrum and Rb. sphaeroides (both contain ubiquinone as QA and QB). These contrasting observations lead us to suggest that at low pH, the slow dark relaxation of pigment in Chromatium is determined by direct electron transfer from QB (not through QA).