A mathematical model for quinone-herbicide competition in the reaction centres of Rhodobacter sphaeroides

A quantitative model describing the amplitude of semiquinone absorption in photosynthetic reaction centres after successive flashes in the presence of increasing inhibitor concentration is presented. By using relatively simple algebra, the semiquinone signals can be calculated and fitted to the oscillation pattern by optimizing only two parameters; the electron and quinone equilibrium constants, Ke and Kq, respectively. In this work we expand our earlier model [Tandori et al. (1991) Photosynthetica 25, 159-166] by introducing the inhibitor equilibrium constant, Ki, describing the best fit of the model to the measured oscillation pattern. We found that there are characteristic parameters of the measured and normalized signal, and of those calculated from the mathematical model, which fit well with competitive Michaelis-Menten kinetics.