The flash-induced turnover of cytochrome b-563, cytochrome f and plastocyanin in chloroplasts. Models and estimation of kinetic parameters

Abstract Flash-induced redox changes of cytochrome b -563, cytochrome f and plastocyanin (PC), and the electrochromic response from chloroplast suspensions in reducing conditions (added dichlorophenyldimethylurea, exogenous quinol, anaerobic) were measured in the time range 0–20 ms by deconvoluting absorbance changes at appropriate wavelenghts. No response attributable to cytochrome b -559 was observed. Various dimethylbenzoquinols with different substituents on the 6-position of the benzene ring gave similar kinetics for these processes. There was no significant effect on cytochrome b -563, f or PC kinetics of adding nonactin to decay the transthylakoid electric potential difference. These data, together with comparable data for proton deposition, were used in a parameter optimisation procedure, the Inverse Method, to produce rate coefficients for some of the partial reactions occurring when cytochrome f is oxidised by plastocyanin and quinol is subsequently oxidised by cytochrome b / f complexes. Models such as the Q-cycle or semiquinone (SQ) cycle were used to formulate differential equations describing the time-dependencies of various forms of the cytochrome b / f complex containing reduced or oxidised cytochrome f . Rieske centre, cytochrome b -563, and so forth. The Inverse Method minimised the error between data and corresponding model predictions by adjusting parameter values. A model with the two b -563 cytochromes not directly connected electronically was unsatisfactory; Q-cycle and SQ-cycle models could not be differentiated by the available data. A Q-cycle model gave a close match in all respects between data and model predictions using 8 rate coefficients. The following average, reduced rate-coefficients (s −1 ) were estimated for the chloroplast cytochrome b / f complex under the conditionsused: k pp (between cytochrome f and P 2000; k pp (reverse) 220; k OR (between quinol and Rieske centre) 200; k HQ (between cytochrome b -563 (high potential) and quinone at n-sites) 380; k QH (reverse) 150. The Rieske centre and cytochrome f appeared to be in rapid equilibrium, with an equilibrium constant of about 3, Rieske positive to f .