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

Flash-induced redox changes of cytochrome b-563, cytochrome f{hook} 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{hook} 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{hook} is oxidised by plastocyanin and quinol is subsequently oxidised by cytochrome b f{hook} 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{hook} complex containing reduced or oxidised cytochrome f{hook}. 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{hook} complex under the conditionsused: k_pp (between cytochrome f{hook} 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{hook} appeared to be in rapid equilibrium, with an equilibrium constant of about 3, Rieske positive to f{hook}.