The Kinetics of Reactions in and Near the Cytochrome b/f Complex of Chloroplasts. II. Cytochrome b-563 Reduction

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Abstract
The reduction of cytochrome b-563 was measured following flash-induced electron transfer from duroquinol to methyl viologen, in the presence of 3(3,4-dichlorophenyl)-1,1-dimethyl urea and 1 .mu.M nonactin or 1 .mu.M valinomycin (plus 10 mM K+). The apparent rate constant of this reduction (nonactin present) increased from about 100 s-1 to 460 s-1 as the external concentration of duroquinol was varied from 0.01 to 0.5 mm. The corresponding maximum extent of reduction of cytochrome b-563 varied from 0.13 to 0.27 molecules per b/f complex. Rate constants in the presence of valinomycin were lower at all concentrations of duroquinol by a factor of about 1.5. The mean enthalpy of activation calculated from Arrhenius plots of apparent rate constant for cytochrome b-563 reduction was 60 kJ mol-1, for temperature variation betwee 23 and 4.degree.C. The above, and further data in oxidizing conditions, and with added 2-n-nonyl-4-hydroxyquinoline N-oxide, together with data on proton deposition, were compared with the predictions of kinetic model. In this model, flash-generated oxidised plastocyanin oxidised Rieske centres during random diffusion, and plastoquinol reduced the Rieske centres and cytochrome b-563 sequentially, subsequent electron and proton transfer followed those in a Q-cycle. Many observations were predicted by the model, in which the rate-limiting step was the first electron transfer from plastoquinol to the Rieske centre, subsequent steps being much faster. The rate and extent of reduction of cytochrome b-563 were fully consistent with a reaction between it and a radical form of plastoquinone formed after oxidation of the latter by Rieske centres (referred to as ''oxidant-induced reduction''), under a wide range of conditions.

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