Identification of the site where the electron transfer chain of plant mitochondria is stimulated by electrostatic charge screening

Modular kinetic analysis was used to determine the sites in plant mitochond ria where charge-screening stimulates the rate of electron transfer from ex ternal NAD(P)H to oxygen. In mitochondria isolated from potato (Solanum tub erosum L.) tuber callus, stimulation of the rate of oxygen uptake was accom panied by a decrease in the steady-state reduction level of coenzyme Q, and by a small decrease in the steady-state reduction level of cytochrome c. M odular kinetic analysis around coenzyme Q revealed that stimulation of the rate was due to stimulation of quinol oxidation via the cytochrome pathway (cytochrome bc(1), cytochrome c and cytochrome c oxidase). It was not a con sequence of any effect on quinone reduction (by external NADH or NADPH dehy drogenase). This explains the salt-induced decrease in the steady-state red uction level of coenzyme Q. Analysis around cytochrome c revealed that stim ulation by salts was due to a dual effect on the respiratory chain. The kin etic curves for the oxidation and reduction pathways of cytochrome c reveal ed that they were both activated by salt, the simultaneity explaining the s mall variation observed in the steady-state reduction level of cytochrome c . A simple kinetic core model is used to show that changes in the rate of d issociation of cytochrome c from the membrane can explain the observed kine tic changes in both cytochrome c reduction and cytochrome c oxidation. The stimulation is proposed to be the result of an increase in the rate constan t of cytochrome c dissociation from the membrane induced by cation screenin g. We conclude that this type of modular kinetic analysis is a powerful too l to identify and quantitatively characterize multiple-site effects on the mitochondrial respiratory chain.