Diurnal Pattern of Chloroplast Coupling Factor Oxidation Kinetics in Leaves of Intact Sunflower

Abstract

In green plants light energy is stored in the form of redox free-energy and a proton gradient generated by light driven electron transport. The proton motive force (pmf) across the thylakoid membrane, in turn, drives the phosphorylation of ADP to ATP catalyzed by the CF1,-CF0-ATP synthase or coupling factor (ATP-ase). The regulation of the chloroplast ATP-ase is important since this enzyme can catalyze not only the synthesis of ATP from ADP and Pi, driven by a pmf across the membrane, but also the reverse reaction, hydrolysis of ATP with the formation of a pmf (1) to passive dissipation of ΔGaATP as protons across the thylakoid membrane. Work on this regulation using isolated systems has been summarized by a model presented by Junesch and Graber (2). Briefly, this model states as follows. The ATP-ase is not enzymatically active until a pmf, either as a ΔpH or as ΔΨ, or their sum, is imposed across the thylakoid membrane. In addition, regulation is governed by the redox state of a pair of sulfhydryl groups on the -subunit of the ATP-ase. Both the oxidized and reduced forms of the enzyme apparently have the same maximal catalytic rate, but the magnitude of the pmf required to activate the reduced form is less than for the oxidized form.