The kinetics of reactions around the cytochrome bf complex studied in an isolated system

Abstract

The kinetics of oxidation and reduction of P700, plastocyanin, cytochrome f and cytochrome b-563 were studied in a reconstituted system consisting of Photosystem I particles, cytochrome bf complex and plastocyanin, all derived from pea leaf chloroplasts. Decyl plastoquinol was the reductant of the bf complex. Turnovers of the system were initiated by laser flashes. The reaction between oxidised P700 and plastocyanin was non-homogeneous in that a second-order rate coefficient of c. 5×10⁻⁷ M⁻¹ s⁻¹ applied to 80% of the P700⁺ and c. 0.7×10⁷ M⁻¹ s⁻¹ to the remainder. In the presence of bf complex, but without quinol, the electron transfer between cytochrome f and oxidised plastocyanin could be described by a second-order rate coefficient of c. 4×10⁷ M⁻¹ s⁻¹ (forward), and c. 1.6×10⁷ M⁻¹ s⁻¹ (reverse). The equilibrium coefficient was thus 2.5. Unexpectedly, there was little reduction of cytochrome f ⁺ or plastocyanin⁺ by electrons from the Rieske centre. With added quinol, reduction of cytochrome b-563 occurred. Concomitantly, electrons appeared in the oxidised species. It was inferred that either the Rieske centre was not involved in the high-potential chain of electron transfer events, or that, only in the presence of quinol, electrons were quickly passed from the Rieske centre to cytochrome f ⁺. Additionally, the presence of quinol altered the equilibrium coefficient for the cyt f/PC interaction from 2.5 to c. 5. The reaction between quinol and the bf complex was describable by a second-order rate coefficient of about 3×10⁶ M⁻¹ s⁻¹. The pattern of the redox reactions around the bf complex could be simulated in detail with a Q-cycle model as previously found for chloroplasts.