Proton Efflux and Phosphorylation in Flash Groups

Abstract

When thylakoid membranes are illuminated by single turnover flashes, transmembrane redox reactions in photosystem I and II lead to the rapid generation (>1 ns) of a transmembrane electric potential difference, Δφ - This Δφ can be detected by the electrochromic absorption changes which can be measured at 524 nm (1). Whereas the generation of Δφ is due to a transmembrane electron shift, the decay represents different events: in the absence of phosphorylation, it is due mainly to the movement of ions, e.g., K+, Cl−, etc. Under phosphorylating conditions, an additional H+ efflux coupled with ATP synthesis is observed (2). The kinetic analysis of the electrochromic absorption changes permits calculation of the phosphorylation-coupled proton efflux. If the amount of synthesized ATP is measured simultaneously the H+/ATP ratio can be calculated (3). When the ATP-synthase is in the oxidized state, H+/ATP = 3 is obtained (3). We have carried out similar measurements with the ATP-synthase in the ative, reduced state (4).