Relations between the electrical potential, pH gradient, proton flux and phosphorylation in the photosynthetic membrane

Abstract

The transmembrane electrical potential (Δϕ), the proton flux ▪, the rate of electron transport ▪, the pH gradient (ΔpH) and the rate of phosphorylation ( ▪) were measured in chloroplasts of spinach. Photosynthesis was excited periodically with flashes of variable frequencies and intensities. A new method is described for determining the rate of electron transport and proton flux. Under conditions where the rate of electron transport and proton flux are not pH controlled the following correlations were found in the range 50 mV ⩽ Δϕ ⩽ 125 mV and 1.8 ⩽ ΔpH ⩽ 2.7: 1. (1) The pH gradient, ΔpH, increases with ▪ independently of pH out between 7–9. 2. (2) The rate of phosphorylation, ▪, depends exponentially on ΔpH (at constant Δϕ) and is independent of pH out between 7–9. 3. (3) The rate of phosphorylation, ▪, depends also on Δϕ (at constant ΔpH and at constant proton flux ▪. 4. (4) The proton flux via the ATPase pathway, ▪, depends non-linearly on the ratio of the proton concentrations: ▪ ∼ ( H in + H out + ) b , (b = 2.3–2.6). The proton flux via the basal pathway, ▪, depends linearly on the ratio of the proton concentrations: ▪ ∼ ( H in H out ) . 5. (5) The ratio Δ H + ATP ( e ATP ) , i.e. the ratio of the total proton flux, ▪ + ▪, and the rate of ATP formation, ▪, depends strongly on Δϕ and on ΔpH. The ratio is Δ H + ATP ≈ 3 ( e ATP ≈ 1.5) at ΔpH 2.7 and Δϕ = 125 mV. 6. (6) It is supposed that the reason for the dependence of Δ H + ATP on Δϕ and ΔpH is the different functional dependence of the basal proton flux ▪ and the phosphorylating proton flux ▪ on ΔpH and Δϕ. The calculation of Δ H + ATP on the basis of this assumption is in fair agreement with the experimental values. Also the “threshold” effects can be explained in this way. 7. (7) The ratio of Δ H p + ATP , i.e. the ratio of the phosphorylating proton flux ▪ and ▪, is Δ H p + ATP ≈ 2.4 .