A simple, quantitative approach to the coupling of photophosphorylation to electron flow in terms of proton fluxes

Abstract

A simple relationship between observed phosphorylation efficiencies (P/e ratios) and internal proton concentration in spinach chloroplast thylakoids was derived. P/e ratios, varied by either changing the light intensity or by adding the energy transfer inhibitor, 4''-deoxyphlorizin, changed with internal proton concentration in accordance with this relationship. A quantitative prediction of the effect of uncouplers on the P/e ratio can probably be made. By extrapolation of plots of observed P/e ratios against internal proton concentration divided by the overall rate of electron flow, a maximum intrinsic P/e of about 0.66 is obtained. Assuming that 2 protons appear inside thylakoids per electron transferred, a P/e ratio of 0.66 suggests that 3 internal protons are consumed for each ATP formed. Internal protons may be considered to be substrates for the phosphorylation reaction. Hill plots of phosphorylation rate vs. internal proton concentration also indicate that 3 protons are consumed for each ATP synthesized. Thus, the H+ concentration gradient behaves quantitatively, as well as qualitatively, as if it is the connecting link between electron flow and phosphorylation in illuminated thylakoids.