Light-dependent proton efflux from chloroplast thylakoids

Abstract

This work clarifies certain aspects of proton conductivity estimates in the light compared to dark conditions for spinach chloroplast thylakoid membranes. A method is presented, with kinetic analysis to justify it, that permits the separation of the proton influx and efflux rate constants, the sum of which contributes to the measured apparent first-order rate constant for proton efflux linked to basal electron transport. Proton fluxes linked to ATP formation were not dealt with. Using this technique it was shown that dicyclohexylcarbodiimide, an inhibitor of proton channel function, completely blocks a component of proton efflux in the light, as well as partially blocking the proton efflux in the dark. Antibody against purified chloroplast coupling factor (CF 1) inhibits the light-dependent proton efflux, but has no effect on the dark proton efflux. Those data are consistent with there being a proton efflux pathway through the coupling factor complex, both in the light and the dark. The H + efflux through the coupling factor was closely correlated with adenine nucleotide exchange activity. As suggested by others, such exchange activity may be an indication of conformational changes linked to the activation of the coupling factor. A plausible model is that the positive proton electrochemical potential gradient leads to an interaction between protons and the coupling factor, causing a conformational change, which leads to adenine nucleotide exchange linked to the passage of protons through the coupling complex. The nucleotide exchange activity reflects a transition from a higher to a lower binding affinity. Some of the Gibbs free energy lost in the dissipation of the proton gradient must be conserved in the transition to the lower affinity adenine nucleotide binding form of the coupling factor protein complex.