Stimulation of ATP Synthesis by a Membrane Potential in Chloroplasts

Abstract

Chloroplasts preloaded with protons by light-induced proton uptake do not synthesize ATP in a following dark period if the proton concentration gradient (ApH) is lower than 2.5-3.0 pH units. At such suboptimal ApH values synthesis of ATP can be obtained by imposing a diffusion potential across the chloroplast membrane. This potential was realized by providing a high external concentration of either KC1 in the presence of valinomycin or of NaCl in the presence of monactin. Diffusion-potential-induced stimulation of ATP synthesis was observed both in chloroplasts and in bacterial chromatophores.The stimulation could be abolished by the addition of a highly permeant anion, like picrate.It is proposed that the effect of the diffusion potential indicates that an increase in the electrochemical gradient of protons can be coupled to the synthesis of ATP.ATP synthesis coupled to the flow of cations down their gradients has been demonstrated in several systems that contain an ATP-driven ion pump. In erythrocytes high external sodium concentration together with high internal potassium induced ATP synthesis by the reversal of the Na+-Kf pump In chloroplasts proton gradients generate high yields of ATP provided the chloroplasts are loaded with a sufficient number of protons and the pH gradient is large. These requirements are met both in acid-base transition or in post illumination-type experiments [13]. In both cases it can be assumed that proton concentration gradient reverses the ATP-driven proton pump [9,13]. We have recently shown that it is possible to stimulate ATP synthesis both in acid-base transition and postillumination experiments by imposing a diffusion potential on a

Initial formation of ATP in photophosphorylation does not require a proton gradient

“…Such data and denlonstrations that illuminated chloroplasts take up protons that can be used to drive ATP synthesis in a following dark period (post-illumination phosphorylation) [5,6], are consistent with the capacity of the proton gradient to drove ATP formation. Membrane potential created by valinomycin-K' has also been shown to aid ATP fo~atio~l [7,8].…”

Demonstration of ΔpH‐ and Δψ‐induced synthesis of inorganic pyrophosphate in chromatophores from Rhodospirillum rubrum

“…A concentration of 4 mM K + in the assay medium failed to induce ATP synthesis. This has previously been found in chloroplasts [3,[19][20][21][22] and photosynthetic bacteria [23][24][25]. The synthesis of ATP had to be monitored at high sensitivity, which gave noisy recordings and was the reason for the scattering of the points in fig.2.…”

Demonstration of acid—base phosphorylation in chromatophores in the presence of a K+ diffusion potential

“…It is therefore concluded that the imposition of a K' diffusion potential can stimulate acid-base phosphorylation also in chromatophores. In this system, as in postillumination ATP synthesis in chloroplasts [9] and chromatophores [ 151, the degree of stimulation depends on the level of the diffusion potential, since it decreased in the presence of permeant anions which reduce the magnitude of the diffusion potential.…”

“…Recently it has been reported that at suboptimal conditions for postillumination tid acid-base phosphorylation in chloroplasts ATP synthesis could be stimulated by an artificially induced K' diffusion potential [9,10,14]. Similarly, the low level of postillumination ATP formed in chromatophores in the absence of permeant anions or cations in the light stage was found to be markedly stimulated by a K' diffusion potential created by the addition of valinomycin+ KC1 to the dark stage [ 151.…”

“…The yield of ATP obtained in this acid-base phosphorylation [2] as well as in post%mination ATP synthesis in chloroplasts [3] was found to be dependent on the number of protons accumulated inside the particles [4-81 and on the size of the pH gradient (A pH) formed across their membrane [2, 3,9]. When the A pH dropped below a value of 2.5 pH units almost no ATP synthesis was observed in either the acid-base [2,10] or the postillumination [9,10] type experiments. The very low level of postillumination ATP synthesis found in chromatophores [ 1 l-13) has also been traced to the low A pH created during illumination, since an increase in this A pH resulted in a pronounced increase in the yield of ATP [ 131.…”