Shift from localized to delocalized protonic energy coupling in thylakoids by permeant amines
1988; Elsevier BV; Volume: 934; Issue: 3 Linguagem: Inglês
10.1016/0005-2728(88)90095-3
ISSN1879-2650
AutoresClaude Sigalat, Yaroslav de Kouchkovsky, Francis Haraux, Françoise de Kouchkovsky,
Tópico(s)Photoreceptor and optogenetics research
ResumoWe have suggested that in energy-transducing organelles structural constraints may hinder H+ transport from their sites of active, redox translocation to their sites of passive or phosphorylating escape (microchemiosmosis). We could modulate these constraints first by affecting the physico-chemical properties of the medium, and now by adding permeant amine buffers to a suspension of thylakoids. The following results are obtained. (1) Whether driven by Photosystem I or by Photosystem II, phosphorylation is stimulated by amines (imidazole, hexylamine, NH4CI) at concentrations low enough hardly to modify the proton gradient: ΔpH is stable if not reduced, Δψ is slightly increased but still negligible; this extends the observations made by Giersch. (2) The concentration curves of phosphorylation stimulation by amines exhibit a minor peak before the main one previously reported. (3) ATP synthesis and ΔpH are decreased by amines at higher concentrations, but the dependence of the phosphorylation rate (flow) versus ΔpH (force) is then shifted towards lower ΔpH values. (4) The normally less efficient Photosystem II-driven phosphorylation is comparable to Photosystem I with amines. (5) The flow-force curves, which are distinct when traced by limiting ΔpH by an H+ influx decrease (light) or efflux increase (nigericin), are much closer with either photosystem when amines are added. The Photosystem I curve produced by increasing nigericin beyond approx. 200 nM becomes insensitive to amines. (6) In general, Photosystem II requires significantly less nigericin or amines than Photosystem I to obtain similar effects. It is proposed that amines have a double effect. By efficiently carrying protons along the membrane, amines lower the protonic resistances and thereby delocalize the proton gradient. At higher amine concentrations, uncoupling occurs, probably by an indirect backflow of protons more than by a buffering effect, as generally admitted. In conclusion, the multiple-resistances microchemiosmotic scheme which we have proposed earlier is strengthened; it predicts that intermediate states may link delocalized (canonical chemiosmosis) and localized coupling modes, which is established here.
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