Ca++ Gating of Proton Fluxes in Thylakoid Membranes: Regulation of Localised and Delocalised Energy Coupled Proton Gradients
This report gives an overview of recent developments in the study of how proton gradients are generated and utilised for energising ATP formation in spinach or pea chloroplasts. The evidence is reviewed that supports the hypothesis for there being energisation either by membrane domain-localised proton gradients or delocalised proton gradients. Delocalised H+ gradients competent to drive ATP formation form under several different conditions—all of which appear to displace Ca++ ions from binding sites in the thylakoid or on the lumen side of the membrane. A localised proton gradient coupling mode is observed when Ca++ ions are not displaced in the first place (such as by pretreatments which maintain low K+ concentrations or by not over-acidifying the thylakoids) or if Ca++ ions are added to thylakoids which were treated so as to displace Ca++ ions from thylakoid binding sites. Treatments such as 100 mM KCl incubation, over-acidifying the thylakoid by basal electron flow-dependent H+ accumulation, or those in which a lipid-soluble Ca++ chelator is added, lead to delocalised H+ gradients. Ca++ ions added to the first and third treatments listed above reverse the delocalising tendency, and maintain the localised gradient coupling mode.
Intact chloroplasts respond to such treatments in the same manner as isolated thylakoids. Therefore, we conclude that the evidence favours the hypothesis that thylakoid energy coupling proton gradients are regulated by Ca++ binding to a ‘gating structure’ so that either localised or delocalised H+ fluxes can provide the energetic H+ gradient needed to drive ATP formation.
KeywordsThylakoid Membrane Proton Gradient Proton Flux Intact Chloroplast Flash Sequence
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