Abstract
Abstract The treatment of chloroplasts with 1 mm N-ethylmaleimide (NEM) in the light, but not in the dark, resulted in a partial, permanent inhibition of photophosphorylation. Noncyclic electron flow coupled to phosphorylation was also inhibited on incubation of chloroplasts with NEM in the light, but this inhibition was reversed by ammonium chloride. The uncouplers, carbonyl cyanide m-chlorophenylhydrazone and ammonium chloride, largely prevented the inhibition. Adenosine triphosphate and adenosine diphosphate partially protected phosphorylation from inhibition by light and NEM. Light-induced hydrogen ion uptake was unaffected by NEM except that the uptake stimulated by adenosine triphosphate was inhibited. The calcium-dependent adenosine triphosphatase of coupling factor 1 was also inhibited in chloroplasts treated with NEM in the light prior to extraction of the coupling factor. It is suggested that light causes a conformational change in the membrane-bound coupling factor, allowing the reaction of a group (or groups) with NEM. The sensitivity of the inhibition to adenine nucleotides suggests that these nucleotides modify the conformation of coupling factor 1 in the light.
Highlights
The uncouplers, carbonyl cyanide m-chlorophenylhydrazone and ammonium chloride, largely prevented the inhibition
It is suggested that light causes a conformational change in the membrane-bound coupling factor, allowing the reaction of a group with NEM
Uncoupling concentrations of NH&l and CCP protected chloroplasts from the action of NEM in the light (Table III). These results suggest that the light requirement t’o form the i,oo;\wLk
Summary
The treatment of chloroplasts with 1 mM N-ethylmaleimide (NEM) in the light, but not in the dark, resulted in a partial, permanent inhibition of photophosphorylation. It is suggested that light causes a conformational change in the membrane-bound coupling factor, allowing the reaction of a group (or groups) with NEM. We recently reported (1) that low concentrations of ATP enhance the estent of light-dependent H+ uptake (2) in spinach chloroplasts. Because this stimulation was sensitive to an an&. A light-induced conformations[1] change in CF1 might cause the esposure of a sulfhydryl group which in the dark is not accessible to NEAL If the exposed group were required for CF, activity, its react,ion with NE;[11] would be expected to inhibit phosphorylstion. Aliquots of the mixtures were either assayed directly for photophosphorylation or were further treated prior to assay as described in the legends to the figures and tables
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