Ferredoxin-Mediated Electron Transport and Photophosphorylation Reactions in Spinach Chloroplasts.

Abstract

ATP synthesis and electron transport are measured in an aerobic reconstituted system of thylakoid membranes plus purified ferredoxin. Electron transport is supported by substrate levels of ferredoxin with oxygen as the terminal electron acceptor, or by catalytic amounts of ferredoxin with saturating NADP. Both the oxygen and NADP reduction pathways are coupled with a P/O of 1.6 which reflects concurrent noncyclic and PSI cyclic contributions to ATP synthesis. The cyclic component is highly sensitive to antimycin A in the presence of ferredoxin alone (oxygen reduction), however antimycin A has little or no effect on the cycle during NADP reduction. EPR measurements of P700('+), and spectral observations of interchain b cytochromes, show that antimycin A sensitivity is seen only under reducing conditions in the interphotosystem chain. The magnitude of ATP synthesis by the PSI cycle during oxygen reduction is dependent upon the concentration of reduced ferredoxin, while the cycle in the presence of NADP is not. Heparin, a non-redox active ionic analog of ferredoxin which is shown to compete with ferredoxin during NADP reduction, inhibits PSI cyclic photophosphorylation at low concentrations during oxygen reduction, but has little effect on the cycle during NADP reduction. In the presence of ferredoxin alone the P/O is stimulated by partial inhibition of PSII electron flow with DCMU; this effect is not seen in the presence of NADP. Similar stimulation of the P/O is seen with a high NADPH/NADP poise; this increased P/O is partially sensitive to antimycin A. The PSI cyclic reactions during oxygen reduction and NADP reduction show differing characteristics with regard to (1) antimycin inhibition, (2) heparin sensitivity, (3) the requirement for reduced ferredoxin, and (4) partial inhibition by DCMU. This suggests the presence of two phosphorylating cycles around PSI--one operating under reduced interchain conditions, the other under more oxidized conditions (i.e. NADP reduction). The increased P/O seen with a high NADPH/NADP poise seems to result from the concurrent activity of both cycles as the interchain assumes an intermediate redox level.