Evidence for two cyclic photophosphorylation reactions concurrent with ferredoxin-catalyzed non-cyclic electron transport

Addition of ferredoxin to isolated spinach chloroplast thylakoid membranes reconstitutes phosphorylating electron transfer characterized by elevated P / O ratios (1.6). When oxygen is the terminal electron acceptor for reduced ferredoxin, the P / O value is lowered by antimycin A or low concentrations (I0/~M) of heparin (an anionic macromolecule), but not by inhibition of the activity of membrane-bound ferredoxin-NADP reductase. When NADP is present as the terminal electron acceptor for reduced ferredoxin, the elevated P / O value (again 1.6) is unaffected either by antimycin A or low concentrations (I0/~M) of heparin. When ferredoxin-catalyzed cyclic or Q-loop activity is sensitive to antimycin A, the ferredoxin pool and P-700 are both present in a largely reduced state. The opposite result is obtained for antimycin-A-insensitive activity in the presence of NADP. Our results show that conditions exist whereby ferredoxin-catalyzed cyclic electron transport is insensitive to a classical inhibitor of the cytochrome b function. We suggest that the antimycin-A-insensitive pathway of ferredoxin-catalyzed cyclic electron transport may involve the activity of ferredoxin-NADP reductase.