Ferredoxin:NADP⁺ Oxidoreductase Associated with Cytochrome b ₆ f Complex is Highly Active in Plastoquinone Reduction

Abstract

Three methods, differing in the preservation of the cytochrome b ₆ f-associated ferredoxin:NADP⁺ oxidoreductase (FNR), have been applied for the isolation of the cytochrome b ₆ f complex from spinach. The complexes isolated by all three methods showed presence of FNR peptide(s), as revealed immunoblot analysis. However, when incorporated into liposome membranes, the NADPH-PQ (plastoquinone) oxidoreductase activity was not detected for the cytochrome b ₆ f complex isolated with the classical method including NaBr wash. Partial activity was found for the complex isolated with the omission of the wash, but the highest activity was detected for the complex isolated with the use of digitonin. The reaction rate was not significantly influenced by the addition of free FNR or ferredoxin. The reaction was neither affected by triphenyltin nor isobutyl cyanide. On the other hand, the reaction was severely inhibited by NQNO, an competitive inhibitor of PQ at the Q_i site. The obtained data indicate that FNR associated with the cytochrome b ₆ f complex can participate in the cyclic electron transport or chlororepiration as PSI-PQ or NADPH-PQ oxidoreductase, respectively. Moreover, we have shown that PQ in liposomes can be non-enzymatically reduced by ascorbate and this reaction might contribute to dark-reduction pathways of PQ-pool in chloroplasts.