Three substrate binding sites on spinach ferredoxin:NADP+ oxidoreductase.: Studies with selectively acting inhibitors

The effects of phenylmercuric acetate (PMA) and apoferredoxin (apoFd) on the diaphorase activity of spinach ferredoxin:NADP(+) oxidoreductase (FNR) in the presence of dibromothymoquinone (DBMIB) or cytochrome c (Cyt c) were studied. PMA inhibited effectively (I-50 = < 5 mu M) ferredoxin-dependent Cyt c reduction but did not affect evidently the enzyme activity in the presence of DBMIB as an electron acceptor. ApoFd caused also inhibition of Cyt c reduction but slightly stimulated, like ferredoxin, DBMIB reduction. We confirm a hypothesis according to which three binding sites for substrates [NADP(H), Fd-Cyt c, quinone/dichlorophenol indophenol] occur within the molecule of isolated FNR.