Electron transfer reactions of Anabaena PCC 7119 ferredoxin:NADP+ reductase with nonphysiological oxidants

AbstractThe mechanism of single-electron oxidation of ferredoxin-NADP+ reductase (FNR) (EC 1.18.1.2) from cyanobacterium Anabaena PCC 7119 by quinones, aromatic nitrocompounds and inorganic complexes has been studied. In steady-state experiments, the logarithms of bimolecular rate constants of reduction of quinones and nitroaromatics increase with an increase in their single-electron reduction potential, the reactivities of nitroaromatics being markedly lower than of quinones. The absence of inhibition of reaction by ferredoxin and insignificant ionic strength effects suggest that positively charged ferredoxin binding site of FNR is not involved in reduction. In stopped-flow kinetics of oxidation of photoreduced enzyme by 5,8-dihydroxy-1,4-naphthoquinone, the oxidation of FADH. to FAD proceeds much slower than oxidation of FADH- to semiquinone. The patterns of reaction inhibition by NADP+ and 2′,5′-ADP also suggest that oxidation of FAD semiquinone is a rate-limiting step in oxidative half-reaction of steady-state experiments. The analysis of reaction kinetics within the framework of `outer-sphere' electron transfer model gives the values of electron self-exchange constants of FAD/FADH. couple and site-to-surface distances, 0.82–0.95 nm, that seem overestimated in view of available data on the accessibility of FAD to solvent. A possible explanation of poor reactivity of FAD/FADH. redox couple of ferredoxin:NADP+ reductase in comparison to FADH./FADH− couple is that oxidation of FADH− to semiquinone re-presents a `pure' electron transfer, whereas oxidation of FADH. to FAD is electron transfer coupled to a slower proton transfer