ELECTRON-TRANSFER REACTIONS OF ANABAENA PCC-7119 FERREDOXIN-NADP(+) REDUCTASE WITH NONPHYSIOLOGICAL OXIDANTS

The mechanism of single-electron oxidation of ferredoxin-NADP(+) reduc tase (FNR) (EC 1.18.1.2) from cyanobacterium Anabaena PCC 7119 by quin ones, aromatic nitrocompounds and inorganic complexes has been studied . In steady-state experiments, the logarithms of bimolecular rate cons tants of reduction of quinones and nitroaromatics increase with an inc rease in their single-electron reduction potential, the reactivities o f 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 FN R is not involved in reduction. In stopped-flow kinetics of oxidation of photoreduced enzyme by 5,8-dihydroxy-1,4-naphthoquinone, the oxidat ion of FADH to FAD proceeds much slower than oxidation of FADH to semi quinone. 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 t ransfer 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 t o 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 tran sfer coupled to a slower proton transfer.