Kinetic and Potentiometric Characteristics of Ferredoxin: NADP⁺ Oxidoreductase from Chlorobaculum tepidum

Chlorobaculum tepidum ferredoxin: NADP+ oxidoreductase (CtFNR) is a dimeric thioredoxin reductase (TrxR)-type FNR, whose mechanism and redox properties are poorly characterized. In this work, we focused on the reoxidation mechanisms of its flavin adenine dinucleotide (FAD) cofactor using quinones (Q), nitroaromatics (ArNO2), and other nonphysiological oxidants with different single-electron reduction midpoint potentials (E71) and electrostatic charge. Like in other FNRs, the rate-limiting step of the reaction is the reoxidation of FAD semiquinone (FADH•). However, only one FAD per dimer functions in CtFNR due to some nonequivalence of the NADP(H) binding domains in separate subunits. The reactivity of Q increases with increasing E71, while ArNO2 form another analogous series of lower reactivity. The compounds are reduced in a dominant single-electron way. These data are consistent with an "outer sphere" electron transfer mechanism. On the basis of reactions with 3-acetylpyridine adenine dinucleotide phosphate, the two-electron reduction midpoint potential of FAD at pH 7.0 is -0.282 V. In CtFNR, 11% FADH• was stabilized at equilibrium. Calculated electron transfer distances in reactions with Q and ArNO2 were in the range of 2.6-3.4 Å. Taken together with previous studies of Rhodopseudomonas palustris and Bacillus subtilis FNRs, this work allows us to generalize the information on the catalytic ant thermodynamic properties of TrxR-type FNRs. In addition, our data may be valuable from an applied perspective, e.g., the use of redox mediators in photobioelectrochemical systems or microbial cells based on anoxygenic phototrophic bacteria.