Flavodoxin with an air-stable flavin semiquinone in a green sulfur bacterium
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Flavodoxins are small proteins with a non-covalently bound FMN that can accept two electrons and accordingly adopt three redox states: oxidized (quinone), one-electron reduced (semiquinone), and two-electron reduced (quinol). In iron-deficient cyanobacteria and algae, flavodoxin can substitute for ferredoxin as the electron carrier in the photosynthetic electron transport chain. Here, we demonstrate a similar function for flavodoxin from the green sulfur bacterium Chlorobium phaeovibrioides (cp-Fld). The expression of the cp-Fld gene, found in a close proximity with the genes for other proteins associated with iron transport and storage, increased in a low-iron medium. cp-Fld produced in Escherichia coli exhibited the optical, ERP, and electron-nuclear double resonance spectra that were similar to those of known flavodoxins. However, unlike all other flavodoxins, cp-Fld exhibited unprecedented stability of FMN semiquinone to oxidation by air and difference in midpoint redox potentials for the quinone–semiquinone and semiquinone–quinol couples (− 110 and − 530 mV, respectively). cp-Fld could be reduced by pyruvate:ferredoxin oxidoreductase found in the membrane-free extract of Chl. phaeovibrioides cells and photo-reduced by the photosynthetic reaction center found in membrane vesicles from these cells. The green sulfur bacterium Chl. phaeovibrioides appears thus to be a new type of the photosynthetic organisms that can use flavodoxin as an alternative electron carrier to cope with iron deficiency.
KeywordsFlavodoxin Green sulfur bacteria Redox titration Electron transport Iron deficiency ENDOR
- cp-Fld, cp-Fldox, cp-Fldsq, cp-Fldred
Flavodoxin from Chl. phaeovibrioides and its oxidized, semiquinone, and completely reduced forms, respectively
Midpoint redox potential
Electron-nuclear double resonance
Green sulfur bacteria
Photosynthetic reaction center
Quantitative reverse transcription polymerase chain reaction
This work was supported by the Russian Science Foundation research project 19-14-00063. We are indebted to Prof. R.N. Ivanovsky for helpful discussions.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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