| Abstract
| - Flavodoxins catalyze redox reactions using the isoalloxazine moiety of the flavin mononucleotide(FMN) cofactor stacked between two aromatic residues located in two peptide loops. At high FMNconcentrations that favor stacked FMN dimers in solution, isothermal titration calorimetric studies showthat these dimers bind strongly to apo-flavodoxin from Desulfovibrio desulfuricans (30 °C, 20 mM Hepes,pH 7, KD = 5.8 μM). Upon increasing the temperature so the FMN dimers dissociate (as shown by 1HNMR), only one-to-one (FMN-to-protein) binding is observed. Calorimetric titrations result in one-to-one binding also in the presence of phosphate or sulfate (30 °C, 13 mM anion, pH 7, KD = 0.4 μM).FMN remains dimeric in the presence of phosphate and sulfate, suggesting that specific binding of adivalent anion to the phosphate-binding site triggers ordering of the peptide loops so only one isoalloxazinecan fit. Although the physiological relevance of FMN and other nucleotides as dimers has not been explored,our study shows that high-affinity binding to proteins of such dimers can occur in vitro. This emphasizesthat the cofactor-binding site in flavodoxin is more flexible than previously expected.
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