Redox and flavin‐binding properties of recombinant flavodoxin from Desulfovibrio vulgaris (Hildenborough)

Flavodoxin from Desulfovibrio vulgaris (Hildenborough) has been expressed at a high level (3–4% soluble protein) in Escherichia coli by subcloning a minimal insert carrying the gene behind the tac promoter of plasmid pDK6. The recombinant protein was readily isolated and its properties were shown to be identical to those of the wild‐type protein obtained directly from D. vulgaris, with the exception that the recombinant protein lacks the N‐terminal methionine residue. Detailed measurements of the redox potentials of this flavodoxin are reported for the first time. The redox potential, E2, for the couple oxidized flavodoxin/flavodoxin semiquinone at pH 7.0 is –143 mV (25°C), while the value for the flavodoxin semiquinone/flavodoxin hydroquinone couple (E1) at the same pH is –440 mV. The effects of pH on the observed potentials were examined; E2 varies lineraly with pH (slope =–59 mV), while E1 is independent of pH at high pH values, but below pH 7.5 the potential becomes less negative with decreasing pH, indicating a redox‐linked protonation of the flavodoxin hydroquinone. D. vulgaris apoflavodoxin binds FMN very tightly, with a value of 0.24 nM for the dissociation constant (Kd) at pH 7.0 and 25°C, similar to that observed with other flavodoxins. In addition, the apoflavodoxin readily binds riboflavin (Kd= 0.72 μM; 50 mM sodium phosphate, pH 7.0, 5 mM EDTA at 25°C) and the complex is spectroscopically very similar to that formed with FMN. The redox potentials for the riboflavin complex were determined at pH 6.5 (E1=–262 mV, E2=–193 mV; 25°C) and are discussed in the light of earlier proposals that charge/charge interactions between different parts of the flavin hydroquinone play a crucial role in determining E1 in flavodoxin.