Redox properties of wild-type, Cys69Ala, and Cys69Ser <em>Azotobacter vinelandii</em> flavodoxin II as measured by cyclic voltammetry and EPR spectroscopy.

This study deals with the detailed electrochemistry and complete EPR-monitored titrations of flavodoxin II of Azotobacter vinelandii (ATCC 478). Since wild-type flavodoxin dimerises via intermolecular disulphide bond formation between Cys69 residues. Cys69 has been replaced by both an alanine and a serine residue. Redox properties of she C69A and C69S flavodoxin mutants were compared to those of wild-type flavodoxin. In the presence of the promotor neomycin, C69A and C69S flavodoxin showed a reversible response of the semiquinone/hydroquinone couple at the glassy carbon electrode. However, the addition of dithiothreitol proved to be necessary for the stabilisation of the wild-type flavodoxin response. EPR-monitored redox titrations of wild-type and C69A flavodoxin at high and low pH confirmed the redox potentials measured using cyclic voltammetry. The pH dependence of the semiquinone/hydroquinone redox potentials cannot be described using a model assuming one redox-linked pK. Instead, the presence of at least two redox-linked protonation sites is suggested: pKred,1 = 5.39 &#177; 0.08. pKox = 7.29 &#177; 0.14, and pKred,2 = 7.84 &#177; 0.14 with Em,7 = -459 &#177; 4 mV, and a constant redox potential at high pH of -485 &#177; mV. The dependence of the semiquinone/hydroquionone redox potential on temperature is -0.5 &#177; 0.1 mV &#183; K-1, yielding ΔH&#176; = 28.6 &#177; 1.5 kJ &#183; mol-1 and ΔS&#176; = -50.0 &#177; 6.2 J &#183; mol -1 &#183; K-1. No significant differences in redox properties of wild-type. C69A, and C69S flavodoxin were observed. The electrochemical data suggest that replacement of Cys69 in the vicinity of the FMN by either an alanine or a serine residue does not alter the dielectric properties and structure of A. vinelandii flavodoxin 11. [ABSTRACT FROM AUTHOR]