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

Abstract

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 Since wild-type flavodoxin dimerises via intermolecular disulphide bond formation between Cys69 residues, Cys69 has been replaced by both an alanine and a serine residue.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 the 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: pK(red,1) = 5.39 ± 0.08, pK(ox) = 7.29 ± 0.14, and pK(red,2) = 7.84 ± 0.14 with E(m,7) = -459 ± 4 mV, and a constant redox potential at high pH of -485 ± 4 mV. The dependence of the semiquinone/hydroquinone redox potential on temperature is -0.5 ± 0.1 mV · K-1, yielding ΔH° = 28.6 ± 1.5 kJ · mol-1 and ΔS° = -50.0 ± 6.2 J · mol-1 · 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 II.