Redox-dependent structural changes in the superoxide reductase from $Desulfoarculus\ baarsii$ and $Treponema\ pallidum$ : A FTIR study

The redox-induced structural changes at the active site of the superoxide reductase (SOR)from Desulfoarculus baarsii and Treponema pallidum have been monitored by means of FTIR differencespectroscopy coupled to electrochemistry. With this technique, the structure and interactions formed byindividual amino acids at a redox site can be detected. The infrared data on wild-type, Glu47Ala, andLys48Ile mutants of the SOR from D. baarsii provide experimental support for the conclusion that thetwo different coordination motifs observed in the three-dimensional structure of the SOR from Pyrococcusfuriosus [Yeh, A. P., Hu, Y., Jenney, F. E., Adams, M. W. W., and Rees, D. (2000) Biochemistry 39,2499-2508] correspond to the two redox forms of the SOR iron center. We extend this result to thecenter II iron of SOR of the desulfoferrodoxin type. Similar structural changes are also observed uponiron oxidation in the SOR of T. pallidum. In D. baarsii, the IR modes of the Glu47 side chain supportthat it provides a monodentate ligand to the oxidized iron, while it does not interact with Fe2+. Structuralchanges at the level of peptide bond(s) observed upon iron oxidation in wild-type are suppressed in theGlu47Ala mutant. We propose that the presence of the Glu side chain plays an important role for thestructural reorganization accompanying iron oxidation. We identified the infrared modes of the Lys48side chain and found that a change in its environment occurs upon iron oxidation. The lack of otherstructural changes upon the Lys48Ile mutation shows that the catalytic role of Lys, as evidenced by pulseradiolysis experiments [Lombard, M., Houe´e-Levin, C., Touati, D., Fontecave, M., and Nivie`re, V. (2001)Biochemistry 40, 5032-5040], is purely electrostatic, guiding superoxide toward the reduced iron