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Superoxide Reductase: Different Interaction Modes with its Two Redox Partners
Author(s) -
Almeida Rui M.,
Turano Paola,
Moura Isabel,
Moura José J. G.,
Pauleta Sofia R.
Publication year - 2013
Publication title -
chembiochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.05
H-Index - 126
eISSN - 1439-7633
pISSN - 1439-4227
DOI - 10.1002/cbic.201300196
Subject(s) - superoxide , chemistry , redox , electron transfer , rubredoxin , photochemistry , inorganic chemistry , biochemistry , enzyme
Abstract Anaerobic organisms have molecular systems to detoxify reactive oxygen species when transiently exposed to oxygen. One of these systems is superoxide reductase, which reduces O 2 .− to H 2 O 2 without production of molecular oxygen. In order to complete the reduction of superoxide anion, superoxide reductase requires an electron, delivered by its redox partners, which in Desulfovibrio gigas are rubredoxin and/or desulforedoxin. In this work, we characterized the interaction of Desulfovibrio gigas superoxide reductase with both electron donors by using steady‐state kinetics, 2D NMR titrations, and backbone relaxation measurements. The rubredoxin surface involved in the electron transfer complex with superoxide reductase comprises the solvent‐exposed hydrophobic residues in the vicinity of its metal center (Cys9, Gly10, Cys42, Gly43, and Ala44), and a K d of 3 μ M at 59 m M ionic strength was estimated by NMR. The ionic strength dependence of superoxide‐mediated rubredoxin oxidation by superoxide reductase has a maximum k app of (37±12) min −1 at 157 m M . Relative to the electron donor desulforedoxin, its complex with superoxide reductase was not detected by chemical shift perturbation, though this protein is able to transfer electrons to superoxide reductase with a maximum k app of (31±7) min −1 at an ionic strength of 57 m M . Competition experiments using steady‐state kinetics and NMR spectroscopy (backbone relaxation measurements and use of a paramagnetic relaxation enhancement probe) with Fe‐desulforedoxin in the presence of 15 N‐Zn‐rubredoxin showed that these two electron donors compete for the same site on the enzyme surface, as shown in the model structure of the complex generated by using restrained molecular docking calculations. These combined strategies indicate that the two small electron donors bind in different manners, with the desulforedoxin complex being a short lived electron transfer complex or more dynamic, with many equivalent kinetically competent orientations.