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Superoxide reductase (SOR) catalyzes the reduction of O2*- to H2O2. Itsactive site consists of a non-heme Fe2+ center in an unusual square-pyramidal[His4 Cys] coordination. Like many SORs, the electronic absorption bandcorresponding to the oxidized active site of the SOR from Desulfoarculusbaarsii exhibits a pH-dependent alkaline transition changing from ca. 644 to560 nm as the pH increases and with an apparent pKa of 9.0. Variants in whichthe conserved amino acids glutamate 47 and lysine 48 were replaced by theneutral residues alanine (E47A) and isoleucine (K48I), respectively, exhibitedthe same alkaline transition but at lower apparent pKa values of 6.7 and 7.6,respectively. Previous work [Nivi{\`e}re, V.; Asso, M.; Weill, C. O.; Lombard,M.; Guigliarelli, B.; Favaudon, V.; Hou{\'e}e-Levin, C. Biochemistry 2004, 43,808-818] has shown that this alkaline transition is associated with theprotonation/deprotonation of an unidentified base, B-, which is neither E47 norK48. In this work, we show by resonance Raman spectroscopy that at basic pH ahigh-spin Fe3+-OH species is formed at the active site. The presence of the HO-ligand was directly associated with an absorption band maximum at 560 nm,whereas upon protonation, the band shifts to 644 nm. With respect to ourprevious work, B- can be identified with this high-spin Fe3+-OH species, whichupon protonation results in a water molecule at the active site. Implicationsfor the SOR catalytic cycle are proposed.

Fe3+-hydroxide ligation in the superoxide reductase from Desulfoarculus baarsii is associated with pH dependent spectral changes