SPECTROSCOPIC PROPERTIES OF DESULFOFERRODOXIN FROM DESULFOVIBRIO-DESULFURICANS (ATCC-27774)

Desulfoferrodoxin, a non-heme iron protein, was purified previously fr om extracts of Desulfovibrio desulfuricans (ATCC 27774) (Moura, I., Ta vares, P., Moura, J. J. G., Ravi, N., Huynh, B. H., Liu, M.-Y., and Le Gall, J. (1990) J. Biol. Chem. 265, 21596-21602). The as-isolated prot ein displays a pink color (pink form) and contains two mononuclear iro n sites in different oxidation states: a ferric site (center I) with a distorted tetrahedral sulfur coordination similar to that found in de sulforedoxin from Desulfovibrio gigas and a ferrous site (center II) o ctahedrally coordinated with predominantly nitrogen/ oxygen-containing ligands. A new form of desulfoferrodoxin which displays a gray color (gray form) has now been purified. Optical, electron paramagnetic reso nance (EPR), and Mossbauer data of the gray desulfoferrodoxin indicate that both iron centers are in the high-spin ferric states. In additio n to the EPR signals originating from center I at g = 7.7, 5.7, 4.1, a nd 1.8, the gray form of desulfoferrodoxin exhibits a signal atg = 4.3 and a shoulder at g = 9.6, indicating a high-spin ferric state with E /D approximate to 1/3 for the oxidized center II. Redox titrations of the gray form of the protein monitored by optical spectroscopy indicat e midpoint potentials of +4 +/- 10 and +240 +/- 10 mV for centers I an d II, respectively. Mossbauer spectra of the gray form of the protein are consistent with the EPR finding that both centers are high-spin fe rric and can be analyzed in terms of the EPR-determined spin Hamiltoni an parameters. The Mossbauer parameters for both the ferric and ferrou s forms of center II are indicative of a mononuclear high spin iron si te with octahedral coordination and predominantly nitrogen/oxygen-cont aining ligands. Resonance Raman studies confirm the structural similar ity of center I and the distorted tetrahedral FeS4 center in desulfore doxin and provide evidence for one or two cysteinyl-S ligands for cent er II. On the basis of the resonance Raman results, the 635 nm absorpt ion band that is responsible for the gray color of the oxidized protei n is assigned to a cysteinyl-S --> Fe(III) charge transfer transition localized on center II. The novel properties and possible function of center II are discussed in relation to those of mononuclear iron cente rs in other enzymes.