The prismane protein resolved - Mossbauer investigation of a 4Fe cluster with an unusual mixture of bridging ligands and metal coordinations

The prismane protein of Desulfovibrio vulgaris, in its isolated, its one-electron-reduced and its oxidized states, was the subject of a detailed Mossbauer investigation. Measurements were recorded in the range 0.295-77 K and in the field range 0-6.2 T (parallel and perpendicular to the γ beam). The paramagnetic parts of the magnetically split Mossbauer spectra were analyzed with the spin-Hamiltonian formalism, including the nuclear Hamiltonian; the diamagnetic parts result from the nuclear Hamiltonian only. The field-dependent spectra at 295 mK and 4.2 K indicate that the paramagnetic part of the isolated protein represents a spin-coupled 4Fe unit with the spin of one Fe site (5/2) oriented antiparallel to the spins of the other three Fe sites (5/2, 5/2 and 2), yielding a total cluster spin, S(tot) of 9/2. The Mossbauer parameters of the individual Fe sites indicated that this unit represents a 4Fe cluster with an unusual mixture of bridging and terminal ligands and metal coordinations (hybrid cluster). The diamagnetic part of the isolated protein represents an additional 4Fe unit, which, according to its Mossbauer parameters, is a [4Fe^2.5+-4S] cubane. The parameter changes upon one-electron oxidation or reduction and the magnetic properties of the two clusters in the three oxidation states of the protein investigated here reveal that the redox-behavior of the prismane protein is exclusively related with the hybrid cluster. These findings are contrary to the former hypothesis of one or two [6Fe-6S] cluster(s) as the prosthetic group of this protein [Hagen, W.R., Pierik, A.J. and Veeger, C. (1989) J. Chem. Sec. Faraday Trans. 185, 4083-4090; Moura, I., Tavares, P., Moura, J.J.G., Ravi, N., Huynh, B.H., Liu, M.-Y. and LeGall, J. (1992) J. Biol. Chem. 287, 4487-4496]. However, they are in full agreement with the crystal structure of the isolated protein, which, concurrent with our Mossbauer investigation, has been solved.