S= 9/2 EPR signals are evidence against coupling between the siroheme and the Fe/S cluster prosthetic groups in Desulfovibrio vulgaris (Hildenborough) dissimilatory sulfite reductase

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Abstract
Sulfite reductases contain siroheme and iron‐sulfur cluster prosthetic groups. The two groups are believed to be structurally linked via a single, common ligand. This chemical model is based on a magnetic model for the oxidized enzyme in which all participating iron ions are exchange coupled. This description leads to two serious discrepancies. Although the iron‐sulfur cluster is assumed to be a diamagnetic cubane, [4Fe–4S]2+, all iron appears to be paramagnetic in Mössbauer spectroscopy. On the other hand, EPR spectroscopy has failed to detect anything but a single high‐spin heme. We have re‐addressed this problem by searching for new EPR spectroscopic clues in concentrated samples of dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). We have found several novel signals with effective g values of 17, 15.1, 11.7, 9.4, 9.0, 4. The signals are interpreted in terms of an S= 9/2 system with spin‐Hamiltonian parameters g= 2.00, D=−0.56 cm−1, |E/D|= 0.13 for the major component. In a reductive titration with sodium borohydride the spectrum disappears with Em=−205 mV at pH 7.5. Contrarily, the major high‐spin siroheme component has S= 5/2, g= 1.99, D=+9 cm−1, |E/D|= 0.042, and Em=−295 mV. The sum of all siroheme signals integrates to 0.2 spin/half molecule, indicating considerable demetallation of this prosthetic group. Rigorous quantification procedures for S= 9/2 are not available, however, estimation by an approximate method indicates 0.6 S= 9/2 spin/half molecule. The S= 9/2 system is ascribed to an iron‐sulfur cluster. It follows that this cluster is probably not a cubane, is not necessarily exchange‐coupled to the siroheme, and, therefore, is not necessarily structurally close to the siroheme. It is suggested that this iron‐sulfur prosthetic group has a novel structure suitable for functioning in multiple electron transfer.

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