CHARACTERIZATION OF THE PRISMANE PROTEIN FROM DESULFOVIBRIO-VULGARIS (HILDENBOROUGH) BY LOW-TEMPERATURE MAGNETIC CIRCULAR DICHROIC SPECTROSCOPY

The prismane protein of Desulfovibrio vulgaris (Hildenborough) contain s a putative [6Fe-6S] cluster. This novel iron-sulfur cluster has been characterized here by magnetic circular dichroism (MCD) spectroscopy. Three paramagnetic redox states of the cluster, [6Fe-6S](5+), [6Fe-6S ](4+) and [6Fe-6S](3+), each show a distinctive low-temperature MCD sp ectrum which is unlike that observed for any other iron-sulfur cluster s. Magnetization data for the prismane protein in these three redox st ates indicate ground state spins that are in accordance with previous EPR assignments. For the protein as isolated, with the [6Fe-6S](5+) fo rm of the cluster, magnetizations show an exceptionally steep initial slope that can be fit to a ground state of spin S = 9/2. For the semi- reduced protein, the cluster in the [6Fe-6S](4+) form, magnetizations show an initial slope characteristic for a ground state of spin S = 4. For the dithionite-reduced protein, with the [6Fe-6S](3+) form of the cluster, magnetizations are typical for a ground state of spin S=1/2.