Characterization of two dissimilatory sulfite reductases from sulfate-reducing bacteria

Mössbauer, EPR, and biochemical techniques were used to characterize two dissimilatory sulfite reductases: desulforubidin fromDesulfovibrio baculatus strain DSM 1743 and desulfoviridin fromDesulfovibrio gigas. For each molecule of desulforubidin, there are two sirohemes and four 4Fe?4S] clusters. The 4Fe?4S] clusters are in the diamagnetic 2+ oxidation state. The sirohemes are high-spin ferric (S=5/2) and each siroheme is exchanged-coupled to a 4Fe?4S]²⁺ cluster. Such an exchange-coupled siroheme-4Fe?4S] unit has also been found in the assimilatory sulfite reductase fromEscherichia coli/1/ and in a low-molecular weight sulfite reductase fromDesulfovibrio vulgaris/2/. For each molecule of defulfoviridin, there are two tetrahydroporphyrin groups and four 4Fe?4S]²⁺ clusters. To our surprise, we discovered that about 80% of the tetrahydroporphyrin groups, however, do not bind iron.