Siroheme: a new prosthetic group participating in six-electron reduction reactions catalyzed by both sulfite and nitrite reductases.

Ferredoxin-nitrite reductase (EC 1.7.7.1) of spinach, an enzyme that catalyzes the six-electron reduction of nitrite to ammonia, contains siroheme, the new type of prosthetic group recently found in several sulfite reductases (both assimilatory and dissimilatory) that can catalyze the reduction of sulfite to sulfide, also a six-electron reduction. The prosthetic group of sulfite reductase had previously been shown to be an iron-tetrahydroporphyrin of the isobacteriochlorin type (adjacent pyrrole rings reduced) with eight carboxylate side chains. This finding suggests that both types of "multi-electron" reduction processes may share common mechanistic features.