Sulfite and Nitrite reductases are key to both biosynthetic assimilation of sulfur and nitrogen and dissimilation of oxidised anions for energy transduction [1]. Two copies of this repeat are found in Nitrite and Sulfite reductases and form a single structural domain.

1: Sulfite reductase structure at 1.6 A: evolution and catalysis for reduction of inorganic anions. Crane BR, Siegel LM, Getzoff ED; Science 1995;270:59-67. PMID:7569952