The Role of Siroheme in Sulfite and Nitrite Reductases

Abstract

Sulfite and nitrite reductases (SiRs/NiRs) use siroheme, an iron-containing isobacteriochlorin, alongside a [4Fe-4S] cluster to perform the six-electron reduction of sulfite to sulfide or nitrite to ammonia. X-ray crystallographic structures of the catalytic siroheme-containing subunit of Escherichia coli assimilatory SiR provide clues about the relationship between the SiRs and the NiRs, allowing us to use the E. coli enzyme as a model system for other siroheme-containing SiRs and NiRs.1–3 These structures also provide insight into the role of the siroheme in this powerful redox reaction, both as an anchor for the acid/base chemistry that directs substrate formation and as an electronically-flexible cofactor that drives the electron transfer reaction.