Siroheme Explained

Siroheme (or sirohaem) is a heme-like prosthetic group at the active sites of some enzymes to accomplish the six-electron reduction of sulfur and nitrogen.^[1] It is a cofactor at the active site of sulfite reductase, which plays a major role in sulfur assimilation pathway, converting sulfite into sulfide, which can be incorporated into the organic compound homocysteine.^[2]

Biosynthesis

Like all tetrapyrroles, the macrocyclic ligand in siroheme is derived from uroporphyrinogen III. This porphyrinogen is methylated at two adjacent pyrrole rings to give dihydrosirohydrochlorin, which is subsequently oxidized to give sirohydrochlorin. A ferrochelatase then inserts iron into the macrocycle to give siroheme.^[3]

See also

• Ferredoxin-nitrite reductase
• Hydrogensulfite reductase
• Nitrite reductase (NAD(P)H)

Further reading

• 10.1016/S0014-5793(96)01423-8. Siroheme biosynthesis in Saccharomyces cerevisiae requires the products of both MET1 and MET8 genes . Jorgen Hansen . . 1997 . 401 . 20–24 . 9003798. 1. etal. free .

Notes and References

1. 10.1073/pnas.71.3.612. Siroheme: A New Prosthetic Group Participating in Six-Electron Reduction Reactions Catalyzed by Both Sulfite and Nitrite Reductases . Matthew J. Murphy . . 1974 . 71 . 3 . 612–616 . 4595566. 388061. etal. 1974PNAS...71..612M. free .
2. Metabolism of sulfur amino acids in Saccharomyces cerevisiae . Dominique Thomas . Yolande Surdin-Kerjan . . 1997 . 61 . 4 . 503–532 . 10.1128/mmbr.61.4.503-532.1997 . 9409150. 232622.
3. Encyclopedia: Tetrapyrroles in Plants: Chemical Biology of Metal Insertion and Removal. Kaushik Saha . Michaël Moulin . Alison G. Smith . Wiley Encyclopedia of Chemical Biology. 2009. 10.1002/9780470048672.wecb454. John Wiley & Sons. Encyclopedia of Chemical Biology. 978-0-470-04867-2.