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Structure of siroheme

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 dehydrogenase, which plays a major role in sulfur assimilation pathway, converting sulfite into sulfide, which can be incorporated into the organic compound homocysteine.[2]


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]

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  1. ^ Matthew J. Murphy; et al. (1974). "Siroheme: A New Prosthetic Group Participating in Six-Electron Reduction Reactions Catalyzed by Both Sulfite and Nitrite Reductases". PNAS. 71 (3): 612–616. Bibcode:1974PNAS...71..612M. doi:10.1073/pnas.71.3.612. PMC 388061. PMID 4595566.
  2. ^ Dominique Thomas; Yolande Surdin-Kerjan (1997). "Metabolism of sulfur amino acids in Saccharomyces cerevisiae". Microbiology and Molecular Biology Reviews. 61 (4): 503–532. PMC 232622. PMID 9409150.
  3. ^ Kaushik Saha, Michaël Moulin, Alison G. Smith (2009). "Tetrapyrroles in Plants: Chemical Biology of Metal Insertion and Removal". Encyclopedia of Chemical Biology. John Wiley & Sons. doi:10.1002/9780470048672.wecb454.

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