About siroheme biosynthesis

Sulfite and nitrite reductases catalyze the six-electron reduction of sulphite to sulfide and nitrite to ammonia, respectively, which are key enzymatic steps in the assimilation of sulfur and nitrogen into all life forms (6). Siroheme, a modified cyclic tetrapyrrole, similar in structure to heme, chlorophyll and cobalamin, is used as a prosthetic group by sulfite and nitrite reductases (7). Similar to many cyclic tetrapyrroles, siroheme coordinates with a metal in its central cavity. While siroheme and heme coordinate an iron atom, chlorophyll and cobalamin coordinate maganesium and cobalt, respectively.

Assimilatory sulfite reductases are found in bacteria, plants and fungi, but not in animals, while dissimilatory sulfite reductases are found in diverse sulfate-reducing eubacteria and some species of thermophilic archaebacteria. Assimilatory nitrite reductases are also found in bacteria, plants, and fungi, but not in the yeast Saccharomyces cerevisiae. Thus, in S. cerevisiae siroheme is used exclusively in sulfite reductase (6).

The biologically important modified tetrapyrroles, such as siroheme, heme, chlorophyll and cobalamin share a common biosynthetic pathway up to the synthesis of the first macrocyclic intermediate uroporphyrinogen-III (8). Siroheme is biosynthesized from uroporphyringoen-III in four enzymatic steps: two transmethylations, a dehydrogenation, and a ferrochelation (3). In S. cerevisiae the two transmethylations are catalyzed by Met1p, a uroporphyrin III methyltransferase that requires S-adenosyl-L-methionine (AdoMet) as a methyl donor, and the dehydrogenation and ferrochelation reactions are catalyzed by the bifunctional enzyme Met8p (3, 6, 5).

Last updated: 2007-10-03