The enzyme at the heart of sulfur metabolism is sulfite reductase, an oligomeric hemoflavoprotein that is responsible for the six-electron reduction of sulfite to sulfide. Higher eukaryotes cannot perform this reaction, but rely on bacteria and plants to supply us with sulfur in the proper redox state for incorporation into sulfur containing amino acids and cofactors.

At 1.2 angstroms resolution, one can see details about siroheme, the enzyme's central porphyrinoid cofactor.

Siroheme synthase (CysG, shown left) is a homodimeric enzyme involved in siroheme metabolism. CysG transforms uroporphyrinogen III, the precursor to protoporphyrin IX-derived macrocycles like heme and chlorophyll, into siroheme by performing four chemical reactions: two methyl transfers, NAD-dependant ring oxidation, and iron insertion.