Siroheme

Siroheme (or sirohaem) is a heme-like prosthetic group used by some enzymes to accomplish the six-electron reduction of sulfur and nitrogen. Siroheme is synthesized from uroporphyrinogen III, a heme and vitamin B₁₂ precursor. It plays a major role in the sulfur assimilation pathway: converting sulfite to a biologically useful sulfide, which can be incorporated into the organic compound homocysteine.