title: Dihydrosirohydrochlorin
text: # Dihydrosirohydrochlorin
  
  Dihydrosirohydrochlorin is one of several naturally occurring tetrapyrrole macrocyclic metabolic intermediates in the biosynthesis of vitamin $B_{12}$ (cobalamin). Its oxidised form, sirohydrochlorin, is precursor to sirohaem, the iron-containing prosthetic group in sulfite reductase enzymes. Further biosynthetic transformations convert sirohydrochlorin to cofactor F430 for an enzyme which catalyzes the release of methane in the final step of methanogenesis.
  
  ## Biosynthesis
  
  Dihydrosirohydrochlorin is derived from a tetrapyrrolic structural framework created by the enzymes deaminase and cosynthetase which transform aminolevulinic acid via porphobilinogen and hydroxymethylbilane to uroporphyrinogen III. The latter is the first macrocyclic intermediate common to haem, chlorophyll, sirohaem and vitamin $B_{12}.$ Uroporphyrinogen III is subsequently transformed by the addition of two methyl groups to form dihydrosirohydrochlorin.
  
  