Siroheme

Sirohäm consists of a porphyrin with a central iron atom. Sirohäm was originally discovered in 1973 as a prosthetic group of NADPH-dependent sulfite reductase from E. coli and thus is considered the first discovered bacteriochlorin. It differs from the other hemes in the type and number of side chains of the porphyrin. In nature Sirohäm comes as a cofactor found in plant and bacterial Nitritreduktasen and Sulfitreduktasen.

Biosynthesis

Sirohäm is biochemically formed from uroporphyrinogen III.

Uroporphyrinogen III is also a precursor of protoporphyrin IX, which is used for the chlorophyll and heme biosynthesis. Uroporphyrinogen III is an S- adenosylmethionine -dependent methyltransferases Dihydrosirohydrochlorin ( precorrin 1) methylated. After oxidation of Dihydrosirohydrochlorin Sirohydrochlorin ( precorrin 2) is formed, although the responsible dehydrogenase ( in A. thaliana ) has yet to be identified. Finally, created by the incorporation of ferrous iron ( Fe2 ) Sirohäm. This reaction is catalyzed by a Sirohydrochlorin - ferrochelatase.