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Sirohydrochlorin cobalt chelatase (CbiX) and sirohydrochlorin iron chelatase (SirB), N-terminal domain. SirB catalyzes the ferro-chelation of sirohydrochlorin to siroheme, the prosthetic group of sulfite and nitrite reductases. CbiX is a cobaltochelatase, responsible for the chelation of Co2+ into sirohydrochlorin, an important step in the vitamin B12 biosynthetic pathway. CbiX often contains a C-terminal histidine-rich region that may be important for metal delivery and/or storage, and may also contain an iron-sulfur center. Both are found in a wide range of bacteria. This subgroup also contains single domain proteins from archaea and bacteria which may represent the ancestral form of class II chelatases before domain duplication occurred.
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Jiyao W et al.(2023). "The conserved domain database in 2023.",