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Sirohydrochlorin cobalt chelatase (CbiX) and sirohydrochlorin iron chelatase (SirB), C-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 CbiX and SirB are found in a wide range of bacteria.
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