The enzyme sirohydrochlorin cobaltochelatase (EC 4.99.1.3) catalyzes the reaction

cobalt-sirohydrochlorin + 2 H⁺ = sirohydrochlorin + Co²⁺

In the forward direction of reactions towards cobalamin in anaerobic bacteria, the two substrates of this enzyme are sirohydrochlorin and Co²⁺; its two products are cobalt-sirohydrochlorin and H⁺.

This enzyme belongs to the family of lyases, specifically the "catch-all" class of lyases that do not fit into any other sub-class. The systematic name of this enzyme class is cobalt-sirohydrochlorin cobalt-lyase (sirohydrochlorin-forming). Other names in common use include CbiK, CbiX, CbiXS, anaerobic cobalt chelatase, cobaltochelatase [ambiguous], and sirohydrochlorin cobalt-lyase (incorrect). This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B₁₂) in bacteria such as Salmonella typhimurium and Bacillus megaterium. It has also been identified as the enzyme which inserts nickel into sirohydrochlorin in the biosynthesis of cofactor F430, reaction EC 4.99.1.11.^[1]

• Cobalamin biosynthesis

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes 1TJN and 2DJ5.

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