Cobaltochelatase is responsible for the insertion of cobalt into the corrin ring of coenzyme B12 during its biosynthesis. Two versions have been well described. CbiK/CbiX is a monomeric, anaerobic version which acts early in the biosynthesis (pfam06180). CobNST is a trimeric, ATP-dependent, aerobic version which acts late in the biosynthesis (TIGR02257/TIGR01650/TIGR01651). A number of genomes (actinobacteria, cyanobacteria, betaproteobacteria and pseudomonads) which apparently biosynthesize B12, encode a cobN gene but are demonstrably lacking cobS and cobT. These genomes do, however contain a homolog (modelled here) of the magnesium chelatase subunits BchI/BchD family. Aside from the cyanobacteria (which have a separate magnesium chelatase trimer), these species do not make chlorins, so do not have any use for a magnesium chelatase. Furthermore, in nearly all cases the members of this family are proximal to either CobN itself or other genes involved in cobalt transport or B12 biosynthesis.
Jiyao W et al.(2023). "The conserved domain database in 2023.",