Figure 3. Late steps of coenzyme B₁₂ biosynthesis.

This figure schematizes the two branches of the nucleotide loop assembly (NLA) pathway that yields coenzyme B₁₂ in S. enterica. This bacterium can assimilate exogenous, incomplete corrinoids such as cobinamide (Cbi) converting it to AdoCbi and activating it to AdoCbi-GDP via AdoCbi-P using the bifunctional kinase/guanylyltransferase (CobU) enzyme. The base shown in this scheme is 5,6-dimethylbenzimidazole (DMB), which is activated to its α-ribotide by the CobT PRTase (shown inside the box). Condensation of the activated intermediates followed by dephosphorylation yields biologically active coenzyme B₁₂.