One pathway can incorporate either adenine or dimethylbenzimidazole as an α-axial ligand of B12 cofactors in Salmonella enterica

Corrinoid (vitamin B-12-like) cofactors contain various alpha-axial ligands, including 5,6-dimethylbenzimidazole (DMB) or adenine. The bacterium Salmonella enterica produces the corrin ring only under anaerobic conditions, but it can form complete corrinoids aerobically by importing an incomplete corrinoid, such as cobinamide (Cbi), and adding appropriate alpha- and beta-axial ligands. Under aerobic conditions, S. enterica performs the corrinoid-dependent degradation of ethanolamine if given vitamin B-12, but it can make B-12 from exogenous Cbi only if DMB is also provided. Mutants isolated for their ability to degrade ethanolamine without added DMB converted Cbi to pseudo-B-12 cofactors (having adenine as an alpha-axial ligand). The mutations cause an increase in the level of free adenine and install adenine (instead of DMB) as an alpha-ligand. When DMB is provided to these mutants, synthesis of pseudo-B-12 cofactors ceases and B-12 cofactors are produced, suggesting that DMB regulates production or incorporation of free adenine as an alpha-ligand. Wild-type cells make pseudo-B-12 cofactors during aerobic growth on propanediol plus Cbi and can use pseudo-vitamin B-12 for all of their corrinoid-dependent enzymes. Synthesis of coenzyme pseudo-B-12 cofactors requires the same enzymes (CobT, CobU, CobS, and CobC) that install DMB in the formation of coenzyme B-12. Models are described for the mechanism and control of alpha-axial ligand installation.