FIGURE 1.

Proposed mechanism for the reduction and adenosylation of cobalamin. PduO-type ACA enzymes (oval) bind 5-coordinate cob(II)alamin and facilitate its reduction to cob(I)alamin by displacing the lower ligand. The latter event generates a 4-coordinate cob(II)alamin intermediate whose redox potential is raised enough so it can accept one electron from dihydroflavin. The resulting cob(I)alamin nucleophile attacks ATP to generate coenzyme B₁₂ (AdoCbl) and triphosphate. In these studies, we investigated the source of the electron for the reduction of cob(II)alamin to cob(I)alamin.