-!- The corrinoid adenosylation pathway comprises three steps: (i) reduction of Co(III) to Co(II) by a one-electron transfer. -!- This can occur non-enzymically in the presence of dihydroflavin nucleotides or reduced flavoproteins. -!- (ii) Co(II) is bound by corrinoid adenosyltransferase, resulting in displacement of the lower axial ligand by an aromatic residue. -!- The reduction potential of the 4-coordinate Co(II) intermediate is raised by ~250 mV compared with the free compound, bringing it to within physiological range. -!- This is followed by a second single-electron transfer from either free dihydroflavins or the reduced flavin cofactor of flavoproteins, resulting in reduction to Co(I). -!- (iii) the Co(I) conducts a nucleophilic attack on the adenosyl moiety of ATP, resulting in transfer of the deoxyadenosyl group and oxidation of the cobalt atom to Co(III) state. -!- Three types of corrinoid adenosyltransferases, not related by sequence, have been described. -!- In the anaerobic bacterium Salmonella enterica they are encoded by the cobA gene (a housekeeping enzyme involved in both adenosylcobalamin de novo biosynthesis and salvage), the pduO gene (involved in (S)-propane-1,2-diol utilization), and the eutT gene (involved in ethanolamine utilization). -!- Since EutT hydrolyzes triphosphate during catalysis, it is classified as a separate enzyme. -!- The mammalian enzyme belongs to the PduO type. -!- The enzyme can act on other corrinoids, such as cob(II)inamide.