-!- The corrinoid adenosylation pathway comprises three steps: (1) Reduction of Co(III) to Co(II) by a one-electron transfer; this can be carried out by EC 1.16.1.3, or non-enzymically in the presence of dihydroflavin nucleotides. (2) Co(II) is reduced to Co(I) in a second single-electron transfer by EC 1.16.1.4. (3) The Co(I) conducts a nucleophilic attack on the adenosyl moiety of ATP to leave the cobalt atom in a Co(III) state (EC 2.5.1.17). -!- The enzyme responsible for the adenosylation reaction is the product of the gene cobO in the aerobic bacterium Pseudomonas denitrificans and of the gene cobA in the anaerobic bacterium Salmonella typhimurium. -!- In P.denitrificans, the enzyme shows specificity for cobyrinic acid a,c-diamide and the corrinoids that occur later in the biosynthetic pathway whereas CobA seems to have broader specificity. -!- While CobA has a preference for ATP and Mn(2+), it is able to transfer a variety of nucleosides to the cobalt, including CTP, UTP and GTP, in decreasing order of preference and to use Mg(2+) instead of Mn(2+).