Converts cob(I)alamin to adenosylcobalamin (adenosylcob(III)alamin), a coenzyme for methylmalonyl-CoA mutase, therefore participates in the final step of the vitamin B12 conversion (PubMed:12514191). Generates adenosylcobalamin (AdoCbl) and directly delivers the cofactor to MUT in a transfer that is stimulated by ATP- binding to MMAB and gated by MMAA (Probable). . This is the function of MMAB (metabolism of cobalamin associated B, ENSG00000139428).