Adenosylcobalamin-dependent diol dehydratase (DD) undergoes suicide inactivation by glycerol, one of its physiological substrates, resulting in the irreversible cleavage of the coenzyme Co-C bond. The damaged cofactor remains tightly bound to the active site. The DD-reactivating factor reactivates the inactivated holoenzyme in the presence of ATP and Mg 2+ by mediating the exchange of the tightly bound damaged cofactor for free intact coenzyme. In this study, we demonstrated that this reactivating factor mediates the cobalamin exchange not stoichiometrically but catalytically in the presence of ATP and Mg 2+ . Therefore, we concluded that the reactivating factor is a sort of enzyme. It can be designated DD reactivase. The reactivase showed broad specificity for nucleoside triphosphates in the activation of the enzymeAEcyanocobalamin complex. This result is consistent with the lack of specific interaction with the adenine ring of ADP in the crystal structure of the reactivase. The specificities of the reactivase for divalent metal ions were also not strict. DD formed 1 : 1 and 1 : 2 complexes with the reactivase in the presence of ADP and Mg 2+ . Upon complex formation, one b subunit was released from the (ab) 2 tetramer of the reactivase. This result, together with the similarity in amino acid sequences and folds between the DD b subunit and the reactivase b subunit, suggests that subunit displacement or swapping takes place upon formation of the enzymeAEreactivase complex. This would result in the dissociation of the damaged cofactor from the inactivated holoenzyme, as suggested by the crystal structures of the reactivase and DD.

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