Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase

Nicole A. Leal, Horatiu Olteanu, Ruma Banerjee, Thomas A. Bobik

Research output: Contribution to journalArticlepeer-review

43 Scopus citations


The final step in the conversion of vitamin B12 into coenzyme B12 (adenosylcobalamin, AdoCbl) is catalyzed by ATP:cob(I)alamin adenosyltransferase (ATR). Prior studies identified the human ATR and showed that defects in its encoding gene underlie cblB methylmalonic aciduria. Here two common polymorphic variants of the ATR that are found in normal individuals are expressed in Escherichia coli, purified, and partially characterized. The specific activities of ATR variants 239K and 239M were 220 and 190 nmol min -1 mg-1, and their Km values were 6.3 and 6.9 μM for ATP and 1.2 and 1.6 μM for cob(I)alamin, respectively. These values are similar to those obtained for previously studied bacterial ATRs indicating that both human variants have sufficient activity to mediate AdoCbl synthesis in vivo. Investigations also showed that purified recombinant human methionine synthase reductase (MSR) in combination with purified ATR can convert cob(II)alamin to AdoCbl in vitro. In this system, MSR reduced cob(II)alamin to cob(I)alamin that was adenosylated to AdoCbl by ATR. The optimal stoichiometry for this reaction was ∼4 MSR/ ATR and results indicated that MSR and ATR physically interacted in such a way that the highly reactive reaction intermediate [cob(I)alamin] was sequestered. The finding that MSR reduced cob(II)alamin to cob(I)alamin for AdoCbl synthesis (in conjunction with the prior finding that MSR reduced cob(II)alamin for the activation of methionine synthase) indicates a dual physiological role for MSR.

Original languageEnglish (US)
Pages (from-to)47536-47542
Number of pages7
JournalJournal of Biological Chemistry
Issue number46
StatePublished - Nov 12 2004

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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