Dihydroflavin-driven adenosylation of 4-coordinate Co(II) corrinoids: are cobalamin reductases enzymes or electron transfer proteins?

J Biol Chem. 2010 Jan 29;285(5):2911-7. doi: 10.1074/jbc.M109.059485. Epub 2009 Nov 21.


The identity of the source of the biological reductant needed to convert cobalamin to its biologically active form adenosylcobalamin has remained elusive. Here we show that free or protein-bound dihydroflavins can serve as the reductant of Co(2+)Cbl bound in the active site of PduO-type ATP-dependent corrinoid adenosyltransferase enzymes. Free dihydroflavins (dihydroriboflavin, FMNH(2), and FADH(2)) effectively drove the adenosylation of Co(2+)Cbl by the human and bacterial PduO-type enzymes at very low concentrations (1 microm). These data show that adenosyltransferase enzymes lower the thermodynamic barrier of the Co(2+) --> Co(+) reduction needed for the formation of the unique organometalic Co-C bond of adenosylcobalamin. Collectively, our in vivo and in vitro data suggest that cobalamin reductases identified thus far are most likely electron transfer proteins, not enzymes.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adenosine / chemistry
  • Alkyl and Aryl Transferases / chemistry*
  • Bacterial Proteins / chemistry*
  • Carbon / chemistry
  • Catalytic Domain
  • Cloning, Molecular
  • Cobalt / chemistry
  • Corrinoids / chemistry
  • Dose-Response Relationship, Drug
  • Electrons
  • Flavins / metabolism*
  • Humans
  • Kinetics
  • Oxidation-Reduction
  • Thermodynamics
  • Vitamin B 12 / chemistry*


  • Bacterial Proteins
  • Corrinoids
  • Flavins
  • Cobalt
  • Carbon
  • Alkyl and Aryl Transferases
  • ATP-corrinoid adenosyltransferase
  • Adenosine
  • Vitamin B 12