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

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

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Department of Bacteriology, University of Wisconsin, Madison, Wisconsin 53726-1521, USA.


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.

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