title: Dihydroflavin-driven adenosylation of 4-coordinate Co(II) corrinoids: are cobalamin reductases enzymes or electron transfer proteins?
abstract: 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.
authors.affiliation: Department of Bacteriology, University of Wisconsin, Madison, Wisconsin 53726-1521, USA.
authors.forename: Paola E
authors.initials: PE
authors.lastname: Mera
authors.forename: Jorge C
authors.initials: JC
authors.lastname: Escalante-Semerena
mesh_terms: D000241:Adenosine / Q000737:chemistry; D019883:Alkyl and Aryl Transferases / Q000737:chemistry*; D001426:Bacterial Proteins / Q000737:chemistry*; D002244:Carbon / Q000737:chemistry; D020134:Catalytic Domain; D003001:Cloning, Molecular; D003035:Cobalt / Q000737:chemistry; D045728:Corrinoids / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004583:Electrons; D005415:Flavins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D010084:Oxidation-Reduction; D013816:Thermodynamics; D014805:Vitamin B 12 / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D045728:Corrinoids; D005415:Flavins; D003035:Cobalt; D002244:Carbon; D019883:Alkyl and Aryl Transferases; C082269:ATP-corrinoid adenosyltransferase; D000241:Adenosine; D014805:Vitamin B 12