Binding of Cob(II)alamin to the adenosylcobalamin-dependent ribonucleotidereductase from Lactobacillus leichmannii - Identification of dimethylbenzimidazole as the axial ligand

The ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichma nnii catalyzes the reduction of nucleoside 5'-triphosphates to 2'-deoxynucl eoside 5'-triphosphates and uses coenzyme B-12, adenosylcobalamin (AdoCbl), as a cofactor. Use of a mechanism-based inhibitor, 2'-deoxy-2'-methylenecy tidine 5'-triphosphate, and isotopically labeled RTPR and AdoCbl in conjunc tion with EPR spectroscopy has allowed identification of the lower axial li gand of cob(II)alamin when bound to RTPR. In common with the AdoCbl-depende nt enzymes catalyzing irreversible heteroatom migrations and in contrast to the enzymes catalyzing reversible carbon skeleton rearrangements, the dime thylbenzimidazole moiety of the cofactor is not displaced by a protein hist idine upon binding to RTPR.