Adenosylcobalamin-Dependent Ribonucleotide Reductases

Ribonucleotide reductases with an absolute requirement for adenosyl-cobalamin (Fig. 1) as a coenzyme have been demonstrated only in microorganisms (7). These reductases may be classified into two groups based on the nature of the nucleotide substrate utilized ribonucleoside triphosphate reductases, which act upon the ribonucleoside triphosphates 

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Lawrence, C. C., Gerfen, G. J., Samano, V., Nitsche, R., Robins, M. J., and Stubbe, J., 1999, Binding of Cob(II)alamin to the adenosylcobalamin-dependent ribonucleotide reductase from Lactobacillus leichmanniioldentification of dimethylbenzimidazole as the axial ligand, J. Biol. Chem. 274 7039n7042. 

Numerous analogs of adenosylcobalamin have been tested for their ability to replace or to inhibit the action of the coenzyme in the adenosyl-cobalamin-dependent ribonucleotide reductase reaction the enzyme from L. leichmannii has been used in most of these studies. Kinetic studies have been used in most investigations of analog-enzyme interactions and thus the interpretation of data regarding the affinity of analogs for the reductase is subject to the limitations imposed on kinetic studies of a complex reaction. 

With one exception, the known B12-requiring reactions involve either (1) methyl group transfer or (2) adenosylcobalamin-dependent isomerizations. The isomerizations exchange a carbon-bound hydrogen with another carbon-bound functional group as shown here. The one exception is an intermolecular transfer reaction catalyzed by a ribonucleotide reductase of Lactobacillus. 

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