Lactobacillus leichmannii, ribonucleotide reductase

Within these groups there may be further substantial structure differences among individual enzymes. The long-held view that Lactobacillus leichmannii ribonucleotide reductase is typical of all B 12-dependent, and E. coli reductase of all other enzymes including the eukaryotic ones appears no longer warranted. 

Reduction of ATP to the 2 -deoxy-compound has been carried out on a preparative scale using recombinant Lactobacillus leichmannii ribonucleotide triphosphate reductase. ... 

Incorporation of solvent hydrogen into the substrate does not rule out its transfer as hydride if the source of hydride is dithiol, as in the ribonucleotide reductases. In the most understood enzyme of this type, from Lactobacillus leichmannii, the function of the dithiol (45) is to reduce the cobamide coenzyme (46) and provide a source... 

Booker, S., and Stubbe, J., 1993, Cloning, sequencing, and expression of the adenosylcobal-amin-dependent ribonucleotide reductase from Lactobacillus leichmannii. Proc Natl. Acad. Set U.S.A. 90 8352n8356. 

Brown, M. L., and Li, J., 1998, Activation parameters for the carbon-cobalt bond homolysis of coenzyme B12 induced by the B[2-dependent ribonucleotide reductase from Lactobacillus leichmannii. J. Am. Chem. Soc. 120 9466n9474. 

Lin, A. N., Ashley, G. W., and Stubbe, J., 1987, Location of the redox-active thiols of ribonucleotide reductase sequence similarity between the Escherichia coli and Lactobacillus leichmannii enzymes. Biochemistry 26 6905n6909. 

Ribonucleotide reductase has been studied extensively in two bacterial systems Escherichia coli (5) and Lactobacillus leichmannii (6). These two systems differ in their cofactor requirements and in the level of phosphorylation of the substrate. 

The fermentative bacteria, Lactobacillus leichmannii probably possess the most simple of all ribonucleotide reductases, but the protein does require the most complex coenzyme known, 5 -deoxyadenosylcobalamin (Fig. 1). This corrinoid coenzyme catalyzes but few enzymatic reactions, usually involving an intramolecular 1,2-hydrogen shift as found in the common methylmalonylCoA succinylCoA isomerase reaction. Participation in deoxyribonucleotide formation, where hydrogen transfer occurs intermolecularly (Eq. I), was first implicated by the nutritional requirement of lactobacilli for vitamin B12 during DNA synthesis, and was verified in cell-free extracts by Blakley and H. A. Barker in 1964 The reaction has been studied in great detail in the laboratories of Beck Blakley and Hogenkamp ... 

Table 5. Kinetic data and effector specificity for reduction of natural and synthetic, or modified ribonucleotides catalyzed by the triphosphate reductase of Lactobacillus leichmannii and diphosphate reductase of E. coll, respectively. (Data from Ref. 135,136, 142)... 

Fig. 13. Dependence of the specific activities of Lactobacillus leichmannii (left) and Escherichia coli (right) ribonucleotide reductases upon coenzyme B12 and subunit B 2 concentration, respectively, in the absence and presence of allosteric effector nucleotides (dGTP for ATP reduction, dTTP for CDP reduction). Conditions as in Table 5...

The discoveries made with the E. coli system provided the basis for studies of ribonucleotide reduction in other microbial species and in animal cells. The mechanism of ribonucleotide reduction in rat tissues resembles that of E. coli in many ways, whereas ribonucleotide reduction in Lactobacillus leichmannii differs distinctively, in that coenzyme Bk takes part in a reduction accomplished at the nucleoside triphosphate level. Each of the three reductases that have been extensively purified reduces ribonucleotide substrates representing all four of the ribonucleosides of RNA and each displays remarkable allosteric regulatory properties (see below). 

Brown, K. L., and U, j. "Activation Parameters for the Carbon-Cobalt Bond Homolysis of Coenzyme B12 Induced by the B,2-Dependent Ribonucleotide Reductase from Lactobacillus leichmannii." f. Am. Cltcm. Soc., 120,... 

Licht S and Stubbe J (1996) Catalysis of carbon-cobalt bond cleavage by the ribonucleotide reductase of Lactobacillus leichmannii. Abstr Int Symp Vitamin Bn, Zurich, Switzerland, pp 35-36... 

---