Lactobacillus leichmanii ribonucleotide reductase

Lactobacillus leichmanii ribonucleotide reductase has a molecular weight of 76 000, with a single polypeptide chain of about 690 amino acids. The large size of the apoenzyme probably reflects the need for it to have sites to interact with the coenzyme, a dithiol, a substrate and allosteric effectors. A transient radical species was observed during catalysis. 

Ribonucleotide Reductase. The ribonucleotide reductases catalyze the reduction of ribonucleoside-diphosphates (or triphosphates) to the corresponding 2 -deoxyribonucleoside-diphosphates (or triphosphates), processes of preeminent importance for the biosynthesis of DNA (see Table 2, entry 4) (65,86). A variety of metal-containing cofactors have been discovered in the ribonucleotide reductases investigated to date (eg, a binuclear iron center in the mammalian and in the E. coli ribonucleoside diphosphate reductase) and the oxidation of two protein thiols to a disulfide unit is indicated as the direct source of the two reduction equivalents. The reductase from Lactobacillus leichmanii employs coenzyme B12 as cofactor in its (normal) base-on form and acts on purine- or pyrimidine-based ribonucleoside-triphosphates. Its crystal structure reveals not only the arrangement of the bound corrinoid cofactor, but also how the enzyme is... 

The ribonucleotide reductase of Lactobacillus leichmanii catalyses the rapid conversion of enzyme-bound coenzyme B12 to an intermediate which has an absorption spectrum in the visible and u.v. like that of cob(ii)alamin (Biar)- Coffman et al. have recently analysed the e.s.r. spectrum of this intermediate and suggest that it contains low-spin cobalt(ii) with strongly distorted six-fold co-ordination. 

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