The mechanism of Lactobacillus leichmannii ribonucleotide reductase. Evidence for 3' carbon-hydrogen bond cleavage and a unique role for coenzyme B12.

Abstract

Reduction of [3'-3H, U-14C]UTP with ribonucleotide reductase showed isotope effects of 1.6, 1.8, and 1.8 at pH 6.1, 7.3, and 8.3, respectively. Similar studies with [3'-3H, U-14C]ATP gave effects of 2.1, 2.1, 1.7, and 1.9 at pH 5.5, 6.1, 7.3, and 8.3, respectively. During the course of the isotope effect determinations the samples were analyzed for the production of 3H2O. Reduction of [3'-3H]UTP at pH 6.1, 7.3, and 8.3 resulted in 0.08, 0.15, and 0.24% of the total 3H being volatilized at 50% reaction. Reduction of [3'-3H]ATP gave no detectable 3H2O at pH 6.1 or 7.3 and a maximum of 0.02% at pH 8.3. The isotope effects in conjunction with 3H2O production indicate that ribonucleotide reductase catalyzes cleavage of the 3' carbon-hydrogen bond of NTPs during the reduction to dNTPs. The role of the cofactor, adenosylcobalamin (AdoCbl), was also investigated. Incubation of [3'-3H]NTP with prereduced ribonucleotide reductase in the absence of reductant (one-turnover conditions) followed by isolation and analysis of AdoCbl for radioactivity indicated no 3H transfer from substrate to cofactor and no 3H2O production. Similar results were obtained in the presence of DTT (multiple turnover conditions). The role of AdoCbl was further investigated by examination of its potential to mediate hydrogen transfer between two substrates. Incubation of [3'-3H]UTP and unlabeled ATP with ribonucleotide reductase and isolation of the products produced indicated no 3H in dATP. These results allow us to postulate a unique role for AdoCbl in the ribonucleotide reductase reaction, that of a radical chain initiator rather than an intermediary hydrogen transfer agent.