New insights into a critical biological control step of the mechanism of Ribonucleotide reductase

This paper focuses on the radical mechanism of class I ribonucleotide reductases, investigated using the hybrid DFT functional B3LYP. In this work, ionic residues were used where appropriate, in contrast with the neutral models used in previous theoretical studies of the mechanism. The calculations provide further insight into the mechanism by emphasizing the step involving cysteine oxidation; a step not fully understood and for which earlier theoretical and experimental proposals differ. Mechanisms involving cysteine addition to the substrate are ruled out. Instead cysteine oxidation is achieved via a single mechanistic step involving a proton coupled electron transfer with simultaneous formation of the disulfide bridge. Asn437 is demonstrated not to be critical to the radical chemistry as such, but essential due to the arrangement of the active site. We also propose that the biological role of the disulfide bridge is to block the substrate inside the active site, precluding its dissociation into solution until the disulfide bridge is reduced, a condition necessary to allow a new turnover. (C) 2004 Elsevier B.V. All rights reserved.