DFT Calculations for Intermediate and Active States of the Diiron Center with a Tryptophan or Tyrosine Radical in Escherichia coli Ribonucleotide Reductase

Class Ia ribonucleotide reductase subunit R2 contains a diiron active site. In this paper, active-site models for the intermediate X-Trp48•+ and X-Tyr122•, the active Fe(III)Fe(III)-Tyr122•, and the met Fe(III)Fe(III) states of Escherichia coli R2 are studied, using broken-symmetry density functional theory incorporated with the conductor-like screening solvation model. Different structural isomers and different protonation states have been explored. Calculated geometric, energetic, Mossbauer, hyperfine, and redox properties are compared with available experimental data. Feasible detailed structures of these intermediate and active states are proposed. Asp84 and Trp48 are most likely the main contributing residues to the result that the transient Fe(IV)Fe(IV) state is not observed in wild-type class Ia E. coli R2. Asp84 is proposed to serve as a proton-transfer conduit between the diiron cluster and Tyr122 in both the tyrosine radical activation pathway and the first steps of the catalytic proton-coupled ...