Rational reprogramming of the R² subunit of Escherichia coli ribonucleotide reductase into a self-hydroxylating monooxygenase

The outcome of O₂ activation at the diiron(II) cluster in the R2 subunit of Escherichia coli (class I) ribonucleotide reductase has been rationally altered from the normal tyrosyl radical (Y122•)¹ production to self-hydroxylation of a phenylalanine side-chain by two amino acid substitutions that leave intact the (histidine)₂-(carboxylate)₄ ligand set characteristic of the diiron-carboxylate family. Iron ligand Asp (D) 84 was replaced with Glu (E), the amino acid found in the cognate position of the structurally similar diiron-carboxylate protein, methane monooxygenase hydroxylase (MMOH). We previously showed that this substitution allows accumulation of a μ-1,2-peroxodiiron(III) intermediate, which does not accumulate in the wild-type (wt) protein and is probably a structural homologue of intermediate P (H_peroxo) in O₂ activation by MMOH. In addition, the near-surface residue Trp (W) 48 was replaced with Phe (F), blocking transfer of the "extra" electron that occurs in wt R2 during formation of the formally Fe(III)Fe(IV) cluster X. Decay of the μ-1,2-peroxodiiron(III) complex in R2-W48F/D84E gives an initial brown product, which contains very little Y122• and which converts very slowly (t_1/2 ∼ 7 h) upon incubation at 0 °C to an intensely purple final product. X-ray crystallographic analysis of the purple product indicates that F208 has undergone ε-hydroxylation and the resulting phenol has shifted significantly to become a ligand to Fe2 of the diiron cluster. Resonance Raman (RR) spectra of the purple product generated with ¹⁶O₂ or ¹⁸O₂ show appropriate isotopic sensitivity in bands assigned to O-phenyl and Fe-O-phenyl vibrational modes, confirming that the oxygen of the Fe(III) -phenolate species is derived from O₂. Chemical analysis, experiments involving interception of the hydroxylating intermediate with exogenous reductant, and M̈ssbauer and EXAFS characterization of the brown and purple species establish that F208 hydroxylation occurs during decay of the peroxo complex and formation of the initial brown product. The slow transition to the purple Fe(III) -phenolate species is ascribed to a ligand rearrangement in which μ-O^2- is lost and the F208-derived phenolate coordinates. The reprogramming to F208 monooxygenase requires both amino acid substitutions, as very little ε-hydroxyphenylalanine is formed and pathways leading to Y122• formation predominate in both R2-D84E and R2-W48F.