Crystal Structures of Two Self-Hydroxylating Ribonucleotide Reductase Protein R2 Mutants: Structural Basis for the Oxygen-Insertion Step of Hydroxylation Reactions Catalyzed by Diiron Proteins,

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Abstract
The R2 protein of ribonucleotide reductase catalyzes the dioxygen-dependent one-electron oxidation of Tyr122 at a diiron-carboxylate site. Methane monooxygenase and related hydroxylases catalyze hydrocarbon hydroxylation at diiron sites structurally related to the one in R2. In protein R2, the likely reaction site for dioxygen is close to Phe208. The crystal structure of an iron ligand mutant R2, Y122F/E238A, reveals the hydroxylation of Phe208 at the meta, or epsilon-, ring position and the subsequent coordination of this residue to the diiron site. In another mutant, F208Y, the "foreign" residue Tyr208 is hydroxylated to Dopa. The structures of apo and diferrous F208Y presented here suggest that Tyr208 is coordinated to the iron site of F208Y throughout the Dopa generation cycle. Together, the structural data on these two mutants suggest two possible reaction geometries for the hydroxylation reaction catalyzed by these modified R2 diiron sites, geometries which might be relevant for the hydroxylation reaction catalyzed by other diiron sites such as methane monooxygenase. A critical role for residue Glu238 in directing the oxidative power of the reactive intermediate toward oxidation of Tyr122 is proposed.