The Core Structure of X Generated in the Assembly of the Diiron Cluster of Ribonucleotide Reductase: 17O2 and H217O ENDOR

Abstract

The intermediate, designated X, formed during the self-assembly reaction of the tyrosyl radical/μ-oxo-bridged diferric cofactor in the R2 subunit of Escherichia coli ribonucleotide reductase (RNR) is directly involved in the oxidation of Y122 to the catalytically essential ·Y122. Earlier rapid freeze-quench (RFQ) Q-band ENDOR studies led to the formulation of X as a spin-coupled FeIII/FeIV center, with an S = 1/2 ground state, and showed that X contains a single terminal aqua ligand (water molecule or 2-fold disordered hydroxyl) bound to FeIII but does not contain an hydroxyl bridge. That ENDOR data, coupled with RFQ-EXAFS data, plus the strong spin coupling between the iron ions constrain the structure of X to a di- or tribridged species whose inorganic core (defined as iron and exogenous ligands) contains the [(HxO)FeIIIOFeIV] fragment. To determine whether the core contains a second oxo bridge and to establish the fate of the atoms derived from O2, we have now performed CW and pulsed Q-band 17O ENDOR e...