Structural Elucidation of the Reduced Mn(III)/Fe(III) Intermediate of the Radical-Initiating Metallocofactor in Ribonucleotide Reductase.

Ribonucleotide reductases (RNRs) are the sole source of deoxyribonucleotides for DNA synthesis and repair across all organisms and carry out their reaction via a radical mechanism. RNR from generates its turnover-initiating cysteinyl radical by long-range reduction of a Mn(IV)/Fe(III) cofactor, producing a Mn(III)/Fe(III) intermediate. Herein, we characterize the protonation states of the inorganic ligands in this reduced state using advanced pulse electron paramagnetic resonance (EPR) spectroscopy and H-isotope labeling.