Conformationally Dynamic Radical Transfer within Ribonucleotide Reductase.

Ribonucleotide reductases (RNR) catalyze the reduction of nucleotides to deoxynucleotides through a mechanism involving an essential cysteine based thiyl radical. In the E. coli class 1a RNR the thiyl radical (C439•) is a transient species generated by radical transfer (RT) from a stable diferric-tyrosyl radical cofactor located >35 Å away across the α2:β2 subunit interface. RT is facilitated by sequential proton-coupled electron transfer (PCET) steps along a pathway of redox active amino acids (Y122β ↔ [W48β?] ↔ Y356β ↔ Y731α ↔ Y730α ↔ C439α). The mutant R411A(α) disrupts the H-bonding environment and conformation of Y731, ostensibly breaking the RT pathway in α2. However, the R411A protein retains significant enzymatic activity, suggesting Y731 is conformationally dynamic on the time scale of turnover. Installation of the radical trap 3-amino tyrosine (NH2Y) by amber codon suppression at positions Y731 or Y730 and investigation of the NH2Y• trapped state in the active α2:β2 complex by HYSCORE spectroscopy validate that the perturbed conformation of Y731 in R411A-α2 is dynamic, reforming the H-bond between Y731 and Y730 to allow RT to propagate to Y730. Kinetic studies facilitated by photochemical radical generation reveal that Y731 changes conformation on the ns-μs time scale, significantly faster than the enzymatic kcat. Furthermore, the kinetics of RT across the subunit interface were directly assessed for the first time, demonstrating conformationally dependent RT rates that increase from 0.6 to 1.6 × 104 s-1 when comparing wild type to R411A-α2, respectively. These results illustrate the role of conformational flexibility in modulating RT kinetics by targeting the PCET pathway of radical transport.