Deciphering Radical Transport in the Large Subunit of Class I Ribonucleotide Reductase

Incorporation of 2,3,6-trifluorotyrosine (F<sub>3</sub>Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the <i>C</i>-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F<sub>3</sub>Y radical from the [Re]–F<sub>3</sub>Y−βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y–Y established, radical transport occurs with a rate constant of 3 × 10<sup>5</sup> s<sup>–1</sup>. Point mutations that disrupt the Y–Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the colinear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme.