19 F Electron-Nuclear Double Resonance Reveals Interaction between Redox-Active Tyrosines across the α/β Interface of E. coli Ribonucleotide Reductase.

Ribonucleotide reductases (RNRs) catalyze the reduction of ribonucleotides to deoxyribonucleotides, thereby playing a key role in DNA replication and repair. Escherichia coli class Ia RNR is an α 2 β 2 enzyme complex that uses a reversible multistep radical transfer (RT) over 32 Å across its two subunits, α and β, to initiate, using its metallo-cofactor in β 2 , nucleotide reduction in α 2 . Each step is proposed to involve a distinct proton-coupled electron-transfer (PCET) process. An unresolved step is the RT involving Y 356 (β) and Y 731 (α) across the α/β interface. Using 2,3,5-F 3 Y 122 -β 2 with 3,5-F 2 Y 731 -α 2 , GDP (substrate) and TTP (allosteric effector), a Y 356 • intermediate was trapped and its identity was verified by 263 GHz electron paramagnetic resonance (EPR) and 34 GHz pulse electron-electron double resonance spectroscopies. 94 GHz 19 F electron-nuclear double resonance spectroscopy allowed measuring the interspin distances between Y 356 • and the 19 F nuclei of 3,5-F 2 Y 731 in this RNR mutant. Similar experiments with the double mutant E 52 Q/F 3 Y 122 -β 2 were carried out for comparison to the recently published cryo-EM structure of a holo RNR complex. For both mutant combinations, the distance measurements reveal two conformations of 3,5-F 2 Y 731 . Remarkably, one conformation is consistent with 3,5-F 2 Y 731 within the H-bond distance to Y 356 • , whereas the second one is consistent with the conformation observed in the cryo-EM structure. The observations unexpectedly suggest the possibility of a colinear PCET, in which electron and proton are transferred from the same donor to the same acceptor between Y 356 and Y 731 . The results highlight the important role of state-of-the-art EPR spectroscopy to decipher this mechanism.