2,3-Difluorotyrosine at position 356 of ribonucleotide reductase R2: A probe of long-range proton-coupled electron transfer

Escherichia coli class I ribonucleotide reductase catalyzes the conversion of ribonucleotides to deoxyribonucleotides and consists of two subunits: R1 and R2. R1 possesses the active site, while R2 harbors the essential diferric-tyrosyl radical (Y•) cofactor. The Y• on R2 is proposed to generate a transient thiyl radical on R1, 35 Å distant, through amino acid radical intermediates. To study the putative long-range proton-coupled electron transfer (PCET), R2 (375 residues) was prepared semisynthetically using intein technology. Y356, a putative intermediate in the pathway, was replaced with 2,3-difluorotyrosine (F₂Y, pK_a = 7.8). pH rate profiles (pH 6.5-9.0) of wild-type and F₂Y-R2 were very similar. Thus, a proton can be lost from the putative PCET pathway without affecting nucleotide reduction. The current model involving H• transfer is thus unlikely.