Pulsed ELDOR Spectroscopy Measures the Distance between the Two Tyrosyl Radicals in the R2 Subunit of the E. coli Ribonucleotide Reductase

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Abstract
Escherichia coli ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). This RNR is composed of two homodimeric subunits: R1 and R2. R1 binds the NDPs in the active site, and R2 harbors the essential di-iron tyrosyl radical (Y•) cofactor. In this paper, we used PELDOR, a method that detects weak electron−electron dipolar coupling, to make the first direct measurement of the distance between the two Y•'s on each monomer of R2. In the crystal structure of R2, the Y•'s are reduced to tyrosines, and consequently R2 is inactive. In R2, where the Y•'s assume a well-defined geometry with respect to the protein backbone, the PELDOR method allows measurement of a distance of 33.1 ± 0.2 Å that compares favorably to the distance (32.4 Å) between the center of mass of the spin density distribution of each Y• on each R2 monomer from the structure. The experiments provide the first direct experimental evidence for two Y•'s in a single R2 in solution.