EPR Study of the Mixed-Valent Diiron Sites in Mouse and Herpes Simplex Virus Ribonucleotide Reductases. Effect of the Tyrosyl Radical on Structure and Reactivity of the Diferric Center

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Abstract
Reduction of ribonucleotide reductase (EC 1.17.4.1) R2 proteins in a frozen glycerol−buffer solution at 77 K by mobile electrons generated by γ-irradiation produces EPR-detectable iron sites in mixed-valent Fe(II)/Fe(III) states. The primary EPR signals give information about the ligand arrangement of the diferric form of the iron site, whereas secondary signals observed after annealing of the sample show the effects of structural relaxation. In recombinant metR2 proteins (without free radical) from mouse and herpes virus type 1, the mixed-valent sites trapped at 77 K give rise to axial S = 1/2 EPR spectra with g values in the range 1.79−1.94, observable at temperatures up to 110 K. The spectra are assigned to μ-oxo-bridged dinuclear iron sites. In mouse metR2, the primary EPR spectrum is a mixture of two components. Annealing the R2 samples to 160−170 K transforms the primary EPR signals into rhombic spectra, characterized by gav < 1.8, and observable only below 25 K. These spectra are assigned to partially relaxed forms with a μ-hydroxo bridge, formed by protonation of the oxo bridge. Further annealing at 220 K produces new rhombic EPR spectra, which are closely similar with those observed and found to be stable after chemical reduction at room temperature. The EPR signal of the primary mixed-valent iron site in active mouse R2 protein with a tyrosyl radical also has two components. Both are different from those observed in metR2. In herpes simplex virus type 1 protein R2, one primary mixed-valent component was observed for the met protein. The dose−yield curve for the mixed-valent state in active mouse R2 is sigmoidal in shape, indicating that the tyrosyl radical is reduced by mobile electrons before the iron site. Kinetic experiments on the reduction by dithionite on mouse R2 without and with radical show a significantly enhanced rate for reduction of the iron site in the protein without radical. The results suggest that in active mouse R2 only complete diferric sites with neighboring radicals give rise to the mixed-valent spectra, and that these sites may exist in two structurally distinct forms. The results on the mouse R2 proteins confirm and extend previous results obtained on the Escherichia coli protein R2 showing that the presence of the tyrosyl radical significantly affects not only the structure but also the reactivity of the iron site.

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