Evidence by site-directed mutagenesis supports long-range electron transfer in mouse ribonucleotide reductase

Mammalian ribonucleotide reductase consists of two nonidentical subunits, proteins R1 and R2, each inactive alone. The R1 protein binds the ribonucleotide substrates while the R2 protein contains a binuclear iron center and a tyrosyl free radical, essential for activity. The crystal structures of the corresponding Escherichia coli proteins suggest that the distance from the active site in R1 to the tyrosyl radical buried in R2 is about 35 A. Therefore, an electron pathway was suggested between the active site and the tyrosyl radical. Such a pathway could include a conserved tryptophan on the suggested R1 interaction surface of R2 and a conserved aspartic acid hydrogen bonded both to the tryptophan and to a histidine iron ligand. To find experimental support for such an electron pathway, we have replaced the conserved tryptophan in mouse R2 with phenylalanine or tyrosine and the aspartic acid with alanine. All the mutated R2 proteins were shown to bind metal with the same affinity as native R2 and to form the binuclear iron center. In addition, the W103Y and D266A proteins formed a normal tyrosyl free radical while only low amounts of radical were observed in the W103F protein. Neither the kinetic rate constants nor the equilibrium dissociation constant of the R1/R2 complex was affected by the mutations as shown by BIAcore biosensor technique. However, all mutant R2 proteins were completely inactive in the enzymatic assay, supporting the hypothesis that the tryptophan and aspartic acid residues are important links in an amino acid residue specific long-range electron transfer.