Spectroscopic studies of the murine and human p53 induced ribonucleotide reductase p53R2 protein, differences in cooperative binding of Fe(II) with the R2 protein

Ribonucleotide reductase (RNR) is the enzyme that converts ribonucleotides to corresponding deoxyribonucleotides. The mammalian R2 subunit of the enzyme complex reacts with ferrous iron and dioxygen to generate a diferric iron-oxygen cluster and a tyrosyl radical that is essential for enzymatic activity. A p53 induced isoform of the R2 subunit (p53R2) is proposed to be involved in the production of deoxyribonucleotides during DNA repair. The human and murine p53R2 proteins have been studied by electron paramagnetic resonance (EPR), Mössbauer spectroscopy, magnetic circular dichroism (MCD) and CD, as previously published for mouse R2. While the studies of the active diferric iron-oxygen cluster and the tyrosyl radical (also hydrogen binding to the tyrosyl radical) together with the mixed valent form (Fe(II)-Fe(III) cluster) shows little or no variation between the mouse R2 and p53R2 subunits, the MCD and X-band integer spin EPR studies reveals a difference between diferrous forms. In addition no cooperative binding of Fe(II) or Co(II) is seen for p53R2. The mouse Co(II)Co(II) R2 form has a S= 3 ground state. The diferrous form has a S=2 ground state, with one positive and one negative zero field splitting. In its active form R2 is in complex with another subunit R1, no differences is observed in the interaction of R1 with either mouse R2 or p53R2.