The inability of oxidized ribonucleotide reductase to bind substrate has functional
significance. In the absence of substrate, the enzyme’s free radical is stored in the
interior of the R2 subunit, close to its dinuclear iron center. When substrate is
bound, the radical is presumably transferred to it via a series of protein side chains
in both R2 and R1. If the substrate is unable to properly react after accepting this
free radical, as would be the case if the enzyme were in its oxidized state, the free
rsdical could potentially destroy both the substrate and the enzyme. Thus, an
important role of the enzyme is to control the release of the radical’s powerful
oxidizing capability. It does so in part by preventing the binding of substrate
while the enzyme is in its oxidized from.