The Inabillty of Oxidized Ribonucleotide Reductase to Bind Substrate Serves
an Essential Protective Function. Comparison of the X-ray structures of reduced
R1 (in which the redox-active Cys 225 and CYs 462 residues are in their SH froms)
and oxidized R1 (in which Cys 225 and Cys 462 are disulfide-linked) reveals that
Cys 462 in reduced R1 has rotated away from is position in oxidized R1 to become
buried in a hydrophobic pocket, whereas Cys 225 moves into the region formerly
occupied by Cys 462. The distance between the formerly disulfide-linked S atoms
thereby increases from 2.0 Å to 5.7 Å. These movements are accompanied by small
shifts of the surrounding polypeptide chain. R1 Cys 225 in oxidized ribonucleotide
reductase prevents the binding of substrate through steric interference of its S atom
with the substrate NDP’s O2’ atom.