Figure 23-9 class I ribonucleotide reductase from E. coli. (a) Schematic diagram of the quaternary structure. The enzyme consists of two identical pairs of subunits, R12 and R22. Each R2 subunit contains a binuclear Fe(III) complex that generates a phenoxy radical at Tyr 122. The R1 subunits each contain three different allosteric effector sites and five catalytically important Cys residues. The enzyme's two active sites occur at the interface between the R1 and R2 subunits. (b) A ribbon diagram of R22 Asp 84 viewed perpendicularly to its twofold axis with the subunits drawn in blue and yellow. The Fe(III) ions are represented by orange spheres, and the His 118 radical harboring Tyr 122 side chains are shown in space-filling representation with their C and O atoms green and red. (c) The binuclear Fe(II) complex of R2. Each Fe(III) ion is octahedrally coordinated by a His N atom and five O atoms, including those of the O2- ion and the Glu carboxyl group that bridges the two Fe(III) ions. [Part b based on an X-ray structure by Hans Eklund, Swedish University of Agricultural Sciences, Uppsala, Sweden. PDBid 1RIB see Interactive Exercise 33.