Figure 5.

Representative Mn^II and Fe^II coordination geometries in class I RNRs. Class Ia Fe ^II/II₂-β₂ (PDB accession code 1PIY) and class Ib Fe ^II/II₂-β₂ (PDB accession code 3N38) structures were used to generate the Fe^II panels at top left. Class Ib Mn ^II/II₂-β₂ structures from the Ec (white, PDB accession code 3N37) and Bs (blue, PDB accession code 4DR0) enzymes were used to generate the Mn^II panels at top right. Note that the Mn^II sites in the class Ib β₂ structures tend to exhibit higher coordination numbers with more strict adherence to octahedral geometry than the Fe^II coordination environments in the class Ia or Ib β₂s. This trend is more apparent in the class Ib Ec Mn ^II/II₂-β₂ structure than in the class Ib Bs Mn ^II/II₂-β₂, and interestingly, the Ec protein binds Mn^II more tightly than the Bs homolog.⁹ Shown at bottom is a comparison to the coordination environments in class Ic β at site 1 (Mn^II, left) and site 2 (Fe^II, right). The binding sites in class Ic β₂ resemble those in the Mn^II structures from class Ib β₂s in that they are six-coordinate with exogenous water ligands, but the geometric distortion, particularly axial to the His ligand, is similar to Fe ^II/II₂-β₂ metal-binding interactions.