Figure 3.

Structural features of the class I RNR β subunits.A, core four-helix bundle implicated in metal binding and/or radical cofactor assembly (white) and B, auxiliary helices (cyan). C, topology diagram showing the locations of amino acids in class Ia RNR important for metal-binding, Tyr• formation, or radical translocation. D, metal-binding site in Fe^II₂-β of class Ia RNR (PDB accession code 1PIY). Selected amino acids shown in stick format and Fe^II ions shown as green spheres. Coordination interactions or hydrogen bonds shown as dashed lines. E, a sequence alignment showing conservation of important metal-binding (cyan) or radical formation sites (orange) in all known class I RNR subclasses. Amino acids important for oxidant access and suppression of Tyr• formation in class Id RNR are shown in boldface and highlighted in yellow.