3.

Overall architecture of GREs. (A) GREs are made of two antiparallel 5-stranded β-sheets. The first half β-sheet is followed by the Cys loop, the second half β-sheet, and then the Gly loop. The Cys and Gly loops meet in the center of the barrel to form part of the buried active site. Structural architecture demonstrated with PFL (PDB ID: 1H18). (B) Topology diagram of GREs showing the conserved 10-stranded β/α-barrel. (C) The GRD is located at the C-terminus of the GRE at the periphery of the structure such that it can be flexible for post-translational radical installation. The cap helix is conserved across GREs, located between β2 and β3, and covers one end of the barrel.