Comparison of the crystal packing of (a) the wild-type Keap1 Kelch domain in complex with an Nrf2-derived peptide (PDB entry 2flu
; Lo et al., 2006), (b) unliganded Keap1 (PDB entry 1zgk
; Beamer et al., 2005) and (c) the E540A/E542A double mutant. Residues 540 and 542 in monomers A and B are shown as red and orange spheres, respectively. The access to the Nrf2-binding site is blocked in wild-type Keap1 (b) and free in one of the two protomers of the E540A/E542A double mutant (c). This binding site is occupied by the cyclic peptide.