The Keap1-binding motif of ProTa shares a similar sequence with that of the Neh2. Crystal structures of ProTa and Neh2 peptides bound to the Kelch domain of Keap1 further reveal that these two proteins bind to the same site on the Kelch domain and form similar b-turn conformations. The Kelch domain adopts a six-bladed Gefitinib bpropeller structure with each blade composed of four anti-parallel b-strands. Both ProTa and Neh2 bind to the positively charged face of the b-propeller where the inter-blade loops are located and the electrostatic interactions are crucial for the stability of the complexes. Interestingly, despite the high sequence identity and structural similarity of the binding motifs, ProTa seems to have a lower binding affinity to Keap1 compared to Neh2. Atomistic microsecond scale MD simulations were used to investigate the molecular mechanisms by which the intrinsically disordered ProTa and Neh2 interact with Keap1. In particular, we focused on whether their XEEXGE motifs bind to Kelch domain through coupled folding and binding, PSEs or a combination of both mechanisms. Our results show that in their free states, both the Keap1-binding motifs of ProTa and Neh2 display intrinsic propensities to form bound-state-like b-turns, and that the CT99021 residues outside of the motifs may also contribute to the stability of the structural elements. We found that the Keap1- binding motif of Neh2 adopted a b-turn conformation that more closely resembled its bound-state structure than that of ProTa. Based on these results, we propose that binding occurs synergistically via a combination of PSEs and coupled folding and binding with a heavy bias towards PSEs, especially for Neh2. The better understanding of the binding mechanisms may provide insight into developing of therapeutics that can be used to promote cellular response to oxidative stress. All simulations were performed using GROMACS version 4, with the GROMOS96 53a6 united atom force-field parameter set. This force field has been shown to be reliable in simulating proteins, including b-peptide folding. Protonation states of ionizable residues were chosen based on their most probable state at pH 7. Protein and non-protein atoms were coupled to their own temperature baths, which were kept constant at 310 K using the weak coupling algorithm. Pressure was maintained isotropically at 1 bar using the Berendsen barostat. Prior to the production runs, the energy of each system was minimized using the steepest descents method. This was followed by 2 ps of positionrestrained dynamics with all non-hydrogen atoms restrained with a 1000 kJ mol21 force constant. The timestep was set to 2 fs. Initial atom velocities were taken from a Maxwellian distribution at 310 K. All bond lengths were constrained using the LINCS algorithm. Cut-off of 1.0 nm was used for Lennard-Jones interactions and the real part of the long-range electrostatic interactions, which were calculated using the Particle-Mesh Ewald method. 0.12 nm grid-spacing was used for PME.