Functional consequences of retro-inverso isomerization of a miniature protein inhibitor of the p53-MDM2 interaction

Peptide retro-inverso isomerization is thought to be functionally neutral and has been widely used as a tool for designing proteolytically stable D-isomers to recapitulate biological activities of their parent L-peptides. Despite success in a wide range of applications, exceptions amply exist that clearly defy this rule of thumb when parent L-peptides adopt an a-helical conformation in their bound state. The detrimental energetic effect of retro-inverso isomerization of an a-helical L-peptide on its target protein binding has been estimated to be 3.0-3.4 kcal/mol. To better understand how the retro-inverso isomer of a structured protein works at the molecular level, we chemically synthesized and functionally characterized the retro-inverso isomer of a rationally designed miniature protein termed stingin of 18 amino acid residues, which adopts an N-terminal loop and a C-terminal alpha-helix stabilized by two intra-molecular disulfide bridges. Stingin emulated the transactivation peptide of the p53 tumor suppressor protein and bound with high affinity and via its C-terminal a-helix to MDM2 and MDMX-the two negative regulators of p53. We also prepared the retro isomer and D-enantiomer of stingin for comparative functional studies using fluorescence polarization and surface plasmon resonance techniques. We found that retro-inverso isomerization of L-stingin weakened its MDM2 binding by 720 fold (3.9 kcal/mol); while enantiomerization of L-stingin drastically reduced its binding to MDM2 by three orders of magnitude, sequence reversal completely abolished it. Our findings demonstrate the limitation of peptide retro-inverso isomerization in molecular mimicry and reinforce the notion that the strategy works poorly with biologically active a-helical peptides due to inherent differences at the secondary and tertiary structural levels between an L.-peptide and its retro-inverso isomer despite their similar side chain topologies at the primary structural level) (C) 2013 Elsevier Ltd. All rights reserved.