Terphenyl-Based Bak BH3 α-Helical Proteomimetics as Low-Molecular-Weight Antagonists of Bcl-xL

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Abstract
We describe a general method for the mimicry of one face of an α-helix based on a terphenyl scaffold that spatially projects functionality in a manner similar to that of two turns of an α-helix. The synthetic scaffold reduces the flexibility and molecular weight of the mimicked protein secondary structure. We have applied this design to the development of antagonists of the α-helix binding protein Bcl-xL. Using a sequential synthetic strategy, we have prepared a library of terphenyl derivatives to mimic the helical region of the Bak BH3 domain that binds Bcl-xL. Fluorescence polarization assays were carried out to evaluate the ability of terphenyl derivatives to displace the Bcl-xL-bound Bak peptide. Terphenyl 14 exhibited good in vitro affinity with a Ki value of 0.114 μM. These terphenyl derivatives were more selective at disrupting the Bcl-xL/Bak over the HDM2/p53 interaction, which involves binding of the N-terminal α-helix of p53 to HDM2. Structural studies using NMR spectroscopy and computer-aided docking simulations suggested that the helix binding area on the surface of Bcl-xL is the target for the synthetic ligands. Treatment of human embryonic kidney 293 (HEK293) cells with terphenyl derivatives resulted in the disruption of the binding of Bcl-xL to Bax in intact cells.