Apoptosis is a form of cell death essential for proper development, tissue homeostasis, and the elimination of virally-infected and tumorigenic cells in animals. Abnormal suppression of apoptotic cell death is a hallmark of proliferative diseases including cancer. Excessive apoptotic death is implicated in neurodegenerative disorders such as Alzheimer's disease. The execution of apoptosis is carried out by caspases, a group of intracellular cysteine proteases that cleave their substrates adjacent to aspartic acid. In contrast, the inhibitor of apoptosis (IAP) protein family negatively regulates programmed cell death by preventing the activation of pro-caspases and inhibiting the enzymatic activity of mature caspases. Smac (second mitochondria-derived activator of caspases) is a master regulator of apoptosis in mammalian cells where it neutralizes the inhibitory effects of IAPs on caspases. The high-resolution X-ray structure of the Smac homodimer in complex with the third BIR repeat of XIAP shows a highly localized interface between the Smac N-terminus and a surface groove on XIAP-BIR3. Synthetic peptides as short as 4 amino acids (AVPI) antagonize the Smac/XIAP interaction in vitro and sensitize cells in culture to apoptosis in response to chemotherapeutic drug treatment. Using the Smac/XIAP-BIR3 co-crystal structure as a guide, our objective is to design and synthesize small molecule mimetics of the Smac N-terminus that bind the third BIR repeat of XIAP with high affinity. Such molecules could be potentially used in conjunction with conventional chemo- or radiation therapy to kill cancer cells - particularly in tissues that express high levels of IAPs. Methods to achieve this goal are presented in stages with synthetic chemistry being developed for the construction of focused compound libraries -constituents of which are designed as spatial and electronic complements to the target XIAP-BIR3 surface. A sensitive fluorescence polarization-based binding assay is proposed as a reliable readout of compound affinity that is amenable to high-throughput operation.
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