Receptor exposure time

Several proteins are naturally produced as inactive proenzymes and acquire full activity only when cleaved at a specific position by another enzyme. Caspase-3, a cysteine protease, is a key component of the apoptosis signaling pathway. Its inactive form procaspase-3 is cleaved at position Seri 76 by caspase-8 in the death receptor-induced apoptosis pathway, eventually forming the active tetramer. Majima and coworkers artificially reproduced the activation mechanism of procaspase-3 by photoinducing the cleavage of the backbone in a mutant protein containing a Npg residue specifically introduced at position 176 [115]. The incorporation efficiency of Npg by using an in vitro transcription/translation system was only 15%. Nevertheless, photoactivation (366 nm, 0°C, up to 10 min exposure time) of Npg-caspase-3 was followed within 1 min by a clear activation of enzymatic activity as quantified by the change in fluorescence of the peptidic substrate Z-DEVD-rhodamine 110. 

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