Fig. 4. Two qABP working strategies. (A) Schematic illustration of activity-dependent fluorescent labeling of Cys proteases by using qABP. (B) Another strategy of the enzyme activity-dependent qABP reaction. The structure of qABP contains a mandelic acid core which was surrounded by a fluorophore (red), a quencher and an enzyme WH (blue). The resulting intermediate of enzymatic reaction has three possible pathways within the cells: (pathway 1) hydration/quenching by water; (pathway 2) covalent attachment to the target enzyme; (pathway 3) accumulation in the organelle where the enzymatic reaction occurs. Reproduced from ref. 58 with permission from American Chemical Society, copyright 2011.