Esterase Specific Fluorescent Probe: Mechanistic Understanding Using QM/MM Calculation and Cell States Discrimination

Esterases enzymes regulate the body's homeostasis by catalyzing the hydrolysis of various esters. These are also involved in protein metabolism, detoxification, and signal transmission. Most importantly, esterase plays a significant role in cell viability and cytotoxicity assays. Hence, developing an efficient chemical probe is essential for monitoring the esterase activity. Several fluorescent probes for esterase have also been reported targeting cytosol and lysosomes. However, the ability to create efficient probes is constrained due to a lack of understanding of the esterase's active site for hydrolyzing the substrate. In addition, the fluorescent turn-on may limit efficient monitoring. Herein, we have developed a unique fluorescent probe, PM-OAc, to monitor mitochondrial esterase enzyme activity ratiometrically. This probe exhibited a bathochromic wavelength shift with esterase enzyme in alkaline pH (pH similar to 8.0) due to an intramolecular charge transfer (ICT) process. The phenomenon is well supported by TD-DFT calculation. Moreover, the substrate (PM-OAc) binding at the active site of esterase and its catalytic mechanism to hydrolyze the ester bond are elucidated by molecular dynamics (MD) simulation and QM/MM (Quantum mechanics/molecular mechanics) calculations, respectively. Fluorescent image-based analysis of the cellular environment reveals that our probe can distinguish between live and dead cells based on esterase enzyme activity.

Automated summary

Esterase enzymes play a crucial role in regulating the body's homeostasis and are involved in various important cellular processes. Developing efficient chemical probes to monitor esterase activity is essential. In this study, a unique fluorescent probe, PM-OAc, was developed to ratiometrically monitor mitochondrial esterase activity. The probe exhibited a wavelength shift with esterase enzyme in alkaline pH due to an intramolecular charge transfer process. Molecular dynamics simulation and QM/MM calculations were used to elucidate the substrate binding and catalytic mechanism of esterase. The probe showed potential for distinguishing live and dead cells based on esterase enzyme activity.