COVID-19 pandemic caused by SARS-CoV-2 led to the research aiming to find the inhibitors of this virus. Towards this world problem, an attempt was made to identify SARS-CoV-2 main protease (Mpro) inhibitory peptides from ricin domains. The ricin-based peptide from barley (BRIP) was able to inhibit Mpro in vitro with an IC50 of 0.52 nM. Its low and no cytotoxicity upto 50 µM suggested its therapeutic potential against SARS-CoV-2. The most favorable binding site on Mpro was identified by molecular docking and steered molecular dynamics (MD) simulations. The Mpro-BRIP interactions were further investigated by evaluating the trajectories for microsecond timescale MD simulations. The structural parameters of Mpro-BRIP complex were stable, and the presence of oppositely charged surfaces on the binding interface of BRIP and Mpro complex further contributed to the overall stability of the protein-peptide complex. Among the components of thermodynamic binding free energy, Van der Waals and electrostatic contributions were most favorable for complex formation. Our findings provide novel insight into the area of inhibitor development against COVID-19.

The main protease (Mpro) of SARS-CoV-2 has been recognized as an attractive drug target because of its central role in viral replication. Our previous preliminary molecular docking studies showed that theaflavin 3-gallate (a natural bioactive molecule derived from theaflavin and found in high abundance in black tea) exhibited better docking scores than repurposed drugs (Atazanavir, Darunavir, Lopinavir). In this study, conventional and steered MD-simulations analyses revealed stronger interactions of theaflavin 3-gallate with the active site residues of Mpro than theaflavin and a standard molecule GC373 (a known inhibitor of Mpro and novel broad-spectrum anti-viral agent). Theaflavin 3-gallate inhibited Mpro protein of SARS-CoV-2 with an IC50 value of 18.48 ± 1.29 μM. Treatment of SARS-CoV-2 (Indian/a3i clade/2020 isolate) with 200 μM of theaflavin 3-gallate in vitro using Vero cells and quantifying viral transcripts demonstrated reduction of viral count by 75% (viral particles reduced from Log106.7 to Log106.1). Overall, our findings suggest that theaflavin 3-gallate effectively targets the Mpro thus limiting the replication of the SARS-CoV-2 virus in vitro.

Main protease (Mpro) of SARS-CoV-2 is crucial for its replication/infection and has been recognized as an attractive drug target. In this study, we identified theaflavin 3-gallate as an inhibitor of Mpro protein of SARS-CoV-2 with IC50 value of 18.48 ± 1.29 µM. Compared to theaflavin, theaflavin 3-gallate exhibited superior antiviral activity and at a concentration of 200 µM reduced the viral count by 75% (viral particles reduced from 106.7 to 106.1). Time-dependent analyses of conventional and steered MD-simulations revealed stronger interactions of theaflavin 3-gallate with the active site residues of Mpro than the standard molecule GC373 and theaflavin. Taken together, our findings suggest that theaflavin 3-gallate can be developed into a potential lead molecule against SARS-CoV-2.