Targeting SARS-CoV-2 with Quercetin and Its Analogs as Inhibitors of 3CLpro

The COVID-19 pandemic changed our global landscape and created an urgent need to find effective therapies to combat SARS-CoV-2, the causal agent of this disease. Despite the significant reduction achieved through vaccination efforts, the search for effective antivirals remains essential, particularly given the continued emergence of novel strains of the virus.

One potential area of investigation is the use of natural products, particularly flavonoids, to inhibit key viral enzymes. A study conducted by our students in the midst of the COVID-19 outbreak, entitled “Virtual Screening of Quercetin and its Analogs as Inhibitors of 3CLpro in SARS-CoV-2,” investigated the ability of quercetin and its derivatives to act as inhibitors of the primary protease (3CLpro) of SARS-CoV-2, a key enzyme for viral replication.

Why 3CLpro is a Key Target in Combating SARS-CoV-2

3CLpro, also known as the main protease, plays a pivotal role in the life cycle of SARS-CoV-2 by processing the viral polyproteins into functional units necessary for replication and transcription. Inhibiting this enzyme effectively halts the virus’s ability to replicate, making it an attractive target for antiviral drug development. The high sequence homology of 3CLpro among different coronaviruses also suggests that inhibitors targeting this enzyme could have broad-spectrum antiviral activity.

Protein structure of 3CLpro. Image by Purnamasari & Maliza (2021)

Exploring Quercetin and Its Analogs as Natural Inhibitors

Quercetin, a flavonoid found in many fruits and vegetables, has garnered attention for its antiviral properties. Previous studies have shown that quercetin can inhibit various stages of viral infections, including those caused by coronaviruses. Our study takes this a step further by using virtual screening and molecular docking to evaluate the binding affinity of quercetin and 33 of its analogs to 3CLpro.

Quercetin structure. Photo courtesy Yikrazuul

Our Computational Approach: Virtual Screening and Molecular Docking

We employed AutoDock Vina, a widely used molecular docking software, to simulate the interaction between the 3CLpro enzyme and the flavonoid compounds. The crystal structure of 3CLpro (PDB ID 6LU7) was prepared by removing non-standard residues and water molecules and adding hydrogen atoms to ensure accurate docking simulations.

The ligands, including quercetin and its analogs, were sourced from the ZINC database and converted to the PDBQT format for docking using OpenBabel. To validate our docking protocol, we re-docked the co-crystallized inhibitor N3 into the 3CLpro binding site, achieving an RMSD value of 0.692 Å, which confirmed the reliability of the method.

Rutin: A Promising Inhibitor

Among the 33 quercetin analogs tested, rutin emerged as the most promising candidate, exhibiting a binding affinity of -10 Kcal/mol. This is significantly higher than the binding affinities of the known inhibitors N3 (-7.1 Kcal/mol) and Remdesivir (-8.1 Kcal/mol).

Rutin’s strong binding affinity is attributed to its interactions with the catalytic dyad residues His41 and Cys145 of 3CLpro, forming hydrogen bonds and hydrophobic interactions that stabilize the inhibitor-enzyme complex.

The Potential Impact on Antiviral Drug Development

The findings of this study have several important implications for future COVID-19 treatments:

1. Natural Compound Potential: The high binding affinity of rutin underscores the potential of natural compounds, particularly flavonoids, as effective inhibitors of viral enzymes. This aligns with other studies that have highlighted the antiviral properties of flavonoids against SARS-CoV-2.
2. Broad-Spectrum Antivirals: Given the conserved nature of 3CLpro among coronaviruses, inhibitors like rutin could potentially be effective against a range of coronavirus strains, offering a broad-spectrum antiviral solution.
3. Further Research: While the in-silico results are promising, further research, including molecular dynamics simulations and in vitro/in vivo testing, is necessary to confirm the efficacy and safety of these compounds. This step is crucial before any clinical application can be considered.
4. Drug Design: The study provides valuable insights into the structural features that contribute to the binding affinity of flavonoids to 3CLpro. This information can guide the design of more potent analogs with enhanced antiviral activity.

Final Thoughts

Our investigation highlights the potential of quercetin and its analogs, especially rutin, as inhibitors of the major protease of SARS-CoV-2. Through the use of computational methods, we have identified promising candidates that have the potential to facilitate the development of novel antiviral drugs.

The road from virtual screening to clinical application is long, but the promising results of this study offer hope in the ongoing battle against COVID-19 and its variants. With further research and development, natural products like rutin may become an important part of our antiviral arsenal.

Further reading

Purnamasari, E. W. E., & Maliza, R. (2021). Virtual Screening Quercetin dan Analognya sebagai Inhibitor 3CLpro pada SARS-CoV-2. Seminar Nasional Bioteknologi VII, Universitas Gadjah Mada.