The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes COVID-19 (coronavirus disease 2019) has infected over 58.5 million people worldwide and killed over 1.38 million is one of the most significant global health crises in recent memory. There are currently no effective and safe vaccines to prevent SARS CoV-2 infection nor effective medications to treat COVID-19 disease. Researchers around the world are thus trying to find effective molecules and compounds that could in the fight against the virus.
Researchers led by first author Relja Suručić from the Department of Pharmacognosy, Faculty of Medicine, University of Banja Luka, Banja Luka, Bosnia and Herzegovina, have studied the use of pomegranate peel extracts in preventing infections with SARS-CoV-2. Their study titled, “Computational study of pomegranate peel extract polyphenols as potential inhibitors of SARS-CoV-2 virus internalization,” was published in the latest issue of the journal Molecular and Cellular Biochemistry.
Coronaviruses, explain the researchers, are known to cause respiratory tract infections in humans. These include the Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV) and the novel coronavirus SARS-CoV-2. The SARS-CoV-2 is known to cause severe symptoms in some individuals requiring hospitalization, ventilation, and ICU care. The World Health Organization declared the COVID-19 pandemic on the 11th of March this year.
Viral structure, Transmission and infection
The SARS-CoV-2 virus also has a rapid transmission rate between humans. The virus has four main structural proteins along with other accessory proteins:
- spike (S) glycoprotein
- small envelope (E) glycoprotein
- membrane (M) glycoprotein
- nucleocapsid (N) protein
The SARS-CoV-2 enters into the cells of the respiratory tract using the S glycoprotein. The virus interacts with the angiotensin-converting enzyme 2 (ACE2) receptor found abundantly in the respiratory system.
The S glycoprotein has two functional subunits known as S1, which is an amino (N)-terminal subunit, and S2, a carboxyl (C)-terminal subunit. S1 on the surface binds to the ACE2 receptor, and the S2 interacts with the host cell membrane to allow the fusion of the membranes of the cell and the virus so that the viral particles can finally enter into the cell. The host cell contains specific proteases that can break or cleave the S1/S2 to allow the process of fusion and entry. Notable protease is the transmembrane serine protease 2 (TMPRSS2). Both the ACE2 and TMPRSS2 are required for viral entry into the cell.
Furin also leads to cleavage of the S glycoprotein and thus facilitates the entry of the SARS-CoV-2 virus by its binding to the ACE2 receptor. TMPRSS2 inhibitors and furin inhibitors are speculated to prevent virus entry into the host cell.
Pomegranate peel extract
Several natural products are being explored to prevent SARS-CoV-2 infection or treat it effectively. Pomegranate (Punica granatum L., Punicaceae family) consumed worldwide has been known to have beneficial health properties and useful in treating type 2 diabetes, atherosclerosis, cardiovascular diseases, inflammatory diseases, cancers, etc.
Pomegranate peel extracts are known to contain “phytobiotics such as hydrolyzable tannins (ellagitannin, punicalagin, punicalin, gallic and ellagic acid), flavonoids, anthocyanins, and other phenols,” explain the researchers. These polyphenols are known to have several properties, including:
- anti-inflammatory effects
- antioxidant effects
- hypoglycaemic or blood sugar reducing effects
- lipid-lowering or cholesterol lowering effects
- antihypertensive or blood pressure lowering effects
- antimicrobial effects
Pomegranate extracts have been known to be useful against viruses such as “influenza virus, herpes virus, poxviruses, and human immunodeficiency virus,” the researchers state. Molecules in the extract called the “punicalagin, punicalin and ellagic acid”, also show effects against hepatitis C virus (HCV). Pomegranate peel extracts (PoPEx) have also shown effects against the influenza virus by preventing virus entry and transcription of the RNA.
This study targeted four major ellagitannin members present in PoPEx, “punicalagin, punicalin, ellagic acid, and gallic acid,” to see their efficacy against SARS-CoV-2 in lab models (in silico tools).
The researchers tested the binding affinities of ellagic acid, gallic acid, punicalagin, and punicalin on four protein targets that could allow entry of the virus into the host cell. These molecules tested were;
- Camostat, and
- Selected PoPEx constituents (punicalagin, punicalin, ellagic acid and gallic acid)
The four ptoeitn targets were:
- SARS-CoV-2 spike glycoprotein,
- Angiotensin-converting enzyme 2,
- Transmembrane serine protease 2 (TMPRSS2).
The 3D structures of molecules tested were downloaded, and computational techniques were used to check their binding affinity.
The results of this study showed that constituents of pomegranate peel extracts such as punicalagin and punicalin show significant potential in interacting with the selected protein targets and thus could possibly prevent the viral entry into the host cell. This needs to be followed up with in vitro and in vivo studies they write.
- Using the DoGSiteScorer tool, potential binding pockets were determined on the proteins.
- The most druggable pockets for the S glycoprotein, ACE2, furin, and TMPRSS2 were chosen.
- A high apolar amino acid ratio meaning good drug target was found for S glycoprotein (apolar amino acid ratio was 0.63)
- Apolar amino acid ratios of ACE2, furin and TMPRSS2 were 0.38, 0.26 and 0.40, respectively.
- The best drug score values for analyzed targets ranged between 0.73 and 0.84
- Pocket volumes for drug binding determined the potential of the drug to bind with the target. The most significant volume, surface, and depth of the selected pocket were detected for ACE2.
- Punicalagin and punicalin formed the most stable complexes with the protein targets. They also showed intensive interactions with TMPRSS2 amino acid residues
Conclusions and implications
The viral entry prevention into the host cell could effectively prevent the infection. This study showed that PoPEx polyphenols could offer potential inhibitory activity against the SARS-CoV-2, especially during its host cell entry. Authors of the study write, “punicalagin and punicalin are promising candidates for further anti-SARS-CoV-2 in vitro studies”. The team concluded, “Being the ingredients of a natural product that is used as food, these candidates also have a confirmed safety profile which is their additional and important advantage in the disease treatment.”