Structural analysis
Prediction of secondary structure of 2019-nCoV S-protein has been done using SOPMA (Self Optimised Prediction Method with Alignment). The S-protein contains 1288 aa residues comprising 350 α helices (27.17%), 312 β-turns (9.08%), 509 random coils (39.52%). Through ExPASyProtParam, the total number of negatively charged (Asp + Glu) and positively charged residues (Arg + Lys) were determined to be 112 and 100, respectively. The aliphatic index was found to be 81.58. The GRAVY (Grand Average of Hydropathicity) scored to –0.163. The instability index was computed to be 31.58. These features classify the protein as stable. It was also revealed through computational studies that the half-life of S- protein is maximum in case of mammals (mammalian reticulocytes- 30 hours) than in case of yeast (> 20 hours) and bacteria (E. Coli- >10 hours).
Structure alignment
Superimposition of structures of S-proteins of 2019-nCoV and SARS-CoV was evaluated by TM-align (https://zhanglab.ccmb.med.umich.edu/TM-align/) for comparative structural studies. These two viruses were considered for Structure-Structure superimposition due to maximum sequence similarity. From this study, it was observed through structural alignment that2019-nCoV and SARS-CoV only differ in RBD fragment and remaining part of the structure is identical (Suppl. fig. S2). From the structure alignment and phylogenetic analysis, it was observed that SARS-CoV is an ancestor of the newly upsurge virus 2019-nCoV. However, some changes were observed in the RBD fragment of 2019-nCoV compared to SARS-CoV.
Phylogenetic analysis
2019-nCoV shares the highest sequence identity (73.9095 %) with SARS-CoV and the lowest similarity in amino acid sequence was observed with HCoV–229E (10.6077 %).Similarly, the sequence identity is intermediate i.e 22.9037 % and18.5559 % with MERS-CoV, HCoV-
NL63, respectively. Phylogenetic tree in (Suppl. Fig. S3) shows that 2019-nCoV and SARS- CoV have same OTU (Operational Taxonomic Unit) due to the highest sequence similarity.
Molecular docking analysis
The binding modes of curcumin and catechin with S-protein and ACE2 were studied through AutodockVina1.1.2.The binding energy of S-protein with catechin and curcumin scored to be–10.5Kcal/mol and –7.9Kcal/molrespectively (Table1). The binding affinity of curcumin with ACE2 was noted to be –7.8Kcal/mol where as that of catechin was found to be - 8.9Kcal/mol (Table 2). From the docking scores, it can be deduced that both catechin and curcumin have strong binding affinity with S-protein as well as ACE2.AlthoughVan der Waals force, conventional hydrogen bonds and carbon hydrogen bonds facilitates binding between ligands (curcumin or catechin) and S-protein, amino acid residues of the protein that participate for such interactions for different bonds varies between curcumin and catechin (Fig. 2 and 3). Molecular docking experiments showed the affinity or binding capacity of curcumin and catechin with S-protein as well as ACE2. It also provided the evidence that catechin binds with greater affinity than curcumin
Molecular Simulation analysis
The results from Molecular Simulation data throw a light on the interaction of curcumin with S-protein. It was observed that interaction between curcumin and S-protein existed over the time span of 100ns but substantial interaction was seen during the simulation time of 40ns to 100ns (Fig. 4). Local changes along the protein chain were characterised through Root Mean Square Fluctuation (RMSF). The plot indicates curcumin possesses the ability to cause fluctuation of all amino acids of S protein (Suppl Fig. S4).Protein and ligand interaction was strong at RBD site of S-protein from 40ns to 90ns (Suppl. Fig. S6). RBD site of S-protein are
linked with keto group of curcumin with strong affinity at amino acid Leu–335 through hydrophobic bonds. Interaction with this amino acid occurs for 40% of the simulation time (Fig. 5).Molecular simulation studies favour docking studies which state that even though catechin has high binding energy with S-protein, curcumin binds directly to the RBD of the S-protein with greater affinity. At the same time, catechin is seen to cause greater fluctuation in amino acids near the RBD site.
The RBD fragment of 2019-nCoV spans from 319–591 S-residues25. From our studies it is deduced that curcumin directly binds to amino acids in this region Leu C:546, Gly C:548, Phe C:541, Asp C:571, Ala C:570, Thr C:572, Thr C:547, Thr C:573 whereascatechin binds to the S-protein in the near proximity of RBD fragment to Gln B:314, Glu B:309, Lys B:310, Gly B:311, Lys B:304, Tyr B:313, Thr B:302, Ile B:312, Leu B:303 and Ile B:312 residues (Table1).
The average change in displacement of atoms in all frames was recorded through Root Mean Square Deviation (RMSD).The average RMSD is obtainedto be 18 Å and 10 Å for S-protein- CurcuminandS-protein- catechin complex respectively. RMSD plot depicted thebinding RMSD plot depicted theinteraction of S-protein andcatechinwhich indicate their rigid interaction between 10–20ns simulation time out of 100ns trajectory(Fig. 6). Maximum structural fluctuation of S-protein was observed in between 300–500 amino acids and after 1000 amino acids residues (Suppl Fig. S5). The above data supports that S-protein and catechin interaction occurs with amino acids of S-protein near the RBD site (319 aa–591 aa)25. Amino acid residues Arg–634 and Val–635 near the RBD site of S-protein have stronger affinity towards hydroxyl group of catechin with 54% and 35%,respectively, out of 100ns simulation trajectory (Suppl. Fig. S7).
The binding affinity of curcumin with ACE2 was noted to be –7.8Kcal/mol where as that of catechin was found to be –8.9Kcal/mol. The binding of curcuminor catechinwith ACE2 includes conventional hydrogen Bond, carbon-hydrogen bond and Pi-Sigma interactions. The amino acid residues of the protein that take part in above interaction vary for both ligands (Suppl. Fig. S8 and S9, Table.2).
These results depicted that curcumin and catechin bind to S- protein at the site where it was known to get involved in host cell binding. Similarly, it was also seen that these molecules attach to those sites of ACE2 which were involved in serving a medium of viral entry. Thus, it is apparent from the present study that viral infection can be prevented by use of curcumin and catechin. This would rather serve dual inhibitory machinery by blocking host cell receptor to virus and viral protein entry. Moreover, these two polyphenols (Curcumin an catechin) are potent immuno stimulant and have been reported to induce autophagy, another important mechanism of viral clearance13,26. Therefore, availability of curcumin and catechin may facilitate all different mechanisms simultaneously and thereby promote elimination or neutralisation of viral infection.