D. Protein-Peptide Docking:
For cross validation, protein-peptide docking has been performed using three different servers named HPEPDOCK, FIREDOCK and HADDOCK 2.4, followed by Gibbs free energy estimation using Prodigy server (refer Table III, IV).
HPEPDOCK is a web server for protein-peptide blind docking based on a hierarchical algorithm. According to the HPEPDOCK score, Peptide 01 (-241.390) and Peptide 02 (-241.619) have higher docking score compared to other peptides, while ACE2 (-351.74) has the highest docking score with the RBD.
Patchdock is a molecular docking algorithm based on shape complementarity principles and Firedock performs large-scale flexible refinement and scoring of protein-protein docking. After using Patchdock, followed by Firedock, it has been noted that Peptide 05 shows better docking score than that of ACE2 against spike protein RBD, not only that, Peptide 03 and Peptide 04 both show reasonably good docking score which is comparable with ACE2.
Finally, using HADDOCK 2.4 web-server, protein-protein docking has been performed for all the six peptides and ACE2. The model having lowest HADDOCK score has been selected for further studies for each of the peptides.
Three peptides, Peptide 01 (-103.6 +/- 1.3), Peptide 02 (-102.8 +/- 7.2) and Peptide 03 (-109.2 +/- 5.5) are showing relatively better docking score towards SARS-CoV-2 spike protein receptor binding domain (RBD).
PEPTIDE | HPEPDOCK | FIREDOCK | HADDOCK 2.4 |
Peptide 01 | -241.390 | -29.65 | -103.6 +/- 1.3 |
Peptide 02 | -241.619 | -11.50 | -102.8 +/- 7.2 |
Peptide 03 | -224.651 | -34.34 | -109.2 +/- 5.5 |
Peptide 04 | -157.275 | -32.64 | -60.4 +/- 2.3 |
Peptide 05 | -228.438 | -40.30 | -83.1 +/- 1.9 |
Peptide 06 | -206.948 | -10.15 | -98.1 +/- 2.5 |
ACE2 | -351.74 | -36.74 | -134.1 +/- 3.6 |
Among those six peptides, Peptide 03 shows the best docking score with spike protein RBD, i.e., -109.2 +/- 5.5, though ACE2 has a better docking score with RBD compared to Peptide 03, i.e., -134.1 +/- 3.6. From the ΔG (kcal mol− 1) calculation using Prodigy server, it is evident that Peptide 02 (-11.4 kcal/mol) and Peptide 03 (-11.2 kcal/mol) have comparable ΔG values with ACE2 (-11.9 kcal/mol) (refer Table IV). From the protein-peptide docking studies with RBD, it is clear that among all the six peptides, Peptide 03 has the most promising anti-viral activity against SARS-CoV-2 spike protein Receptor Binding Domain (RBD), followed by Peptide 02 and Peptide 01. |
Table III
Protein-Peptide Docking Scores from different servers
To check whether in presence of Peptide 03, the interaction between ACE2 and RBD will be affected or not, protein-protein docking has been performed with and without Peptide 03 bound with RBD. In absence of Peptide 03, the protein-protein docking score predicted by HDOCK was − 310.19. While protein-protein docking between Peptide 03-bound RBD and human ACE2, the interacting residues of ACE2 and RBD (from the 2D Ligplot + figure of the crystal structure 6M0J) are mentioned as binding site residues during docking, it gives a docking score of -247.94; hence, in presence of Peptide 03, the interaction between ACE2 and RBD has been decreased significantly. Apart from that, in the 2D protein-protein interaction map, Peptide 03-bound RBD-human ACE2 complex and Peptide-free spike protein RBD-human ACE2 complex, there are eleven (11) hydrogen bonds present in both the cases.
Peptides in Protein-peptide complexes | ΔG (kcal mol− 1) | Kd (M) at 25.0 ℃ |
1. Peptide 01 | -10.6 | 1.6E-08 |
2. Peptide 02 | -11.4 | 4.6E-09 |
3. Peptide 03 | -11.2 | 5.8E-09 |
4. Peptide 04 | -7.8 | 1.8E-06 |
5. Peptide 05 | -8.4 | 6.5E-07 |
6. Peptide 06 | -8.9 | 2.8E-07 |
7. ACE2 | -11.9 | 1.9E-09 |
Table IV: Binding Energy and Kd calculation using Prodigy server |
In the case of peptide 03, there are eleven hydrogen bonds between Arg5-Glu749 (2), Ser6-Glu749, Ser6-Cys753, Cys10-Tyr770, Val12-Asn766, Ser13-Asn766, Lys19-Gly711, Lys19-Tyr714, Arg31-Asn752 (2) (Fig.4). While in case of peptide 02, there are only nine hydrogen bonds present, Arg5-Asn752, Arg5-Tyr754, Val12-Tyr770, Gln14-Thr765, Lys19-Tyr718, Lys19-Ser759, Trp23-Glu749, Arg40-Tyr770, Ser42-Arg668 (Fig.3) and in the case of peptide 01, there are total eight hydrogen bonds present, Lys2-Glu671, Lys2-Tyr760, Trp6-Ser759, Met10-Tyr714, Gly14-Gln763, Cys20-Glu749, Cys20-Gln758, Arg23-Glu749 (Fig.2). On the other hand, considering SARS-CoV-2 spike protein RBD-human ACE2 interactions, there are total eleven hydrogen bonds are found, Ser1-Ala740, Gly6-Asn752, Asp12-Lys682, Lys13-Gln758, Glu17-Gln758, Asp20-Tyr714, Gln24-Gly711, Gln24-Tyr714, Tyr65-Asn752, Lys335-Gly761, Lys335-Gln763. Among them, three residues Gly711, Tyr714 and Asn752 of RBD also interact with the peptide 03, that’s the probable reason for decreasing the binding affinity of ACE2 towards RBD in presence of peptide 03.
C. Computation of Physicochemical Properties:
All the physicochemical properties of the six peptides have been summed up in Table V. Among the six peptides, Peptide 02 and Peptide 03 have greater half-life (estimated) than the others. A protein having instability index < 40 can be treated as stable, whereas if instability index > = 40, the corresponding protein will be unstable, from that point of view, none of the peptides are stable. As the peptides are unstable, the stability should be enhanced using various biochemical techniques such as cyclization of the peptide, replacement of natural peptide bonds by pseudo-peptide bonds, selective chemical modification etc. before experimental validation against spike protein [42]. Otherwise, peptidomimetic drugs can be produced based on the functionality of the peptides, which are basically small organic molecules mimicking the characteristics of the peptides, but a way more stable than the peptides [43]. In brief, some processing techniques, such as chemical modification or incorporation of synthetic amino acids, can be applied to increase peptide stability and, consequently, lower susceptibility to hydrolysis by proteases.
The aliphatic index of a protein is the relative volume occupied by aliphatic side chains (alanine, valine, isoleucine, and leucine). The GRAVY value for a peptide or protein is calculated as the sum of hydropathy values of all the amino acids, divided by the number The heat stability of peptide was indicated by its aliphatic index. The higher aliphatic index means higher heat stability. The hydrophilicity and hydrophobicity of peptide were predicted by GRAVY. The peptide was hydrophobic when the GRAVY value was plus; otherwise, it was hydrophilic. Interestingly, in the case of Peptide 05, aliphatic index is zero (0), signifies the absence of aliphatic side chains (Ala, Val, Ile, Leu).
Peptide | MW | Theo. pI | Estimated half-life | Instability index | Aliphatic index | GRAVY |
1. Peptide 01 | 3125.82 | 11.84 | 1.1 hours (mammalian reticulocytes, in vitro). 3 min (yeast, in vivo). 2 min (Escherichia coli, in vivo). | 77.92 | 50.80 | -0.576 |
2. Peptide 02 | 5544.50 | 11.24 | 30 hours (mammalian reticulocytes, in vitro). >20 hours (yeast, in vivo). >10 hours (Escherichia coli, in vivo). | 97.10 | 53.83 | -0.851 |
3. Peptide 03 | 4003.65 | 11.88 | 30 hours (mammalian reticulocytes, in vitro). >20 hours (yeast, in vivo). >10 hours (Escherichia coli, in vivo). | 88.33 | 32.42 | -1.288 |
4. Peptide 04 | 802.93 | 4.37 | 1 hour (mammalian reticulocytes, in vitro). 30 min (yeast, in vivo). >10 hours (Escherichia coli, in vivo). | 40.43 | 130.00 | -0.583 |
5. Peptide 05 | 1551.84 | 11.71 | 1.1 hours (mammalian reticulocytes, in vitro). 3 min (yeast, in vivo). 2 min (Escherichia coli, in vivo). | 165.81 | 0.00 | -1.830 |
6. Peptide 06 | 2076.39 | 10.00 | 1.3 hours (mammalian reticulocytes, in vitro). 3 min (yeast, in vivo). 3 min (Escherichia coli, in vivo). | 40.67 | 51.76 | -1.618 |
Table V: Physicochemical properties of the peptides |
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According to the ToxinPred web server, none of the peptides are toxic. For toxicity prediction, SVM (Swiss-Prot) + Motif based method was used and other options were set as default.
HemoPI server was used to identify the hemolytic potency of the peptides and hybrid method (SVM + Motif based) was used for prediction. Hemolytic potency of a peptide is decided by |
PROB score, which is the normalized SVM score and ranges between 0 and 1, where 1 means the peptide is very likely to be hemolytic and 0 means it is likely to be non-hemolytic. For the aforementioned peptides, the PROB score ranges from 0.46 (peptide 06) to 1.00 (Peptide 01 and 05), which signifies that there is a high probability for them of being hemolytic. |
Peptide | SVM | RF | IBk |
Peptide 01 | 22.21 | 39.55 | 9.71 |
Peptide 02 | 24.53 | 17.85 | 9.71 |
Peptide 03 | 26.99 | 3.17 | 9.71 |
Peptide 04 | 36.67 | 60.11 | 11.41 |
Peptide 05 | 23.8 | 9.06 | 0.9 |
Peptide 06 | 30.5 | 36.52 | 17.01 |
Table VI: IC 50 values predicted by different ML algorithms in micromolar unit |
According to the FDA, IC50 represents the concentration of a drug that is required for 50% inhibition in vitro experiments. Using AVP-IC50Pred server and choosing hybrid model I as prediction model, IC50 values of the peptides have been predicted by three different machine learning algorithms named, Support Vector Machine, Random Forest, IBk (Weka); details of the values are given in Table VI (values are in micromolar unit):