Validation of the SARS-CoV-2 and human ACE-2 models
All models were built and validated using the SWISS-MODEL Homology modeling tool and the SWISS-MODEL quality assessment tool, respectively. For this study, the Chain A monomer of the SARS-CoV-2 S-protein in the open state conformation was used for all analysis. The hACE-2 and the selected neutralizing antibodies all interact with the S-protein in the open state. The S-protein and hACE-2 models were generated using the crystallographic coordinates of PDB entries 7DK3 and 6LZG, respectively. The global model quality estimation (GMQE) and qualitative model energy analysis (QMEAN) scores were used to decide the model selection. Ramachandran plot of each protein model was generated to confirm their quality (Figure 1a). Comparison of the built models to a set of non-redundant protein structures of high quality in the PDB database also showed that the models were suitable for this study (Figure 1b). We then performed a pair-wise alignment of the model structure and their respective templates. The alignments of the model-template structures are shown in Figure 1c. Having assessed the suitability of the model for this study, we used the PyMol v2.4 mutagenesis tool to create the 476S variant by replacing Gly-476 with Ser-476 in the RBD (Figure 1d).
Effect of the mutation on RBD structure dynamics
To determine the impact of the mutation on S-protein structure dynamics, we first characterized the degree of evolutionary conservation of Gly-476 using the ConSurf tool29. Usually, the natural tendency of an amino acid mutating largely depends on its importance to the protein’s structure and functions. The ConSurf conservation profile for the S-protein show that most of the conserved residues are in the S2-subunit, with the most variable residues in the RBD and the NTD (Figure 2). The profile shows a ConSurf score of 1 for Gly-476, suggesting that it is highly variable and easily mutable.
To assess whether G476S mutation induces any conformation change in the S-protein, we used root-mean-square deviation (RMSD), RMSD distribution and root-mean-square fluctuation (RMSF) to compare the structural dynamics of the wild-type and 476S variant S-proteins (Figure 3a-3e). The RMSD describes the conformation stability of the protein system during the simulation by indicating the average displacement of backbone atoms in the protein structure with respect to the starting structure (equilibrated structure, t = 0 ns). The backbone-RMSD value of the wild-type S-protein averaged 1.08 nm, which increased to 1.18 nm in the case of the 476S variant S-protein, indicating a destabilization impact of the mutation (Figure 3a). To determine the most affected regions of the S-proteins, domain-specific backbone-RMSD profiles were generated (Figure 3b). The destabilization effect of the mutation was largely observed in the RDB than the NTD and the S2-subunit. The backbone-RMSD of the wild-type RBD averaged 0.39 nm, which increased to 0.45 nm for the 476S variant RBD.
The structures from the simulation trajectories were clustered using RMSD cut-off of 0.1 nm and the gromos method, resulting in several cluster groups. The group-centre structure of the most populated cluster for both wild-type and 476S variant S-proteins were selected and superimposed. The structure superposition shows that the wild-type and 476S variant S-proteins differ by RMSD of 5.3 Å (Figure 3d). The difference in the RBD is also emphasized. The increased backbone-RMSD of the 476S variant can be attributed to destabilization effect of the mutation which is most pronounced in the RBD.
The residue-specific flexibility of protein structure is indicated by the RMSF. The RMSF was calculated to evaluate the residue flexibility to compare the residue-by-residue variations between the wild-type and 476S S-proteins. Although both the systems assumed a similar residue flexibility profile, the residue fluctuations were higher in the 476S variant RBD, compared to the wild-type RBD (Figure 3e). The most affected regions ranged from residues 347-350, 370-393, 413-428, 456-459 and 471-487. This further indicates the destabilization effect of the mutation in the RBD, resulting in increased flexibility for most of the local residues.
Impact of the mutation on stabilization of the RBD-hACE-2 complex
To investigate the potential impact of the mutation on the interaction between the S-protein and hACE-2 receptor, we first examined the dynamics of the RBD upon binding to the hACE-2 receptor. Comparing RMSD of the RBD in bound and unbound states, we observed that the RBD was destabilized upon binding to the hACE-2 receptor (Figure 4a), for both the wild-type and 476S variant RBD-hACE-2 complexes. Further, the residue-specific flexibility was significantly affected upon binding to hACE-2, most especially in the RBD. The residues in the RBD demonstrated less flexibility upon binding to the hACE-2 receptor (Figure 4b). The most affected regions were residues 330-395 and the least affected region ranged from residues 473-503, including Gly-476 and Ser-476. This suggests a major participation in the interactions with hACE-2 for the residues 330-395.
We examined the interacting interface of the RBD-hACE-2 complex to determine whether Gly-476 and Ser-476 were located within the hACE-2 interacting residue hotspot during the simulation. The middle-structure of the most populated cluster group was selected for this activity. All amino acids that are within 5 Å of the hACE-2 receptor were considered, assuming that most forces of interaction are captured within this distance. Both Gly-476 and Ser-476 were located in the interacting residue hotspot of the RBD-hACE-2 complex during the simulation (Figure 5).
To characterize how the mutation affect RBD-hACE-2 interactions, the binding energy and the individual energy terms governing the RBD-hACE-2 stabilization were calculated. The energy terms for each complex were calculated using 225 structure frames sampled over three independent 15 ns simulations. The estimated binding energies were highly comparable for the wild-type (-2068.268 ± 138.127 kJ/mol) and 476S variant (-1953.245 ± 128.816 kJ/mol), suggesting that the interactions with hACE-2 is not largely affected by the G476S mutation in the RBD. The computed individual energy terms governing the complex stabilization also show high similarity for the wild-type and 476S variant complexes (Table 2).
Table 2: Energy terms governing the interactions between the RBD and human ACE-2 receptor.
|
RBD-hACE-2 interaction energy (kJ/mol)
|
Energy term
|
wild-type
|
476S variant
|
van der Waal
|
-673.925 ± 128.840
|
-555.784 ± 85.479
|
Electrostatics
|
-3186.796 ± 224.890
|
-2860.553 ± 164.651
|
Polar solvation
|
1871.273 ± 273.396
|
1527.539 ± 179.273
|
Non-polar solvation
|
-78.820 ± 11.486
|
-64.447 ± 8.023
|
Total energy
|
-2068.268 ± 138.127
|
-1953.245 ± 128.816
|
To determine the contribution of the Gly-476 and Ser-476 to the overall binding energy, we computed the residue contribution to the stabilization of the interaction between the S-protein and hACE-2. The energy contribution profile of the RBD residues are shown in Figure 6a. The contributions of Gly-476 and Ser-476 to the overall stabilization of the RBD-hACE-2 complexes were -5.7 kJ/mol and -2.7 kJ/mol, respectively. These contributions are insignificant to the overall stabilization of the RBD-hACE-2 complexes. Based on their energy contributions, the residues ranging from 475-503, play no significant roles in the RBD-hACE-2 interactions. The largest contributors to the complex stabilization were from residues 346-424. Although Gly-476 is replaced with a polar residue in the 476S variant, individual energy terms such as electrostatic and van der Waals energy contributions were larger in the wild-hACE-2 complex than the 476S-hACE-2 complex. We, therefore, generated a distance plot for the Gly-476/Ser-476 and hACE-2. The minimum distance between the hACE-2 receptor and Gly-476 averaged 0.23 nm, which increased to 0.43 nm for Ser-476 (Figure 6b).
Impact of the mutation on interactions with H014 and P2B-2F6
Previous studies have established that the 476S variant resists neutralization by particular antibodies such as S2E1222 and CC6.2923. Thus, we studied the impact of the mutation on interactions with two neutralization antibodies that target the RBD but have different mechanisms of neutralization. The antibodies H014 and P2B-2F6 were selected for this particular study. Although both antibodies target the RBD, H014 neutralizes by inducing sterical clashes to the RBD and interfere with hACE-2 binding34. P2B-2F6, however, directly competes with the RBD for hACE-2 binding35 (Figure 7a).
Analysis of the individual residue contributions to the complex interactions revealed that both Gly-476 and Ser-476 have insignificant contributions to the stabilization of both RBD-H014 and RBD-H014 complexes (Figure 7b). Moreover, residue contributions for both RBD-H014 and RBD-P2B-2F6 complexes were similar for the wild-type and 476S variant. The energy terms governing the RBD and neutralizing antibody interactions are summarized in Table 3. The estimated binding energy and individual energy terms did not vary significantly between the wild-type and 476S variant interactions with the neutralizing antibodies. These suggest that the mutation has no effect on the interactions between the RBD and the neutralizing antibodies studied.
Table 3: Energy terms governing the interactions between the RBD and neutralizing antibodies (H014 and P2B-2F6).
|
RBD-H014 interaction energy (kJ/mol)
|
RBD-P2B-2F6 interaction energy (kJ/mol)
|
Energy
|
wild-type
|
476S variant
|
wild-type
|
476S variant
|
van der Waal
|
-634.710 ± 69.288
|
-627.918 ± 55.067
|
-409.078 ± 62.653
|
-465.034 ± 55.456
|
Electrostatics
|
-1019.975 ± 156.196
|
-1108.485 ± 136.660
|
109.956 ± 30.520
|
91.482 ± 132.519
|
Polar solvation
|
1297.268 ± 195.574
|
1370.605 ± 142.078
|
660.597 ± 86.951
|
861.206 ± 149.536
|
Non-polar solvation
|
-67.686 ± 4.429
|
-69.454 ± 4.976
|
-46.876 ± 5.749
|
-53.460 ± 4.034
|
Total energy
|
-425.103 ± 106.969
|
-435.252 ± 101.365
|
394.599 ± 83.723
|
434.194 ± 77.513
|