Docking and scoring of the designed compounds
The molecular docking study was carried out to define the binding free energy between the designed ligands and the NDM-1 receptor and to investigate their binding interactions [71]. During the docking process, molecules with the 10 highest binding free energies were selected. The screening results of twenty designed molecules and the positive control used to predict comparative ΔGbinding are shown in Table 1. The T016 compound had the highest calculated binding free energy (-10.64 kcal/mol), T008 had the second highest energy, and T014 had the lowest energy (-7.56 kcal/mol). The designed molecules possessed a higher ΔGbinding than meropenem, except for T011 (-7.90 kcal/mol) and T014 (-7.56 kcal/mol).
Table 1 The chemical formula, binding free energy, ClogP, Molar mass, and the rule of five compliance of the designed ligands and meropenem
Compound |
Chemical formula |
ΔGbinding (kcal/mol) |
CLogP |
Rule of 5 criteria |
Molar mass (g/mol) |
T001 |
C14H11NOS |
-9.39 |
3.88 |
passed |
241.31 |
T002 |
C14H11NS |
-9.04 |
3.89 |
passed |
225.31 |
T003 |
C14H9Cl2NS |
-9.80 |
5.19 |
passed |
294.20 |
T004 |
C14H9Cl2NS |
-9.80 |
5.19 |
passed |
294.20 |
T005 |
C14H11NOS |
-8.96 |
3.38 |
passed |
241.31 |
T006 |
C15H13NO2S |
-9.49 |
3.39 |
passed |
271.33 |
T007 |
C15H13NOS |
-9.28 |
3.90 |
passed |
255.33 |
T008 |
C13H9NO4S2 |
-9.83 |
1.92 |
passed |
307.34 |
T009 |
C14H13NO2S2 |
-9.40 |
2.95 |
passed |
291.39 |
T010 |
C13H11NO2S2 |
-9.60 |
2.56 |
passed |
277.36 |
T011 |
C14H10ClNS |
-7.90 |
4.54 |
passed |
259.75 |
T012 |
C14H10N2O2S |
-8.83 |
3.80 |
passed |
270.31 |
T013 |
C15H12ClNS |
-9.50 |
4.73 |
passed |
273.78 |
T014 |
C15H11NS |
-7.56 |
4.47 |
passed |
237.32 |
T015 |
C17H13NO4S |
-8.88 |
2.85 |
passed |
327.35 |
T016 |
C17H13NO4S |
-10.64 |
2.85 |
passed |
327.35 |
T017 |
C13H13NOS |
-8.29 |
2.52 |
passed |
231.31 |
T018 |
C14H15NO2S |
-8.00 |
2.53 |
passed |
261.34 |
T019 |
C12H11NOS |
-8.71 |
2.13 |
passed |
217.29 |
T020 |
C11H9NO3S |
-8.30 |
1.94 |
passed |
235.26 |
Meropenem |
C17H25N3O5S |
-7.94 |
-0.31 |
passed |
383.46 |
Calculated water-octanol partition coefficient
An initial prediction of the lead molecule’s physicochemical properties is necessary to identify a viable lead during drug discovery projects [72]. LogP is one of the significant evaluators of a lead compound’s physical properties and pharmacokinetic properties [73]. In the current study, Manifold platform (https://postera.ai/mannifold) was used to estimate the calculated LogP (ClogP) and the rule of five criteria for the designed molecules and the positive control molecule. The designed molecules had a similar ClogP to most marketed antibacterial drugs (1.9–5.19) [74]. The highest lipophilic compounds were T003 and T004, followed by T013. However, the most hydrophilic molecules were T008 and T020, with ClogP values of 1.92 and 1.94, respectively, and none of the designed molecules were more hydrophilic than meropenem. Maintaining a balance between hydrophilicity and hydrophobicity was necessary for the designed molecules because the NDM-1 pocket is surrounded by loops that contain hydrophobic residues, such as Leu65, Met67, Phe70, and Val73, that are intended to interact with the ligand’s hydrophobic functional groups. In addition, L10 of NDM-1 includes Cys208, Lys211, and Asn220. While Cys208 coordinates with Zn2, the other residues interact with the substrate polar groups [31]. Thus, the designed ligands require both high lipophilicity and low to moderate hydrophilicity to effectively inhibit the NDM-1 enzyme.
Drug-likeness and Lipinski’s rule of five
Due to their favorable pharmacokinetic properties, small molecules serve as the gold standard for the development of new medications [75]. Drug-likeness properties are applied to small molecules to identify orally bioavailable drugs [76]. These properties can be measured using Lipinski’s rule of five criteria or their variants [77]. In the current study, all the designed ligands were small molecules (MW < 500 Da) and followed the rule of five. The highest MW belonged to compounds T015, T016 (327.35 g/mol), and T008 (307.34 g/mol), when they did not exceed the meropenem MW (383.46 g/mol) (Tables 1 and 2). However, this MW cutoff may not be a strict condition for oral bioavailability given that several antibiotics with MWs up to 900 Da have oral bioavailability while others with small MWs do not. Indeed, when meropenem has a small MW and passes Lipinski’s rule of five, it cannot be administered orally due to its significant hydrophilicity and poor gastrointestinal tract absorption [78].
Table 2 Lipinski’s rule of five criteria of the designed molecules
Compound |
Rotatable bonds |
HB Donor |
HB Acceptor |
TPSA |
T001 |
3 |
0 |
3 |
22.12 |
T002 |
2 |
0 |
2 |
12.89 |
T003 |
2 |
0 |
2 |
12.89 |
T004 |
2 |
0 |
2 |
12.89 |
T005 |
2 |
1 |
3 |
33.12 |
T006 |
3 |
1 |
4 |
42.35 |
T007 |
3 |
0 |
3 |
22.12 |
T008 |
2 |
0 |
6 |
73.33 |
T009 |
3 |
0 |
4 |
47.03 |
T010 |
2 |
0 |
4 |
47.03 |
T011 |
2 |
0 |
2 |
12.89 |
T012 |
3 |
0 |
4 |
56.03 |
T013 |
3 |
0 |
2 |
12.89 |
T014 |
2 |
0 |
2 |
12.89 |
T015 |
3 |
2 |
3 |
97.46 |
T016 |
3 |
2 |
3 |
97.46 |
T017 |
1 |
1 |
3 |
33.12 |
T018 |
2 |
1 |
4 |
42.35 |
T019 |
1 |
1 |
3 |
33.12 |
T020 |
2 |
1 |
5 |
51.58 |
Meropenem |
5 |
3 |
6 |
110.18 |
An increase in the MW of orally administered drugs has occurred in recent years [79,80] because some researchers argue that performing drug discovery campaigns beyond the rule of five could allow new medications to be identified [81,82]. Thus, the current study investigated the correlation between the binding free energy and the MW of the designed compounds. A significant P-value (0.0036) and moderate correlation (R2=0.38) were observed between the compound’s weight and binding affinity (Fig. 4), suggesting that increasing the size of the ligands may improve the ligand-receptor binding affinity [83].
Analysis of binding interactions between the selected compounds and NDM-1
Several studies have found that ligands with sulfone or sulfamoyl functional groups, including naphthalene and thiazole nuclei, exhibit significant activity and an ability to inhibit MBLs (especially NDM-1) by forming H-bonds and combining with zinc ions in the active site [52,84-86]. Thus, these functional groups were added to the designed compounds. Sulfamoyl is among the most effective chemical groups to coordinate with zinc ions and produce H-bonds with polar residues (Asn220, Asp124, and Gln123) within the active site. Moreover, because the NDM-1 active site has hydrophobic residues adjacent to the zinc center, all the designed molecules had hydrophobic components and were able to form nonpolar interactions with the ligands [87]. Docking was assessed for the crystal structure of a reference complex and two ligand-receptor complexes with a promising outcome (T008 and T016). The main functional groups of T008 were thiazole and naphthalene moieties. T016 also had a naphthalene functional group but possessed a sulfone functional group instead of thiazole (Fig. 3).
Meropenem and NDM-1 binding interactions
In this study, meropenem was considered a positive control to evaluate the activity of the other compounds. The binding interactions between the crystal structure of hydrolyzed meropenem and the NDM-1 complex were assessed, [88] as well as the docking results of meropenem with the protein. Both the crystal structure and docked results confirmed that the ligand was entirely buried in the NDM-1 active site (Fig. 5). In addition, the two positively charged zinc ions in the protein pocket accounted for the hydrolysis of the β-lactam rings and were actively involved in the interactions between the ligand and NDM-1 through a metal coordination bond [89,90]. These results illustrated that various types of binding interactions were available between the ligand and the NDM-1 binding substrate. The crystal structure results showed that three residues were engaged in hydrophobic interactions with meropenem (Val73, Trp93, and Gly219) (Fig. 5A). Meanwhile, the docked results showed that three NDM-1 residues were also involved in hydrophobic interactions with the ligand. Instead of Val73 and Gly219 residues, Gln151 and Asn220 interacted (Fig. 5B). The residues of the NDM-1 active site that interacted with meropenem were similar in both the crystal and docked results, except that the H-bond interactions of the Lys211 side chain in the crystal structure were replaced by main-chain H-bond interactions between Gln123 and meropenem in the docked results. However, binding interactions between several residues and meropenem differed due to changing conformations of the ligand (Figures 5A and 5B).
T016 and NDM-1 binding interactions
T016 could be considered a lead compound because it had the highest binding free energy (-10.64 kcal/mol) and optimum ClogP (2.85) and passed Lipinski’s rule of five (Table 1). According to the docking results, this ligand had several interactions with the receptor’s active site. Its sulfamoyl group interacted with both Asp124 and Asn220 residues on NDM-1 using H-bonds. Moreover, the carbonyl oxygen and hydroxyl atoms of the ligand’s benzoic acid residue formed an H-bond with Asn220 and metal coordination with the zinc ions, respectively. Pi-pi stacking was also observed between the imidazole side chain of His250 and one of the benzene rings of the naphthalene functional group. A second pi-pi interaction (T shaped) belonged to the His120 residue and benzene ring of the ligand’s benzoic acid. Finally, the compound’s hydrophobic naphthalene moiety resulted in a significant increase in binding free energy since it was buried in the sequence of the receptor’s hydrophobic residues (Met67, Phe70, Ala72, Val73, Ala74, and Leu218). Given the availability of hydrophobic residues adjacent to the zinc ions in the NDM-1 active site, the designed ligands may be improved by introducing hydrophobic substituents (Fig. 6) [91].
T008 and NDM-1 binding interactions
T008 had the second highest binding free energy among the designed ligands (9.83 kcal/mol) when the ΔGbinding was significantly higher than the positive control. This compound also had an optimum ClogP (1.94), a small MW (307.34 g/mol), and passed the rule of five (Table 1). Thus, this molecule could also be considered a lead compound. One of the essential binding interactions was the zinc coordination bond between the nitrogen atom of the ligand’s thiazole group and the zinc ion of the receptor pocket (Fig. 7). There was also an H-bond between the carbonyl oxygen of the ligand’s ester functional group and the side chain carboxylic acid of Asp124. In addition, the amide side chain of Gln123 interacted with the sulfonyl functional group of T008. The main hydrophobic contact was a pi-pi stacking between the ligand and the imidazole side chain of the His122. The ligand’s naphthalene functional group formed hydrophobic interactions with NDM-1 residues such as Trp93, Ala121, Met154, and Cys208.
MD simulation studies of T008 and T016 with NDM-1
MD simulations were used to predict the stability and dynamics of T008 and T016 with NDM-1 under physiological conditions [90]. These ligands were selected because they had the highest ΔGbinding rank of the designed ligands. The root-mean-square deviation (RMSD) of the protein-ligand complexes was used to compute the scalar distance between the Ca backbone of the protein and the ligand’s throughput trajectory to determine its conformational stability. According to the RMSD graph, the protein-ligand complexes fluctuated between ~0.5 Å for the T008-NDM-1 complex and 0.2 Å for the T016-NDM-1 complex after passing 2 ns and stabilizing the complex structures. These fluctuations are a normal behavior of globular proteins and were significantly below the upper limit (2.0 Å) [90]. The results suggested that both compounds, especially T016, formed strong interactions with the NDM-1 active site residues and stable complex systems (Fig. 8B).
The root-mean-square fluctuations (RMSF), which were calculated for both complex systems based on the Ca atoms of NDM-1, showed that the fluctuation intensity remained below 0.2 Å, except for Phe70, Asn176, and Lys216, which were represented as either loop or turn in the protein structure (Fig. 8C). Thus, the RMSF plots were able to identify the stability of the protein structure and the availability of flexible regions needed to acquire the superlative conformations. Since the number of H-bonds between a protein and ligand can influence the stability of a ligand-receptor complex, it was critical to probe these bonding interactions during the MD simulations [92]. The H-bonds created between the ligands and NDM-1 during 100 ns MD simulations are shown in Fig. 8A. The bonds maintained a constant interaction during the MD simulation time with an average of one H-bond for the T008-NDM-1 complex and two H-bonds for the T016-NDM-1.