SAR and ADMET analysis
We performed the in silico analysis of 14 molecules to understand their structural features required to interact with the selected target by using ADMET and binding affinity prediction tools. We calculated the physicochemical and ADMET properties of the selected dataset by using Molinspiration and Osiris property explorer. All molecules satisfied Lipinski’s rule of five and have shown positive enzyme inhibitor constant but the DL scores were very poor (Table 1 and 2). The poor DL scores represented that these molecules are highly toxic and not a good pharmacophore. We therefore moved to design novel pharmacophore analogs with improved pharmacological properties.
Table 1
Calculated SAR properties of the dataset obtained from the Molinspiration program.
Molecules
|
TPSAa
|
natoms
|
M.Wtb
|
nONc
|
nOHNHd
|
n
rotbe
|
Volume
|
Enzyme inhibitor
|
7a
|
109.08
|
26
|
364.33
|
8
|
2
|
5
|
303.06
|
0.22
|
7b
|
99.84
|
25
|
348.33
|
7
|
2
|
5
|
294.07
|
0.24
|
7c
|
99.84
|
26
|
362.36
|
7
|
2
|
5
|
310.88
|
0.24
|
7d
|
99.84
|
27
|
376.38
|
7
|
2
|
5
|
327.46
|
0.19
|
7e
|
126.15
|
31
|
434.42
|
9
|
2
|
8
|
372.23
|
0.09
|
7f
|
128.52
|
35
|
478.48
|
9
|
3
|
6
|
407.13
|
0.21
|
7g
|
111.87
|
26
|
363.35
|
8
|
3
|
5
|
306.48
|
0.24
|
7h
|
103.08
|
27
|
377.37
|
8
|
2
|
5
|
323.42
|
0.22
|
7i
|
123.31
|
29
|
407.40
|
9
|
3
|
7
|
348.48
|
0.19
|
7j
|
128.86
|
32
|
441.42
|
10
|
2
|
6
|
369.95
|
0.21
|
7k
|
103.08
|
33
|
453.47
|
8
|
2
|
7
|
395.07
|
0.17
|
7l
|
103.08
|
32
|
439.44
|
8
|
2
|
6
|
378.26
|
0.15
|
7n
|
99.84
|
27
|
376.38
|
7
|
2
|
5
|
327.68
|
0.23
|
7o
|
154.96
|
30
|
420.40
|
10
|
5
|
6
|
353.57
|
0.13
|
7p
|
137.14
|
29
|
406.37
|
9
|
3
|
6
|
337.90
|
0.19
|
aTopological polar surface area; bMolecular weight; cNumber of H-bond donors; dNumber of H-bond acceptors; e Number of rotatable bonds |
Table 2
Physicochemical parameters and drug-likeness properties of the dataset obtained from the Osiris program
Molecules
|
clogP
|
Solubility
|
Drug likeness
Score
|
Overall drug
Score
|
7a
|
-0.72
|
-2.47
|
-5.52
|
0.45
|
7b
|
0.10
|
-3.09
|
-4.91
|
0.44
|
7c
|
0.44
|
-3.36
|
-6.41
|
0.43
|
7d
|
0.71
|
-3.52
|
-3.32
|
0.43
|
7e
|
1.02
|
-3.98
|
-4.94
|
0.37
|
7f
|
1.51
|
-4.08
|
-2.27
|
0.37
|
7g
|
-2.93
|
-2.33
|
-4.06
|
0.46
|
7h
|
-1.54
|
-1.97
|
-0.53
|
0.61
|
7i
|
-2.06
|
-1.76
|
-1.14
|
0.54
|
7j
|
-0.31
|
-3.08
|
-1.41
|
0.48
|
7k
|
-0.12
|
-3.29
|
-1.45
|
0.46
|
7l
|
0.89
|
-3.81
|
-1.02
|
0.47
|
7n
|
0.79
|
-3.63
|
-8.91
|
0.41
|
7o
|
-1.72
|
-3.34
|
-7.91
|
0.24
|
7p
|
-0.56
|
-2.99
|
-4.12
|
0.43
|
Molecular Docking Analysis
We have performed molecular docking simulations on S. epidermidis TcaR to understand the enzyme-ligand interaction at the molecular level and to find a suitable orientation of each ligand within the active site. The fitness scores obtained from the GOLD program were high for active molecules when compared to those of least active and inactive molecules. In the docking results of the selected dataset, the fitness scores and binding energies did not correlate with the inhibitory activity of the molecules whereas the hydrophilic character (H-bond score) of molecules played an essential role and also exhibited a good correlation with their inhibitory activities. The most active molecules 7a, 7b, and 7g have shown the highest protein-ligand H-bonding scores 6.27, 6.73, and 6.40, fitness score of 56.33, 57.47, and 56.90, and binding energies of -8.7, -9.0, and -9.2 kcal/mol (Table 3) respectively. The inactive molecules had low protein-ligand H-bonding scores except molecule 7o. Accordingly, a significant correlation has been found between the protein-ligand H-bond score and the inhibitory activity for the selected dataset.
Table 3
Molecular docking results of selected dataset against S. epidermidis TcaR
Molecules
|
Binding Energy
(kcal/mol)
|
Fitness score
|
S(hb_ext)a
|
S(vdw_ext)b
|
S(vdw_int)C
|
7a
|
-8.7
|
56.33
|
6.27
|
45.78
|
-12.89
|
7b
|
-9.0
|
57.47
|
6.73
|
44.81
|
-13.19
|
7c
|
-8.9
|
61.30
|
2.01
|
49.34
|
-8.53
|
7d
|
-9.3
|
59.35
|
3.52
|
50.67
|
-13.84
|
7e
|
-8.3
|
60.04
|
6.09
|
52.25
|
-12.65
|
7f
|
-9.6
|
58.97
|
1.94
|
51.08
|
-13.22
|
7g
|
-9.2
|
56.90
|
6.40
|
47.19
|
-14.39
|
7h
|
-8.7
|
58.52
|
3.01
|
50.26
|
-22.09
|
7i
|
-8.8
|
61.34
|
2.70
|
54.17
|
-15.85
|
7j
|
-9.7
|
61.63
|
4.68
|
52.34
|
-15.01
|
7k
|
-8.5
|
62.09
|
2.26
|
55.70
|
-16.26
|
7l
|
-7.6
|
60.19
|
4.72
|
51.96
|
-15.98
|
7n
|
-4.3
|
58.06
|
4.76
|
47.51
|
-12.02
|
7o
|
-9.2
|
59.58
|
5.97
|
51.99
|
-17.88
|
7p
|
-9.2
|
57.13
|
6.04
|
51.89
|
-16.03
|
aProtein-ligand H-bond scores; bProtein-ligand van der Waals scores; CIntramolecular van der Waals strain within the ligand |
The binding interactions of the active molecules 7a, 7b, and 7g with the target protein are shown in Figures 1, 2, and 3, respectively. The two carboxyl groups of molecule 7a had five H-bond interactions with ARG110 (H-bond length (BL) 2.61 Å, 2.60 Å), GLU13 (BL 2.28 Å), and ASN20 (BL 2.44 Å, 2.55 Å). In molecule 7a, the π cloud of quinolone ring involved in two π- π stacked interactions (BL 3.74 Å, 3.97 Å) and two π-cation interactions (BL 4.10 Å, 3.74 Å) with HIS42 residue, and the Pyrone ring showed one π-Alkyl interaction with ALA38 (BL 4.41 Å), and one carbon-hydrogen bond interaction with GLN61 (BL 2.88 Å). Similarly, molecule 7b had five H-bond interactions with ARG110 (BL 2.63 Å), GLU13 (BL 2.84 Å), ASN17 (BL 2.97 Å), and ASN20 (BL 2.54 Å, 2.57 Å). Two carboxylic acid groups and one keto group were involved in these H-bond interactions. The π cloud of quinolone ring has shown two π-π stacked interactions (BL 3.86 Å, 4.00 Å) and two π -cation interactions (BL 3.82 Å, 4.28 Å) with HIS42 residue, and the substituent pyrrole ring was involved in π-Alkyl interactions with VAL63 (BL 5.45 Å), and ALA38 (BL 4.17 Å). The keto group, two carboxyl groups, and six-membered piperazine ring of molecule 7g have shown eight H-bond interactions with ARG110 (BL 2.61 Å, 2.33 Å), ASN45 (BL 2.62 Å), GLU13 (BL 2.39 Å), ASN17 (BL 2.61 Å), ASN20 (BL 2.49 Å, 2.72 Å), and GLU61 (BL 2.06 Å). The π cloud of the quinolone ring was involved in two π- π stacked interactions (BL 3.88 Å, 3.95 Å) and one π -cation interaction (BL 3.66 Å) with HIS42 residue. The present molecular docking analysis is helped us to understand how each substituent affects the binding affinity with the target.
Lead Optimization Studies
The initial pharmacological analysis of the selected dataset has shown poor DL properties. Therefore, we have carried out lead optimization studies by considering the most active inhibitors (7a, 7b, and 7g) as leads to develop novel molecules with improved pharmacological properties. We designed forty new pharmacophore analogs (Figure S1) by substituting various functional groups at different positions (1st, 6th, and 7th ) of the basic skeleton of quinolone. Also, we analyzed the importance of each substituent and how the substituent enhances their medicinal values in the basic skeleton by using the in silico tools.
SAR and ADMET analysis of designed molecules
The physicochemical properties and in silico drug-relevant properties of designed molecules are summarized in Tables S2 and S3. The designed molecules satisfied Lipinski’s rule of five, a rule of thumb to evaluate the drug-likeness of a molecule. The lipophilicity values of the designed molecules were less than five. The number of the H-bond donors and acceptors were not more than five and 10, respectively. The molecular weight was less than five hundred Dalton. The designed molecules have shown a positive enzyme inhibition constant, signifying that the molecules act as enzyme inhibitors.
The molecules of our design have shown significantly higher DL scores (except Mol37 to Mol40 over the selected dataset. The positive DL scores (4.83 to 7.20) of these molecules confirm that these pharmacophore analogs qualify as potential commercial drugs. Molecules Mol36, Mol35, Mol32, Mol24, and Mol33 have shown the highest DL scores of 7.20, 6.86, 6.37, 6.34, and 6.33, respectively. Interestingly, the in silico ADMET predictions indicated that the loss of one –COOH group (at 1st position) and the substitution of fluorine with chlorine can increase the DL properties and reduce the toxicity risks comparatively.
Molecular Docking Analysis of designed molecules
As per the docking results, all the designed molecules have shown good fitness scores (<50) (Table S4) against the target. In the selected dataset, the H-bond score has shown a very good correlation with the inhibitory activity. Therefore, molecules Mol1, Mol2, Mol4, Mol9, Mol17, Mol24, Mol34, and Mol35 with the highest H-bond scores were taken to be the most active inhibitors of the target.
We have drawn in Figure 4 the binding conformation of the best candidate molecule Mol34 in the active site of S. epidermis TcaR. The molecule (binding energy of -10.6 kcal/mol, H- bond score of 7.98, and fitness score of 62.95) took a conformation that fits well in the entire groove of the binding site of S. epidermis TcaR. The carboxyl and keto groups of Mol34 played a significant role in the binding by forming three H-bond interactions with the active site residues. The carboxyl group had one H-bond interaction with the ARG110 (BL 2.48 Å) and the keto group had two H-bond interactions with ASN20 (BL 2.71 Å, 2.06 Å). We observed two π-π stacked interactions with HIS42 (BL 4.79 Å, 4.22 Å) and several hydrophobic π-alkyl interactions with VAL63 (BL 4.89 Å), ALA67 (BL 4.53 Å), ALA38 (BL 5.12 Å, 3.75 Å), ALA24 (BL 4.34 Å, 3.36 Å), and LEU27 (BL 5.15 Å). In addition, Mol34 formed a π-cation interaction with ARG71(BL 3.97 Å) and C-H interaction with ASN20 (BL 3.47 Å).