Description of the generated Pharmacophore
The Pharmacophore model was generated using a set of ten active (pIC50 > 7.730) ligands in the dataset with the help of PHASE module available in Schrodinger suite. These active set of ligands may contain structural features which are essential for binding to the active site of pim-3 [50]. The structural and biological activity of pim-3 inhibitors taken for our analysis is depicted in Table 1. The variant list was determined by assigning maximum and minimum number of site to 5 after creating pharmacophore sites for all pim-3 inhibitors. The pharmacophore hypotheses (CPH's) identified in this process were scored and ranked according to the survival active and inactive scores by a scoring method in PHASE module. The inactive ligands though not used in CPH production they are used to eliminate the hypothesis that cannot extricate between actives and inactives [50]. Hence to achieve this purpose the obtained CPH's will be mapped onto the inactive ligands and scored. The hypothesis with good difference in survival active and survival inactive scores ADRRR was taken for further analysis as it clearly discriminates between active and inactive ligands. Among the generated models ADRRR was identified as the best hypothesis by scoring algorithm. The CPH with one H-bond acceptor (A2), one H-bond donor (D3) and three aromatic ring features (R9, R10 and R11) is shown in Figure 1. The distances and angles between various sites of the model are depicted in Figures 2 and 3, and tabulated in Supplementary Tables Tab S1 and Tab S2, respectively. Further, a 3D QSAR model was derived by pharmacophore based alignment of the ligands.
Table 1 Structures and biological activities of pim-3 inhibitors along with fitness scores based on pharmacophore hypothesis ADRRR
Analysis Of 3d Qsar Model
The PHASE module of Schrodinger Suite was used to derive a pharmacophore based 3D QSAR model using the best CPH i.e. ADRRR. The derived 3D QSAR model will help in identifying all the features that are crucial for an inhibitor for its gross inhibitory activity against target pim-3 [50]. The dataset was randomly divided into 38 training and 19 test set (2:1) prior to the model generation. Subsequently with a grid spacing of 1Å the QSAR model was developed using PLS analysis. The t-set or eliminate variable was fixed to <2.0 during PLS regression analysis in order to eliminate those variables whose regression coefficients are very sensitive to tiny changes in the training set [51]. Three PLS factors in total were taken as further increase in the number did not improve the model statistics but over fitted [52]. The dominant statistics was observed with third PLS factor therefore it was chosen to create the QSAR model. To ensure the robustness of the developed model it was validated both internally and externally against training and test sets using R2 and Q2respectively [53, 54]. The R2and Q2 were found to be 0.913 and 0.748. The R2 and Q2 values greater than 0.6 and 0.5 indicates the model is sound and has high ability to predict inhibitory activities in training and test set [53, 55]. The model exhibited good statistical significance with Pearson R value of 0.880, variance ratio; F value of 101.2, low standard deviation; SD of 0.263and root mean square error; RMSE value of 0.286 [54]. The results are tabulated in Table 2. The experimental and predicted values of the entire dataset ligands are shown in Table 1. The 3D QSAR model was taken further for insilico screening in order to identify novel compounds with good inhibitory activity against pim-3.
Table 2
The statistical parameters of various pharmacophore hypotheses obtained through PHASE
ID
|
SD
|
R-squared
|
F
|
RMSE
|
Q-squared
|
Pearson-R
|
ADDPR
|
0.361
|
0.810
|
48.4
|
0.397
|
0.512
|
0.731
|
DDPRR
|
0.395
|
0.772
|
38.4
|
0.427
|
0.436
|
0.663
|
ADRRR
|
0.263
|
0.913
|
101.2
|
0.286
|
0.748
|
0.880
|
AADRR
|
0.401
|
0.766
|
37.1
|
0.440
|
0.401
|
0.667
|
3d Qsar Visualization Of Best Active Compound
The relationship between structure and activity relationship (SAR) and biological activity can be best understood with the help of three dimensional envision of best hypothesis ADRRR and selected indoleligands in aspect of developed QSAR. The 3D QSAR envision of best active compound (compound 39) with biological activity of 6.5 nM (pIC50 = 8.187) is shown Figure 4. In Figure 4a, b andc, blue and pink cubes represent hydrogen bond acceptor favoured and disfavoured regions respectively, yellow and orange cubes represent hydrogen bond donor favoured and disfavoured regions while violet and green cubes represent hydrophobic favoured and disfavoured regions respectively.
The atom based 3D QSAR model while predicting the biological activity takes into the account steric clashes apart from pharmacophore features where as pharmacophore based 3D QSAR predicts the biological activity depending on pharmacophore sites and their areas. From Figure 4a it can be seen that the NH of the indole ring, amino group of pyrimidine ring, NH attached to pyrazine ring and dimethyl amine groups were superposed onto the H-bond donor favoured pink contours.One of the nitrogen‘s of pyrazine ring is projecting into the blue H-bond acceptor favoured contours (Figure 4a). It matched with the H-bond acceptor feature A2 in the hypothesis. The NH of indole ring superposed onto the H-bond donor feature (yellow contour) is depicted in Figure 4b. It explicitly matched with the pharmacophoric feature D3. The dimethyl amino group was onto the H-bond donor favoured yellow contour (Figure 4b). The nitrogen’s of pyrazine and pyrimidine rings were placed towards hydrogen bond disfavoured orange contours (Figure 4b). In Figure 4c it can be observed that the hydrophilic favoured amine group and nitrogen's of the pyrimidine ring, one of the nitrogen's of pyrazine and the dimethyl amine were superposed onto the green hydrophilic favoured contours (Figure 4c).The presence of extra H-bond acceptor, donor and hydrophilic features is advantageous for the biological activity.
Pharmacophore Model Based Virtual Screening
Asinex and Otava lead library databases prepared earlier using LigPrep was used to screen in order to identify potential pim-3 inhibitors using ADRRR common pharmacophore hypothesis as the template. In this process of searching the conformers generated from the database were screened to match the CPH based on the site distance. Five out of five pharmacophore sites without partial matches were matched with the hypothesis in the present study. The search process resulted in the 1507 molecules out of total 186473 molecules from both Asinex and Otava lead library databases. The obtained hits will definitely act as promising pim-3 inhibitors since the model was developed from the active set of compounds with high inhibitory activity against pim-3. Therefore screened out ligands were further subjected to virtual screening process.
Validation Of 3d Model
The modelled protein was validated using ramachandran plot (PROCHECK), ERRAT, Verify 3D and ProSA (Protein Structure Analysis Server). Initially the detailed stereo chemical quality of each amino acid residue was evaluated by ramachandran plot employing PROCHECK program. The statistics of the modelled protein is shown in Figure 5. The modelled protein has 295 amino residues out of which 240 (96%) residues were in the most favoured regions, 9 (3.6%) residues were in additionally allowed regions and one (0.4%) residue was in generously allowed regions. Interestingly none of the amino acid residues were in disallowed regions excluding proline and glycine. The results from PROCHECK were promising hence the generated model is stereo chemically sterling. The information pertaining to non-bonded interactions between unlike atoms can be known by ERRAT. It is "overall quality factor", higher the score higher the quality. The acceptable range is >50 for a high quality model [56]. The overall quality factor i.e. ERRAT score for generated model was 73.801 (Figure 6) indicating dependability and good stability of the protein [57]. The compatibility of an atomic model (3D) with its sequence (1D) can be analyzed by Verify 3D. The assigned 3D-1D score should never be below zero and should be above 0.2. In present study, the value was above 0.2 most part (Figure 7) indicating good compatibility of the model with its own sequence of amino acids. ProSA was used to compute the interaction energy per residue. In present case, the interaction energy of each amino acid residue was determined with respect to remaining protein in order to judge the energy criteria. The overall quality of the model can be assessed with ProSA Z-score. The quality of the protein using the Z-score is actually estimated by comparing the experimentally determined proteins with similar number of amino acid residues that were deposited in the protein databank by X-ray crystallographic method (light blue) and NMR spectroscopy (dark blue) (Figure 8a). The Z-score of the modelled protein was -7.1 (Figure 8b). This negative value indicates that the overall quality of the protein is good. All the above investigations suggest that a high quality model of pim-3 (Figure 9) was obtained which can be further used for docking and MD simulations.
Virtual Screening Using Glide
The 1507 hits filtered out from the Asinex and Otava lead library databases were docked into the active site of pim-3 in two stages. Initially all the hits restituted from the pharmacophore based screening were taken for docking using SP (standard precision) protocol. Subsequently, the selected top scoring 297 (20%) molecules obtained from SP docking were subjected to more rigorous XP (extra precision) docking. Finally binding energies were calculated and the top 6 (2%) molecules having binding energy values greater than the best active compound in the dataset were retrieved.
Binding Free Energy Calculations
The binding free energies of top 6 hits were computed and tabulated in the Table 3. The binding energies of the hits ranged from -94.698 to -96.128 Kcal/mol. The binding energy value of the best active compound (Table 3) was -94.419 Kcal/mol. All the screened ligands exhibited good binding energies greater than the best active compound. Therefore it is clear that these molecules can definitely have greater probability of becoming potent inhibitors against pim-3.
Table 3
Binding free energies of top hits retrieved from Asinex and Otava lead library databases
S. No.
|
Compound
|
Binding Energy
(Kcal/mol)
|
1.
|
39
(Best active)
|
-94.419
|
2.
|
N1
|
-96.128
|
3.
|
N2
|
-96.075
|
4.
|
N3
|
-95.659
|
5.
|
N4
|
-95.270
|
6.
|
N5
|
-94.836
|
7.
|
N6
|
-94.698
|
Interaction Studies Of Screened Hits
The interaction patterns of the screened hits were analyzed from Ligand Interaction Diagram (LID) available in the Schrodinger suite. The interactions are shown in Figures 10 and 11. The purple lines indicate hydrogen bond interactions, green lines indicate π-π stacking interactions, red lines indicate π-cationic interactions and blue-pink line indicates salt bridges. Initially, the pim-3 best active complex was examined with the help of LID. The result is illustrated in Figure 10. The best active compound formed three hydrogen bond interactions with Glu 124, Asp 131 and Glu 174 and formed a salt bridge with Asp 189. The results of the screened hits are illustrated in Figure 11 and Supplementary Figure S1. The screened hits formed hydrogen bond interactions with Phe 51, Glu 124, Arg 125, Pro 126, Asp 131, Asp 134, Glu 174, Asp 189 and Phe 190, salt bridges with Asp 131and Asp189 and showed π-π stacking and π-cationic interactions with Phe 51.
Molecular Dynamics Simulations
The molecular interactions which are actually accountable for the stability are explored with the help of molecular dynamics simulation. To investigate the stability and molecular interactions of the screened hit (N1) obtained from Asinex and Otava lead library databases, MD simulations were performed for 10 ns. The RMSD of screened hit (N1) complexed with pim-3 is shown in the Figure 12. The RMSD initially reached to 2.64Å at 0.74 ns from 1.57Å around 50 ps. The RMSD after fluctuating aroused to 3.54 Å around 4.76 ns and retained between 2.68 and 3.99 Å throughout the simulation up to 10 ns. The averaged RMSD of the complex was 3.11Å indicating that the screened hit N1 formed a stable complex with pim-3.
Protein-ligand interactionsis one of the major factors that is analyzed during the course of simulation. They are useful in understanding the dynamic changes and binding modes during MD simulations. There are four substantial protein-ligand contacts or interactions in MD simulations namely hydrogen bonding, ionic, hydrophobic and water bridges. The binding site of pim-3 consisted the following amino acid residuesPhe 51, Glu 124, Arg 125, Pro 126, Asp 131, Asp 134, Glu 174, Asp 189 and Phe 190.
The screened hit N1 showed hydrogen bond interaction with Glu 124 (96.90%), hydrophobic interactions with Val 54 (19.38%), Ala 67 (45.25%), Leu 123 (58.44%), Leu 177 (34.06%) and Ile 188 (49.45%) and formed water mediated hydrogen bonds with Leu 46 (18.68%), Pro 126 (10.78%), Lys 172 (10.18%), Glu 174 (35%) and Asp 189 (6.69%) (Figures 13 and 14). It also formed π-π stacking and π-cationic interactions with Phe 51 for 71% and 54% of simulation time respectively (Figure 13 and 14). Surprisingly ionic interactions were lost and not observed in the protein complex N1. Therefore it can be noted that the screened hit N1 has good interactions with pim-3.