Preparation of the structures
The three dimensional crystal structure of AChE (PDB ID: 1ACJ), BuChE (4BDS), and BACE-1 (1W51) were retrieved from protein data bank (
18). Water and co-crystal ligand molecules were excluded from the structures and the PDBs were corrected in terms of missing atom types by modeller 9.12 (
19). An
in-house application (MODELFACE) was used for generation of python script and running modeller software. The enzymes were then converted to PDBQT by adding gasteiger partial charges using MGLTOOLS 1.5.6 (
20).
Designing of the ligands
More than 500 ligands were designed based on
Scheme 1 using MTDLs strategy. The tacrine fragment was selected for its inhibition of AChE and BuChE. The coumarin scaffold was chosen for its β-secretase 1 (BACE-1) inhibitory and antioxidant activities. Based on the literature survey, the hydroxyethylamine linker was selected to have BACE-1 inhibitory activities.
Designing of tacrine-coumarin hybrids using MTDLs strategy
Optimization of the ligands
The two dimensional structures of the ligands were drawn using ChemBioDraw Ultra v.13 software (Cambridge Software). Then, the ligands were subjected to minimization procedures by means of an in house TCL script using Hyperchem (Version 8, Hypercube Inc., Gainesville, FL, USA). Each ligand was optimized using molecular mechanics method (MM+) followed by quantum based semiemprical method (AM1) utilizing HyperChem 8. The output structures were thereafter converted to PDBQT by means of MGLtools 1.5.6 for docking procedure.
Drug-likeness analysis
Drug-likeness rules are set of principles for the structural properties of compounds, used for fast calculation of drug-like properties of a molecule. They can be quite effective and efficient. Using DruLiTo (
21), an open source virtual screening tool, as it was shown in
Table 1, drug-likeness descriptors such as Molecular Weight (MW), logP, AlogP, H-Bond Acceptor (HBA), H-Bond Donor (HBD), Total Polar Surface Area (TPSA), Atom Molar Refractivity (AMR), number of Rotable Bond (nRB), number of Atom, number of Acidic group, Rotatable bond Count (RC), number of Rigid Bond (nRigidB), nAtomRing, and nHB for all of the ligands were calculated. DruLiTo calculations is based on the various drug likeness rules like Lipinski›s rule, Veber rule, Ghose filter, BBB rule, CMC-50 like, rule and Quantitative Estimate of Drug-likeness (QED). The compounds that pass the drug-likeness filter were subjected to docking studies.
| No. | MW | logP | AlogP | HBA | HBD | TPSA | AMR | nRB | nAtom | RC | nRigidB | nAromRing | nHB |
|---|
| 1 | 439.99 | 2.684 | 0.175 | 6 | 0 | 47.89 | 142.83 | 6 | 35 | 6 | 34 | 4 | 6 |
| 2 | 470.93 | 2.336 | 0.872 | 6 | 0 | 81.03 | 139.92 | 7 | 34 | 5 | 31 | 3 | 6 |
| 3 | 490.95 | 2.346 | -0.391 | 7 | 0 | 64.96 | 146.22 | 9 | 37 | 5 | 32 | 3 | 7 |
| 4 | 474.95 | 2.317 | 0.251 | 6 | 0 | 55.73 | 144.04 | 8 | 36 | 5 | 32 | 3 | 6 |
| 5 | 472.96 | 2.781 | 0.278 | 6 | 0 | 38.66 | 149.23 | 6 | 36 | 6 | 35 | 4 | 6 |
| 6 | 470.96 | 3.39 | 0.712 | 5 | 0 | 38.66 | 148.45 | 6 | 36 | 6 | 35 | 4 | 5 |
| 7 | 490.93 | 3.442 | 1.289 | 5 | 0 | 63.96 | 153.73 | 6 | 36 | 6 | 35 | 4 | 5 |
| 8 | 461.92 | 2.907 | 0.368 | 6 | 0 | 47.89 | 134.07 | 6 | 33 | 5 | 31 | 3 | 6 |
| 9 | 457.93 | 3.036 | 0.43 | 5 | 0 | 38.66 | 134.89 | 6 | 33 | 5 | 31 | 3 | 5 |
| 10 | 477.9 | 3.088 | 1.006 | 5 | 0 | 63.96 | 140.17 | 6 | 33 | 5 | 31 | 3 | 5 |
| 11 | 487.93 | 2.419 | 0.139 | 7 | 0 | 55.73 | 139.34 | 7 | 35 | 5 | 32 | 3 | 7 |
| 12 | 470.96 | 3.39 | 0.712 | 5 | 0 | 38.66 | 148.45 | 6 | 36 | 6 | 35 | 4 | 5 |
| 13 | 447.98 | 1.135 | -0.69 | 8 | 0 | 64.96 | 134.84 | 8 | 35 | 5 | 31 | 3 | 8 |
| 14 | 463.95 | 1.53 | -0.132 | 7 | 0 | 81.03 | 141.29 | 8 | 35 | 5 | 31 | 3 | 7 |
| 15 | 389.99 | 2.124 | -0.635 | 6 | 0 | 38.66 | 126.32 | 6 | 31 | 5 | 29 | 3 | 6 |
| 16 | 387.99 | 2.093 | -0.201 | 5 | 0 | 38.66 | 125.54 | 6 | 31 | 5 | 29 | 3 | 5 |
| 17 | 391.99 | 1.964 | -0.369 | 6 | 0 | 47.89 | 124.64 | 6 | 31 | 5 | 29 | 3 | 6 |
| 18 | 407.96 | 2.356 | 0.376 | 5 | 0 | 63.96 | 130.81 | 6 | 31 | 5 | 29 | 3 | 5 |
| 19 | 387.99 | 2.093 | -0.308 | 5 | 0 | 38.66 | 125.46 | 6 | 31 | 5 | 29 | 3 | 5 |
| 20 | 407.96 | 2.145 | 0.269 | 5 | 0 | 63.96 | 130.74 | 6 | 31 | 5 | 29 | 3 | 5 |
| 21 | 455.98 | 1.98 | -0.76 | 7 | 0 | 64.96 | 141.5 | 9 | 36 | 5 | 31 | 3 | 7 |
| 22 | 443.98 | 1.819 | 0.32 | 7 | 0 | 64.96 | 137.52 | 8 | 35 | 5 | 31 | 3 | 7 |
| 23 | 439.99 | 1.951 | -0.118 | 6 | 0 | 55.73 | 139.32 | 8 | 35 | 5 | 31 | 3 | 6 |
| 24 | 441.99 | 1.288 | -0.224 | 7 | 0 | 55.73 | 138.67 | 8 | 35 | 5 | 31 | 3 | 7 |
| 25 | 459.98 | 1.456 | -0.625 | 8 | 0 | 74.19 | 139.88 | 9 | 36 | 5 | 31 | 3 | 8 |
| 26 | 455.98 | 1.588 | -1.063 | 7 | 0 | 64.96 | 141.68 | 9 | 36 | 5 | 31 | 3 | 7 |
| 27 | 457.98 | 0.925 | -1.169 | 8 | 0 | 64.96 | 141.03 | 9 | 36 | 5 | 31 | 3 | 8 |
| 28 | 469.98 | 1.691 | -0.587 | 8 | 0 | 64.96 | 145.64 | 10 | 37 | 5 | 31 | 3 | 8 |
| 29 | 435.99 | 3.024 | 0.343 | 5 | 0 | 38.66 | 143.73 | 6 | 35 | 6 | 34 | 4 | 5 |
| 30 | 455.96 | 3.076 | 0.92 | 5 | 0 | 63.96 | 149.01 | 6 | 35 | 6 | 34 | 4 | 5 |
| 31 | 480.95 | 0.97 | -0.865 | 8 | 0 | 55.73 | 140.71 | 8 | 36 | 5 | 32 | 3 | 8 |
| 32 | 442.93 | 2.722 | 0.744 | 5 | 0 | 63.96 | 135.53 | 6 | 32 | 5 | 30 | 3 | 5 |
| 33 | 424.96 | 1.85 | -0.373 | 6 | 0 | 38.66 | 130.96 | 6 | 32 | 5 | 30 | 3 | 6 |
| 34 | 461.92 | 2.696 | 0.262 | 6 | 0 | 47.89 | 134 | 6 | 33 | 5 | 31 | 3 | 6 |
Docking procedure
The docking simulations were carried out using an
in house batch script (DOCKFACE) for automatic running of AutoDock 4.2 and AutoDock Vina. In all experiments genetic algoritm search method was applied to determine the best pose of each ligand in the active site of the target enzymes. The genetic algorithm and grid box parameters for our three targets are listed in
Table 2. Random orientations of the conformations were generated after translating the center of the ligand to a specified position within the receptor active site, and making a series of rotamers. This process was recursively repeated until the desired number of low-energy orientations was obtained. The docking was carried out on flexible ligands and rigid receptors.
| Parameter Name | AChE | BuChE | BACE-1 | GAa Parameters | Value |
|---|
| PDB ID | 1ACJ | 4BDS | 1W51 | Number of GA Runs | 100 |
| No. of points in x | 40 | 50 | 50 | Population Size | 150 |
| No. of points in y | 40 | 50 | 50 | Max. No. of evaluations | 2500000 |
| No. of points in z | 40 | 50 | 50 | Max. No. of generations | 27000 |
| Grid spacing | 0.375 | 0.375 | 0.375 | | |
| Box X center | 4.395 | 55.7 | 63 | | |
| Box Y center | 69.901 | 46.5 | -3.763 | | |
| Box Z center | 65.807 | 81 | 75 | | |
Analysis of docking results
Having finished the docking process, the protein–ligand complex was analyzed in order to understand the type of interactions. Top ranked binding energies (kcal/mol) in AutoDock dlg output file were considered as response in each run.
AutoDock Vina is a surrogate of AutoDock 4.2 and has a new knowledge-based, statistical scoring function instead of the semiempirical force field of AutoDock 4.2. Due to great prediction accuracy and speed over AutoDock 4.2, Vina results were selected as the best docking binding energies. Docking results were supported almost by high cluster populations. The best docking result in each case was considered to be the conformation with the lowest binding energy.
Table 3 revealed the ligands with the best docking results in terms of its binding free energy to the receptors.
In case of AChE and BuChE, the co-crystal ligand is Tacrine.
Protein ligand interaction fingerprint (PLIF)
In order to perform PLIF studies on docking results, by means of preAuposSOM application (
22), the poses of docking were extracted from dlg files. The resulted PDBQTs and the receptor were converted to MOL2 be means of a batch script using Open Babel 2.3.1. The resulted mol2 files were subjected to AuposSOM 2.1 web server (
23). Two training phases with 1000 iterations were set in the self-organizing map settings of AuposSOM conf files. Other parameters of the software were remained as default. The output files were subjected to Dendroscope 3.2.10 (
24,
25) for visualization of the results. Dendroscope is a phylogenetic relationship software that is able to visualize rooted phylogenetic trees and networks efficiently.