Density Functional theory studies (DFTs)
In current study, Quantum chemical calculations i.e., Molecular geometry optimizations, HOMO-LUMO, Molecular electrostatic potential, frontier molecular orbital energies, energetic parameters and global reactivity descriptors of Dacomitinib and Neratinib were carried out with Gaussian 09 and Gauss view 6 software package using DFT/B3LYP method. The SVP basis set was used for all calculations [38]
Optimized geometries
In present study, geometries of FDA approved drugs Dacomitinib and Neratinib were completely optimized in gas and solvent phase using DFT/B3LYP method and 6-31g (d,p) basis set.. No negative frequencies were obtained after the geometry optimization which demonstrating that current geometries are true local minima. Optimized structures of both drugs are presented in Figure 1.
Table 1. Optimization energy of FDA approved drugs
Code
|
Gas
|
Methanol
|
Optimization Energy (hatree)
|
Polarizability
a.u (α)
|
Dipole Moment (debye)
|
Optimization Energy (hatree)
|
Polarizability
a.u (α)
|
Dipole Moment
(debye)
|
Dacomitinib
|
-1912.731301
|
326.801422
|
2.696072
|
-1912.748676
|
435.057775
|
3.885410
|
Neratinib
|
-2173.707581
|
419.348279
|
6.749889
|
-2173.728460
|
560.189099
|
8.520631
|
Frontier molecular orbital analysis (FMOs)
The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) were generated using Gaussian 9 and Gauss view 6 software package. The energy calculations were performed using SVP basis set in gas and solvent phase. The FMOs of both drugs are shown in figure 2.
FMOs have high significance as they determine the reactivity of the compound. The outermost electrons are directly involve in interaction between ligand and target protein so calculation of FMOs become inevitable. The HOMO represents the ability of a compound to donate electrons while LUMO is the ability to accept or withdraw electrons. The small energy gap ΔE (EHOMO-ELUMO) between HOMO and LUMO represents efficient charge transfer and making molecule more polarizable. Whereas, molecules having large ΔE gap (EHOMO-ELUMO) are least reactive and non-polarizable [39].
The calculated values for HOMO and LUMO and ΔE gaps (EHOMO-ELUMO) are tabulated in table 2.
Table 2. Values for HOMO and LUMO and ΔE gaps (EHOMO-ELUMO)
Compound
|
EHOMO
(eV)
|
ELUMO
(eV)
|
∆Egap
(eV)
|
Potential
Ionization I(eV)
|
Affinity A(eV)
|
Electron
donating power (ω-)
|
Electron
accepting Power (ω+)
|
Electrophilicity (Δω±)
|
Dacomitinib
|
Gas
|
-0.21996
|
-0.06775
|
0.152
|
0.21996
|
0.06775
|
0.217
|
0.074
|
0.291
|
Sol
|
-0.22054
|
-0.04304
|
0.178
|
0.22054
|
0.04304
|
0.217
|
0.074
|
0.291
|
Neratinib
|
Gas
|
-0.22254
|
-0.07332
|
0.149
|
0.22254
|
0.07332
|
0.230
|
0.082
|
0.312
|
Sol
|
-0.21939
|
-0.07378
|
0.146
|
0.21939
|
0.07378
|
0.230
|
0.082
|
0.312
|
As shown in table 2 and figure 2, the energy band gap between (EHOMO-ELUMO) for compound Dacomitinib and Neratinib were found to be 0.152eV and 0.149eV in gas phase whereas it was 0.178eV and 0.146eV in solvent phase respectively in B3LYP/DFT SVP level of theory. It was observed that small energy gap was corresponding to more reactivity of the compound. Here we can say that docking score and binding affinities of drugs increases with decrease in the energy gap. Furthermore, chemical hardness and softness is measure of stability of molecular structure. [40] Compound having the large (EHOMO-ELUMO) energy gap refers to hardness of molecule whereas compound having small energy gap are usually more reactive and show high binding energies and binding affinity. Dacomitinib showed softness value of 7.40 in solvent phase whereas Neratinib showed softness value of 6.25 which demonstrate that Dacomitinib is more reactive drug than Neratinib. The Koopman’s theorem was used to express ionization energy and electron affinity of both drugs [41].
I = -EHOMO
A= - ELUMO
We evaluated the following parameters by using their respective formulas: Hardness: η = 1/2(ELUMO - EHOMO); Softness: S = 1/2η; Electronegativity: χ = -1/2(ELUMO + EHOMO); Chemical potential: µ = - χ; Electrophilicity index: ω = µ/2η.
Table 3. Global reactivity descriptors for FDA approved drugs
Compound
|
Hardness (η)
|
Softness (S)
|
Electronegativity (X)
|
Chemical Potential (μ)
|
Electrophilicity Index (ω)
|
Dacomitinib
|
Gas
|
0.076
|
6.57
|
0.144
|
-0.144
|
0.136
|
Sol
|
0.089
|
5.63
|
0.132
|
-0.132
|
0.098
|
Neratinib
|
Gas
|
0.075
|
6.70
|
0.148
|
-0.148
|
0.147
|
Sol
|
0.073
|
6.87
|
0.147
|
-0.147
|
0.148
|
It is known that outermost electrons are involved in interaction between ligand and targeted protein so it is important to calculate electron density of highest occupied and lowest unoccupied molecular orbitals. In present study we have calculated the electron density and molecular electrostatic potential (MEP) of both drugs. The electron density of HOMO in gas phase for Dacomitinib was localized over morpholine and piperdinyl ring of Dacomitinib compound whereas electron density of LUMO are localized to carbo nitrile and benzocarbazole moiety of the compound. The electron density of HOMO and LUMO for Dacomitinib remained nearly same for solvent phase but it was more localized toward carbonitrile ring as compared to gas phase.
Molecular Docking
In-silico molecular modelling studies were conducted using MOE 2015.10 to evaluate and investigate stable protein-ligand interactions. FDA approved inhibitors i.e., Dacomitinib and Neratinib were docked into activation loop of NEK7 (PDB ID: 2WQN). Molecular docking studies showed binding affinity and interaction model of drug/protein candidate. The binding affinities of best pose was assessed through predicted inhibitory constant (ki) values and docking scores obtained through molecular docking. Activation loop of NEK7 protein is consist of intact HRD and DFG motifs which must remain intact to retain the active conformation of a protein. Both approved drugs were assessed for their interactions with amino acid residues of active site. Visualization of best pose was carried out using Discovery visualizer 17.2 [42]. The FDA approved drug showed excellent docking scores and formed stable protein-ligand conformation.
The docked conformation of Drug Dacomitinib with in active pocket of NEK7 formed stable protein-ligand complex with least binding energy of-26.77 kJ/mol and ki 20.37 µM. The amino acid residues which were involved in bonding and nonbonding interactions with Dacomitinib were as follows; ARG42, GLY41, ASP115, ILE40, ILE113, ALA61, LEU111, GLU112, ALA114, VAL48, ILE95, ASP179, LYS63 and PHE168. Dacomitinib showed important molecular interactions including hydrogen bonding, carbon hydrogen bonding, alkyl, pi-alkyl and van der waal interactions. All these interactions were involved in stabilizing protein-ligand complex. Briefly, it was observed that 1-ethyl piperidine ring of Dacomitinib was forming carbon hydrogen bond with ARG42 and GLY41. The formamide ring was making pi-sigma interaction with ILE40, pi-alkyl interaction withALA61 and VAL48, it is also an unfavorable acceptor-acceptor interaction with ASP179. Moreover, dimethylformadine ring is forming hydrogen bond with ASP179. Fluorobenzene ring is making pi-cation interaction with LYS63 and alkyl interaction with VAL48. Furthermore, among non-bonding interactions Van der wall interactions were observed with following residues; ASP115, LEU113, GLU112, ILE95, PHE168.
The bonding and non-bonding interactions of drug Neratinib involved important amino acid residues of activation loop of NEK7. The amino acid residues which were involved in molecular interactions with Neratinib were as follows; LEU180, LYS63, ASP179, VAL48, ILE40, ALA61, LEU113, GLU112, ALA114, ILE95, LEU111, ARG121, ASP115, GLY117, PHE168, ASP118, ALA165, LYS163 and ASN166. Neratinib showed important molecular interactions which contributed to binding affinity and binding score of protein-ligand complex. Most important interactions were observed between piperidinyl, pyrazol and 2,4 dichloro-4-floro phenyl ring of Crizotinib. Trimethylamine ring was involved in major stabilizing interaction i.e., conventional hydrogen bonding with LYS63 and ASP179 respectively of activation loop. 5-methyl-3-methylene-N-vinyl-2,3-dihydropyridin-4-amine is involved in making pi-alkyl interaction with ALA65. Moreover, chlorobenzene ring was involved in making pi-anion interaction with ASP179, pi-alkyl interaction with ALA161 and VAL48. Pyridine ring is forming pi-cation interaction with ASP179, pi-alkyl interaction with ALA114, ALA61, ILE95 and LEU111. These interactions were majorly contributing toward increasing the binding energy. Meanwhile, ethoxyethane ring was involved in carbon-hydrogen bonding with ASP115 amino acid residue. Neratinib showed excellent binding energy of -27.78 kJ/mol courtesy of important molecular interactions. Van der Walls interaction are important hydrophobic interactions which were observed with following amino acid residues; LEU180, ILE40, LEU113, GLU112, ARG121, GLY117, PHE168 and ASN166.
Molecular Dynamic simulation
The simulation paths of Desmond were examined. MD trajectory analysis was used to calculate the root mean square deviation (RMSD), root mean square fluctuation (RMSF), and protein–ligand interactions.
The Root Mean Square Deviation (RMSD) is a metric for calculating the average change in displacement of a group of atoms in relation to a reference frame. It is calculated for each and every frame of the trajectory. Throughout the simulation, monitoring the protein's RMSD can provide insight into its structural configuration. If the simulation has equilibrated, the RMSD analysis can show if the fluctuations at the end of the simulation are around some thermal average structure. The RMSD of a ligand is shown in the plot when the protein-ligand complex is first aligned on the reference protein backbone and then the RMSD of the ligand heavy atoms is measured. If the measured values are much larger than the protein's RMSD, the ligand has most certainly diffused away from its initial binding site.
Figure 5 depicts the evolution of RMSD values for the C-alpha atoms of NEK7 protein over time. The 2wqn-Dacomitinib complex's RMSD plot (Figure 5) shows stability of the protein ligand complex throughout simulation time which show that the complex possessed stable molecular interactions. It was observed that complex reaches stability at 5 ns and remained under 2 angstrom which is perfectly acceptable. After being equilibrated, protein RMSD values fluctuate within 2 Angstrom. After 35 ns Protein RMSD was increased to 2.1 angstrom and dropped again after 50 ns but it remained within limits and remained in equilibrium for remaining simulation time. These results show that the ligands remained securely attached to the receptor throughout the simulated time. RMSD for C-alpha atoms of NEK 7 also remained under 2 angstrom which depict the structural stability of the protein.
The protein-ligand complex showed any structural changes occurred during simulation time. Trajectory analysis of protein-ligand complex showed that RMSD values remained stable. During simulation very minute changes were occur which was insignificant. It was observed that complex was stable initially but after 30 ns it showed fluctuation and RMSD reached to 2.7 angstrom. After 50 ns, complex RMSD again dropped to 1.5 angstrom and remained stable majority of the simulated time. The average value for RMSD showed that complex remained stable without significant fluctuations. It terms of Protein RMSD, it showed slight fluctuations at end of trajectory but still average value for RMSD was perfectly acceptable.
For characterizing local changes along the protein chain, the Root Mean Square Fluctuation (RMSF) is useful. Figure 6 Peaks show sections of the protein that fluctuate the greatest during the simulation on the RMSF plot. Typically, the tails (N- and C-terminal) of proteins change more than any other portion of the protein. RMSF graph depict that amino acid residues ranging from 180 to 220 showed fluctuations in RMSF value. These residues belong to C terminal lobe. Alpha helices and beta strands, for example, are usually stiffer than the unstructured section of the protein and fluctuate less than loop portions. According to MD trajectories, the residues with greater peaks belong to loop areas or N and C-terminal zones. (Figure 7). The stability of ligand binding to the protein is shown by low RMSF values of binding site residues.
Peaks show sections of the protein that fluctuate the greatest during the simulation on the RMSF plot. Typically, the tails (N- and C-terminal) of proteins change more than any other portion of the protein. Secondary structural parts such as alpha helices and beta strands are usually more rigid than the unstructured portion of the protein and fluctuate less than loop areas. The residues with higher peaks belong to loop areas or N and C-terminal zones, as determined by MD trajectories shown in Figure 7. The stability of ligand binding to the protein is shown by low RMSF values of binding site residues.