For understanding the characteristics of binding (ligand) and protein-ligand complex, molecular docking studies were performed. This computational study is the most commonly used technique in the drug discovery research field. In this present study, acetylcholinesterase AChE target was employed for the molecular docking studies with bioactive compounds from the brown marine algae.
Molecular Docking Analysis Of Marine Algae Bioactive Compounds With Ache
The results of the docking studies, namely, docking score and Glide energy have been portrayed in Table 1 which reflect the binding affinity of analysed ligands and the co-crystal. By observing the interactions of natural bioactive compounds and target protein, it was found that, except the co-crystal, remaining all 14 natural compounds produced significant number of hydrogen bond with the conserved active site residues Tyr72, Tyr124 and Phe295. In the docking results, all 14 compounds showed the higher binding energy (docking score and glide energy) compared with co-crystal inhibitor and also were found to have several hydrogen bonds interacting by catalytic sites of AChE. Among fourteen compounds, two compounds (Phlorofucofuroeckol and Tetrafucol) had higher docking score and glide energy compared with the co-crystal inhibitor of AChE (Table 1).
The co-crystal (Dihydrotanshinone I) forms hydrogen bonds with conserved active site residues Tyr72, Tyr124 and Phe295 with docking score and glide energy of -11.63 and − 45.71 kcal/mol, respectively. Similarly, phlorofucofuroeckol also forms hydrogen bonds with Tyr72 and Phe295 along with Gly122, Gly342, Tyr337, Arg296, Ser203 and Glu202 with docking score and glide energy of -16.79, and − 88.10 kcal/mol, respectively. Another compound, Tetrafucol forms a hydrogen bond with Tyr72 and Phe295 along with Ser293, His287 and Asn283 with docking score and glide energy of -14.37, and − 78.65 kcal/mol, respectively. Furthermore, hydrophobic interactions between residues like Tyr341, Phe297, Val294, Tyr337, Phe338, Tyr124, Trp286, Val365, Leu289 and Ala204 as well as π-π stacking were aided by the preservation of aromatic rings. Likewise, the compounds Bieckol, Eckol, Sesquiterpenes, 2-phloroeckol, Eckstolonol, Dieckol, 4-(3,5-Dihydroxyphenoxy) diphlorethohydroxycarmalol, Diphlorethohydroxycarmalol, Fucofuroeckol, Phlorofucofuroeckol B form hydrogen bonds with catalytic residues Tyr72, Tyr124 and Phe295 with the docking energy of -18.20, -13.78, -10.00, -14.97, -12.56, -16.76, -16.83, -17.22, -14.30 and − 13.30 kcal/mol, respectively. Interestingly, all marine algae bioactive compounds have common H-bond interactions with Tyr72, Tyr124 and Phe295, which are also similar to that of the co-crystal inhibitor. Docking results reveal that all marine algae bioactive compounds have strong binding affinities with AChE target protein. The ligand interaction plots of 2D maestro and 3D Pymol representation of protein-ligand complexes are shown in (Fig. 1). Based on the binding energy and key interactions profile analysis, the two best compounds Phlorofucofuroeckol and Tetrafucol complexes were chosen for molecular dynamics simulations to understand the structural and conformational stability. Results presented in Fig. 2 show the superposition analysis of protein-ligand docked complexes and it reveals that seaweed bioactive compounds bind at the active site of AChE target protein.
Table 1
Docking score, glide energy of seaweed bioactive compounds and the co-crystal docked complexes with AChE.
Compounds | Docking score (kcal\mol) | Glide energy (kcal\mol) | Hydrogen bond interactions (D-H…A) | Distances (Å) |
Co-crystal (Dihydrotanshinone I) | -11.63 | -45.71 | (TYR124)OH-H…O20 (TYR72)OH-H…O21 (PHE295)N-H…O17 | 2.84 2.90 3.21 |
Bieckol | -18.20 | -83.41 | (HIS287)NE2-H...O17 (TYR124)OH-H...O18 (TYR124)OH-H...O8 (LEU76)N-H...O6 O6-H...(THR75)OG1 (THR75)N-H...O6 (TYR72)OH-H...O1 | 2.99 2.86 2.99 3.03 2.58 2.98 3.02 |
Phlorofucofuroeckol | -16.79 | -88.10 | (TYR337)OH-H…08 06-H…(ARG296)O (PHE295)N-H…O6 O6-H…(SER203)OG (O12)-H…(SER203)OG (011)-H…(GLU202)OE2 (GLY122)N-H…O12 (TYR72)-H…O9 | 2.98 2.98 2.97 2.76 3.23 2.87 3.01 2.76 |
Eckol | -13.78 | -56.31 | O6-H...(ARG96)O (PHE295)N-H...O6 O9-H...(SER293)O O8-H...(TRP286)O O4-H...(TYR72)OH | 3.08 2.93 3.06 2.78 2.77 |
Sesquiterpenes | -10.00 | -38.28 | (PHE295)N-H…O2 | 2.89 |
Phytol | -9.74 | -41.72 | (THR75)N-H…O1 | 3.01 |
2-phloroeckol | -14.97 | -59.34 | O10-H...(TYR341)O (PHE295)N-H...O5 (PHE295)N-H...O2 (SER293)OG-H...O5 O9-H...(TRP286)O O11-H...(ASP74)OD1 O6-H...(ASP74)OD1 | 2.88 3.15 3.09 2.96 2.67 3.02 2.85 |
Eckstolonol | -12.56 | -60.84 | (PHE295)N-H...O5 (GLY121)N-H...O12 (THR75)N-H...O10 O6-H...(ASP74)OD2 O10-H...(ASP74)OD1 | 2.91 2.68 3.14 2.51 2.95 |
Halogenated Monoterpenes | -7.23 | -32.08 | --- | --- |
Dieckol | -16.76 | -76.85 | (PHE295)N-H...O9 O12-H...(GLU292)OE2 (TYR124)OH-H...O11 (THR75)N-H...(O18) | 2.82 2.77 2.86 3.05 |
4-(3,5-Dihydroxyphenoxy)diphlorethohydroxycarmalol | -16.83 | -77.81 | (PHE295)N-H...O7 (PHE295)N-H...O5 O5-H...(SER293)O (TYR133)OH-H...O15 (THR75)OG1-H...O13 O13-H...(THR75)O O6-H...(TYR72)OH | 2.91 2.97 3.22 2.76 3.02 2.90 3.00 |
Diphlorethohydroxycarmalol | -17.22 | -71.33 | (PHE295)N-H...O7 (PHE295)N-H...O5 O5-H...(SER293)O (TYR133)OH-H...O15 (THR75)OG1-H...O13 O13-H...(THR75)O O6-H...(TYR72)OH | 2.91 2.97 3.22 2.76 3.02 2.90 3.00 |
Fucofuroeckol | -14.30 | -70.04 | O10-H...ARG296)O (PHE295)N-H...O10 O6-H...(GLN291)O (TYR124)OH-H...O9 O9-H...(ASP74)OD2 O7-H...(TYR72)OH | 2.70 3.05 3.12 3.08 2.67 3.05 |
Phlorofucofuroeckol B | -13.30 | -80.24 | O12-H...(PHE338)O O9-H...(LEU289)O (HIS287)ND1-H...O10 (HIS287)N-H...O13 O13-H...(ASN283)O (TYR124)OH-H...O8 | 3.21 2.76 2.83 3.22 2.90 2.78 |
Tetrafucol | -14.37 | -78.65 | (PHE295)N-H...O9 (PHE295)N-H...O5 (SER293)OG-H...O3 O4-H...(SER293)O (HIS287)ND1-H...O8 O12-H...(ASN283)O (TYR72)OH-H...O10 | 3.12 3.26 3.33 2.87 3.07 3.05 3.17 |
Molecular Dynamics Simulations Of The Best Compounds And Co-crystal Complex
To understand the structural stability of the AChE complexes, MD simulations were performed for 100 ns which included AChE-Phlorofucofuroeckol and AChE-Tetrafucol as well as AChE-co-crystal complexes. The RMSD, RMSF, and Rgyr graphs were used to assess the steadiness of the complexes. From MD outcomes of the RMSD as shown in Fig. 3, it can be seen that the trajectory of the AChE in complex with the Phlorofucofuroeckol compound showed lesser structural deviations from the initial to 80 ns and the rmsd value is ~ 0.15–0.2 nm. Afterwards, it reached stable convergence till 100 ns trajectories. Similarly, AChE-Tetrafucol complex also showed very lesser deviations during the course of the MD simulations and the average rmsd value is ~ 0.15 nm in the overall MD simulations. Whereas, AChE-co-crystal complex showed higher structural deviations during the 0–52 ns simulation and it is observed that rmsd is ~ 0.25–0.3 nm. Finally, it reached convergence and then gradually increased in the rmsd at 80–100 ns, rmsd being 0.3–0.35 nm, respectively.
For predicting the dynamic behaviour of residual vacillation with docked complexes all through simulation, root mean square fluctuation analysis was carried out with AChE-Phlorofucofuroeckol and AChE-Tetrafucol as well as AChE-co-crystal complexes. The RMSF graph in Fig. 4 shows that upon binding of bioactive compounds (Tetrafucol and Phlorofucofuroeckol) simulated complexes have lesser residual fluctuations at the active site of AChE. In particular, catalytic sites residues of Tyr72, Tyr124 and Phe295 regions do not have fluctuations as in the co-crystal bound complex. Radius of gyration governs the mass-weight of atoms collection and mutual centre of mass during a molecular dynamics simulation. Rgyr was plotted as per Cα atoms of the protein. Results in Fig. 5 indicate that AChE-Phlorofucofuroeckol and AChE-Tetrafucol complex structures were stable and folded throughout the MD run and rg values range is 2.30 to 2.32 nm. AChE-Co-crystal complex showed fewer structural deviation and Rgyr value is 2.26 to 2.20 nm. The total number of hydrogen bond contacts was also analysed for the 100 ns simulation run. Phlorofucofuroeckol marine bioactive compound with AChE protein has formed 6 H-bonds whereas Tetrafucol compound has 9 H-bonds with target protein and it maintains these throughout MD simulations. The cocrystal has 3 H-bonds, which depict that, the our best bioactive compounds show strong stability with AChE protein (Fig. 6).
Free Energy Calculations
To understand the affinity of ligand binding with AChE protein in energetic parameters, MM-PBSA approach was used to calculate the binding free energy (Table 2). In this calculation, only enthalpy terms are incorporated, due to high computational complexity and entropic parameters not included. The known cocrystal inhibitor has − 14.88 (± 9.93) kJ/mol of binding energy whereas best bioactive compounds Pholorofucofuroeckol and Tetrafucol showed more favourable binding energy − 37.55 (± 5.58) kJ/mol and − 14.92 (± 16.45) kJ/mol respectively. The higher binding affinity in the Pholorofucofuroeckol and Tetrafucol is due to more favorable van dar Waal and electrostatics energetics. In addition, polar solvation effects played a significant role in ligand binding in the Pholorofucofuroeckol.
Table 2
Binding Free Energy Calculations of AChE-cocrystal and best compounds using MMPBSA method.
Protein –Ligand Complexes | ΔE VdW (kJ/mol) | ΔE Elec (kJ/mol) | ΔEGB (kJ/mol) | ΔESURF (kJ/mol) | ΔGGAS (kJ/mol) | Binding Free Energy (ΔG) (kJ/mol) |
Co-crystal (Dihydrotanshinone I) | -20.37 (± 12.07) | -16.82 (± 12.84) | 25.02 (± 14.72) | -2.71 (± 1.55) | -37.20 (± 22.28) | -14.88 (± 9.93) |
Tetrafucol | -18.79 (± 20.27) | -18.69 (± 21.46) | 25.08 (± 27.40) | -2.53 (± 2.72) | -37.47 (± 40.89) | -14.92 (± 16.45) |
Pholorofucofuroeckol | -50.38 (± 4.54) | -38.93 (± 9.48) | 57.76 (± 7.48) | -6.00 (± 0.56) | -89.31 (± 10.86) | -37.55 (± 5.58) |