Experiment 05Molecular DockingDrug Discovery
In Silico Molecular Docking & Binding Affinity Profiling: AutoDock Vina Grid Search & Binding Free Energy ($Delta G$)
Duration: 5 Hours • Computational Drug Design Lab
🎯 Experimental Aim & Learning Objectives
To prepare macromolecular receptor targets and small-molecule ligand files; define 3D active site affinity search grid boxes; execute flexible-ligand semi-flexible receptor docking simulations using AutoDock Vina; cluster conformational binding poses; and calculate binding free energies ($Delta G_{text{bind}}$), dissociation constants ($K_d$), and intermolecular non-bonded interaction networks.
🔬 Theoretical Foundation & Thermodynamic Principles
Molecular docking predicts the preferred orientation (pose) and binding affinity of a small molecule drug candidate within an enzyme’s binding pocket:
- AutoDock Vina Scoring Function: Evaluates the empirical binding free energy ($Delta G_{text{bind}}$) using an optimized scoring function:
$$Delta G_{text{bind}} = Delta G_{text{vdW}} + Delta G_{text{hbond}} + Delta G_{text{electrostatic}} + Delta G_{text{desolvation}} + Delta G_{text{torsional}}$$
Vina combines a machine-learning-inspired scoring function with an iterated local search (Broyden-Fletcher-Goldfarb-Shanno, BFGS) optimization algorithm. - Thermodynamic Relation to Affinity: Binding free energy is directly proportional to the thermodynamic equilibrium association constant ($K_a$) and dissociation constant ($K_d$):
$$Delta G = – R cdot T cdot ln(K_a) = R cdot T cdot ln(K_d)$$
At $T = 298.15text{ K}$, a more negative $Delta G$ signifies tighter binding. Every $-1.36text{ kcal/mol}$ change in $Delta G$ corresponds to a 10-fold increase in binding affinity ($K_d$). - Preparation Steps:
• Receptor: Remove crystallographic waters, add polar hydrogens, assign Kollman/Gasteiger partial charges, save as PDBQT format.
• Ligand: Define rotatable bonds, detect root, assign partial charges, save as PDBQT format.
Figure 1.5: AutoDock Vina receptor 3D affinity grid box setup, intermolecular hydrogen-bonding contacts, and binding free energy ($Delta G_{ ext{bind}}$) pose ranking table.
🧪 Computational Tools & Structural Data
- Software: AutoDock Vina 1.2+, AutoDockTools (ADT / MGLTools), Open Babel, and PyMOL / Discovery Studio Visualizer.
- Benchmark Target-Ligand System: Human Abl Kinase (PDB ID:
1IEP) and the anti-leukemia small molecule inhibitor Imatinib (Gleevec).
📋 Step-by-Step Computational Protocol
- Receptor Preparation: Download
1IEP.pdb. In PyMOL, strip crystallographic water molecules, co-crystallized buffers, and alternative conformations:remove solvent. In ADT, add polar hydrogens and assign Kollman charges. Export asreceptor.pdbqt. - Ligand Preparation: Acquire 3D SDF structure of Imatinib from PubChem. Convert to PDBQT using Open Babel:
obabel imatinib.sdf -O ligand.pdbqt --gen3d -p 7.4. Check that all rotatable single bonds are parameterized. - Grid Box Definition: Center the 3D affinity box on the ATP-binding pocket (coordinates around co-crystallized ligand):
center_x = 10.5, center_y = 18.2, center_z = 56.4; size_x = 22, size_y = 22, size_z = 22 Å. Save invina_config.txtwithexhaustiveness = 16. - Vina Execution: Run simulation:
vina --receptor receptor.pdbqt --ligand ligand.pdbqt --config vina_config.txt --out docked_poses.pdbqt --log docking.log - Pose Analysis & Hydrogen Bond Profiling: Load top pose (Mode 1) and receptor in PyMOL. Identify hydrogen bonds ($< 3.2text{ AA}$) and hydrophobic interactions with pocket gatekeeper residues (Thr315, Glu286, Asp381).
📊 Representative Docking Affinity & Interaction Data
| Conformation Mode | $Delta G_{text{bind}}$ (kcal/mol) | Predicted $K_d$ | RMSD lower bound ($text{AA}$) | Key Interacting Residues |
|---|---|---|---|---|
| Mode 1 (Top Pose) | −11.2 kcal/mol | $6.1 text{ nM}$ | 0.000 | H-bonds: Glu286 (2.1 Å), Asp381 (2.4 Å), Thr315 (2.8 Å) |
| Mode 2 | −10.5 kcal/mol | $20.1 text{ nM}$ | 1.284 | H-bonds: Glu286, Met318 backbone |
| Mode 3 | −9.8 kcal/mol | $65.8 text{ nM}$ | 2.140 | Hydrophobic contact with Phe382 of DFG |
Pharmacological Assessment: Mode 1 exhibits high sub-nanomolar affinity ($Delta G = -11.2text{ kcal/mol}$), accurately reproducing the crystallographic binding mode of Imatinib within the inactive DFG-out conformation of Abl kinase.
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