Docking a ligand into a protein pocket
Steer a small molecule into a model binding pocket with the ξ slider and watch which contacts must line up for binding — the geometric heart of structure-based drug design.
Goal
Recognise why a bound pose needs both shape complementarity (the ring fills the pocket) and chemical complementarity (the ligand –OH sits by a donor), and how docking scores rank poses.
Apparatus and reagents
Virtual docking station: a model pocket with labelled donor/acceptor atoms, one ligand, and a slider for the docking coordinate ξ.
Procedure
- Set ξ = 0 and rotate: the ligand sits far right with no contacts — like a molecule in bulk solvent.
- Push ξ to ~0.5: the ligand is at the pocket mouth but the contact sticks have not formed — orientation still wrong.
- Reach ξ = 1: two new sticks appear — a hydrogen bond to the blue donor and a contact to the red acceptor. This is the bound pose.
- Rotate the bound complex edge-on: judge how much solvent the ring has excluded — the desolvation that drives binding.
- Imagine methylating the ligand so the –OH vanishes: which contact is lost, and how would the docking score change?
What to observe
- Below ξ ≈ 0.7 no contact stick exists: proximity alone does not bind — the pose must satisfy geometry.
- In the bound pose the ring lies flat in the pocket arc, showing shape complementarity.
- The ligand’s –OH ends near the pocket’s labelled donor: matched donor–acceptor pairs make specific, directional contacts.
Explanation
Binding free energy rewards hydrogen bonds, buried hydrophobic surface and desolvation, and punishes lost rotational freedom. Docking programs generate poses, score them with approximations of these terms, and keep the best — the same logic explains the lock-and-key idea of Fischer (1894) refined into induced fit by Koshland. SAR series then test the model: a substituent that removes a key hydrogen bond should cost ~4–20 kJ/mol of affinity.
Chemists behind it
Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.