A protein–protein docking run can spend time exploring orientations that are unlikely to be relevant. If you already have a reasonable idea of where the proteins interact, you can use that information to restrict the search instead of sampling every possible orientation. The key is to treat the starting pose as a guide for sampling—not as proof that the proteins are correctly docked.
This workflow uses the Hex Extension in SAMSON. It is especially useful when you can orient the ligand toward a suspected binding region and want to focus the search around that starting arrangement.
Prepare the structures before narrowing the search
Begin with clean receptor and ligand structures. The tutorial recommends removing alternate-location atoms, water, monatomic ions, and ligands, then adding hydrogens if they are missing. If the structures have missing residues or heavy atoms, the PDBFixer extension can be used to repair them. Avoid changing hydrogens when they were already added according to a chosen protonation state.
For a guided example, the tutorial sample contains two proteins, 2PTC_E and 2PTC_I. In the Hex app, assign one as the receptor and the other as the ligand. Saving the ligand’s initial conformation before docking makes it possible to return to that starting arrangement if you want to try different settings.
Use the starting pose to define a smaller search domain
Hex can search orientations using spherical coordinates. With the Range angles sampling method, angular limits define spherical cones around the intermolecular axis connecting the protein centers. A range of 180 degrees applies no angular restriction; a smaller range limits the orientations considered.
If you know the approximate binding region, first move and orient the ligand toward that region using a Move editor. Then open Advanced parameters in Hex and set the receptor and ligand range angles. The tutorial uses 45 degrees for each as an example of a loose restriction around the starting orientation.
Think of the angle limits as a way to tell the search where to look, not as a way to specify the final interface. If your initial orientation is far from the relevant region, a narrow range can exclude useful poses. When the binding location is uncertain, retaining a broader search is safer than imposing a constraint you cannot justify.
You can also limit the ligand’s twist rotation about the intermolecular axis. This is a separate control from the receptor and ligand range angles, so adjust it only when you have a reason to restrict that rotational motion.
Balance search coverage and run time
Hex offers a fast, low-resolution Steric scan followed by a higher-resolution Final search. The first phase discards most orientations, allowing the final phase to examine promising candidates more finely. The documentation notes that a Steric scan order of 16–18 is sufficient in many cases, and gives combinations such as N=16 for the scan and N=25 for the final search as practical starting points. A higher final order may be appropriate for high-resolution crystal structures when binding-related conformational changes are expected to be small; more flexible or model-built structures may call for a lower order.
These are starting points, not universal settings. More restrictive angles and a coarse-first search can reduce the amount of search needed, but they do not establish that a resulting pose is biologically correct. Check the results rather than treating the top-ranked solution as a conclusion.
Inspect the output, not just the score
After docking, review the clustered modes in the Results tab. You can inspect the best mode in each cluster or view all modes, restore a conformation by selecting its row, and create conformations in the document for further comparison. Visual inspection and interaction analysis can help you assess plausible candidates; SAMSON extensions such as the Protein-Ligand Interaction Analyzer and Hydrogen Bond Finder can support that follow-up work.
The practical lesson is simple: use what you know about the binding region to guide sampling, keep constraints broad enough to reflect your uncertainty, and analyze multiple resulting poses before deciding what to investigate next.
For the complete workflow and parameter details, see the Protein docking with Hex tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.