Two structures of the same protein can show different conformations, but comparing their endpoints does not reveal how the protein might move between them. For molecular modelers, the practical challenge is to generate a useful path without losing track of whether the input structures are compatible, how the search is guided, or what the resulting path actually represents.
SAMSON’s Protein Path Finder provides a way to explore candidate transition paths between two conformations. It uses ART-RRT, combining T-RRT for pathway search with ARAP for motion generation. Constrained minimization is applied during the search to keep generated motions physically reasonable. The result is a set of candidate paths to inspect—not, by itself, proof of a unique biological mechanism.
Start with compatible structures
Before tuning search parameters, make sure the two conformations describe the same system: the chains, residues, and atoms should match. If the conformations come from separate files, prepare them individually and combine them as different models in one PDB file. For your own structure, preparation may include removing alternate locations, ligands, solvent, and ions, and adding hydrogens. If atoms or residues are missing, the PDBFixer extension can help repair the model.
This compatibility check matters because the workflow uses the two conformations as endpoints for one protein’s motion. The tutorial’s sample document is already prepared: it contains Adenylate Kinase chain A in the 4AKE and 1AKE conformations, labeled start and goal.
Set up the calculation before searching
Load the sample document through Home > Download, then open Protein Path Finder from Home > Apps > Biology or find it with Find everything…. The system should already be minimized. In the Settings tab, select Universal Force Field (UFF) as the interaction model and FIRE as the state updater. The tutorial specifies a FIRE step size of 1 fs and one step; when UFF asks whether to use existing bonds, choose that option.
Choose what guides the motion
In Set up the system, retrieve the conformations from the active document and assign the start and goal. Next, choose active ARAP atoms: these are the atoms that control the protein motion, while the remaining atoms follow according to ARAP. In the sample, two alpha-carbon atoms—one in GLY 12 and one in ARG 123—are used. Selecting a small set of meaningful control points is a practical way to define where the motion is guided; it is also worth checking the selection in the document before adding it to the app.
The sampling box defines the region in which active atoms are sampled. Its size and position bias their motion, and therefore influence the paths the planner explores. The example uses a 200 Å cube. Treat this as a setting for the example, not a universal value: choose a region that suits the system and motion you want to investigate.
Balance exploration and repeatability
The tutorial’s example settings include two runs, 20 ARAP-modeling iterations, and 20 FIRE minimization iterations for each generated state. T-RRT parameters include an initial temperature of 0.001 K, a temperature factor of 2, and one failure before increasing the temperature; the RRT extension step size is 1 Å. A seed can be fixed to help reproduce a run with the same setup. Without a fixed seed, the planner uses a random one.
These settings are starting points rather than guarantees that a path will be found within a particular time. The maximum elapsed time per run can limit the search. While it runs, monitor the number of tree nodes, elapsed time, run number, and paths found. Pause and resume the search as needed, or stop it when you have enough results to inspect.
Inspect paths, not just the endpoints
Each discovered path appears in the Results tab with information such as its number of states, minimum and maximum energy, and elapsed search time. Select a path to view its energy curve, then move the slider to inspect individual conformations in the viewport and their corresponding UFF energies. Comparing multiple paths can help you see how different searches behaved; the table also lets you record remarks and copy path data or energy values.
Use the results as candidates for further analysis. The documentation describes refining paths with the P-NEB app, and exporting paths as trajectories or exporting conformations at a chosen interval. Those next steps make it easier to examine or share a path, while keeping the search result distinct from a validated physical explanation.
For the full workflow and related tutorials, see the Protein Path Finder documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.