Ligand Docking

Bold text means that these files and/or this information is provided.

Italicized text means that this material will NOT be conducted during the workshop

fixed width text means you should type the command into your terminal

If you want to try making files that already exist (e.g., input files), write them to a different directory! (mkdir my_dir)

In addition to following this sample docking problem, the user is encouraged to review the Rosetta user guide including the section on ligand-centric movers for use with RosettaScripts.

https://www.rosettacommons.org/docs/latest/

Ligand Docking with a G-Protein Coupled Receptor

The experimental data for this tutorial is derived from: Chien, E. Y. T. et al. Structure of the human dopamine D3 receptor in complex with a D2/D3 selective antagonist. Science 330, 1091-5 (2010).

This particular D3/eticlopride protein-ligand complex was used as a target in the GPCR Dock 2010 assessment, the results of which are discussed here: Kufareva, I. et al. Status of GPCR modeling and docking as reflected by community-wide GPCR Dock 2010 assessment. Structure 19, 1108-1126 (2011).

If you are interested in more information on the performance of Rosetta in modeling and docking D3/GPCRs in general, please consult Nguyen, E. D. et al. Assessment and challenges of ligand docking into comparative models of g-protein coupled receptors. PLoS One 8, (2013).

Dopamine is an essential neurotransmitter that exhibits its effects through five subtypes of dopamine receptors, important members of class A G-protein coupled receptors (GPCRs). Both subtype two (D2R) and subtype three (D3R) function via inhibition of adenyl cyclase, and modulation of these two receptors has clinical applications in treating schizophrenia. However, the high degree of binding site conservation between D2R and D3R makes it difficult to generate pharmacological compounds that selectively bind one or the other, and thereby reducing side effects. Today, we will examine how eticlopride, a D2R/D3R antagonist, binds to human D3R.

A crystal structure is available for the D3R and eticlopride complex (3PBL), but for the purposes of this exercise, we will model the protein-ligand interactions. In reality, you will most likely be using a comparative model rather than a crystal structure for membrane protein ligand docking, but the steps in this tutorial will apply to both.

For this exercise, we'll be doing our pre-docking preparations in the protein_prep and ligand_prep folders. The modeling will be done in the docking folder. All necessary files are also prepared in the answers directory in case you get stuck.

Navigate to the ligand docking directory where you will find the ligand_prep, protein_prep, docking, and answers folders
```
cd ~/rosetta_workshop/tutorials/ligand_docking 
```
Prepare a human dopamine 3 receptor structure. We will do this by obtaining the crystal structure (3PBL) and removing the excess information.
1. Change into the protein_prep directory with the cd command
```
cd protein_prep 
```
2. The clean_pdb.py script will allow you to automatically download a PDB file and strip it of information other than the desired protein coordinates. The 'A' option tells the script to obtain chain A only. The full crystal structure consists of two monomers as a crystallization artifact.
```
~/rosetta_workshop/Rosetta/tools/protein_tools/scripts/clean_pdb.py 3PBL A
```
3. There are two output files from clean_pdb.py: 3PBL_A.pdb contains a single chain of the protein structure and 3PBL_A.fasta contains the corresponding sequence. 3PBL_A.pdb is the receptor structure we will be using for docking, copy this into the docking directory.
```
cp 3PBL_A.pdb ../docking
```
  Note: This structure has a T4-lysozyme domain instead of the third cytoplasmic loop as a stabilizing feature for crystallography. Normally, we would truncate this lysozyme segment and perform loop modeling as discussed in the comparative modeling tutorial to regenerate the intracellular loop. However in the interest of time, we will use the lysozyme containing structure as the eticlopride binding site is far from the intracellular domain.
Next, we will prepare the ligand files by generating params using a eticlopride conformational library.
1. cd into the directory named ligand_prep
```
cd ../ligand_prep 
```
2. In the directory, you will find a pair of already prepared files: eticlopride.sdf and eticlopride_conformers.sdf
  1. eticlopride.sdf: This contains the eticlopride structure found in the 3PBL protein complex.
  Note: You can also find the ligand file from this particular PDB structure by going to the 3PBL page and scrolling down to the "Ligand Chemical Component" section. From there, you can click "Download" under the ETQ identifier.
  1. eticlopride_conformers.sdf: This is a library of conformations for eticlopride generated outside of Rosetta. The downloaded ligand .sdf file only contains conformations found in the PDB so we must expand the library to properly sample the conformational space. We also need to add hydrogens since they are not resolved in the crystal structure. Feel free to open the file in Pymol and use the arrow keys to scroll through the different conformations:
```
pymol eticlopride_conformers.sdf
```
    This particular conformational library was generated using the Meiler lab's BioChemicalLibrary (BCL). The BCL's conformer generator is currently under development but will be made available for distribution in the future. Other methods of ligand conformer generation include OpenEye's MOE software and web-servers such as Frog 2.1 or DG-AMMOS. The generated libraries will differ depending on the chosen method.
3. Generate a .params file and associated PDB conformations with Rosetta atom types for eticlopride. A params file is necessary for ligand docking because Rosetta does not have records for custom small molecules in its database.
  1. Type
```
~/rosetta_workshop/Rosetta/main/source/src/python/apps/public/molfile_to_params.py -h
```
    to learn more about the script for generating the params file.
  2. Type
```
 ~/rosetta_workshop/Rosetta/main/source/src/python/apps/public/molfile_to_params.py \
 -n ETQ -p ETQ --conformers-in-one-file eticlopride_conformers.sdf  
```
    Note: You may encounter a warning about the number of atoms in the residue. This is okay as Rosetta is merely telling you that the ligand has more atoms than an amino acid.
  ETQ.params contains the necessary information for Rosetta to process the ligand, ETQ.pdb contains the first conformation, and ETQ_conformers.pdb contains the rest of the conformational library.
4. If you use the tail command on ETQ.params, you will notice the PDB_ROTAMERS property line that tells Rosetta where to find the conformational library. Make sure this line has ETQ_conformers.pdb as the property.
```
tail ETQ.params 
```
5. Now that we have the necessary files for ligand docking, let's copy them over to the docking directory.
```
cp ETQ* ../docking
```
Now we want to make our final preparations in the docking directory.
1. Switch over to our docking directory
```
cd ../docking
```
2. Open up our prepared receptor and ligand structures to examine the complex
```
pymol 3PBL_A.pdb ETQ.pdb
```
3. Tip: 'All->Action->preset->ligand sites->cartoon' will help you visualize the protein/ligand interface. The All button is denoted by a single letter "A" in Pymol
  
  Since this is a rudimentary exercise, we will start with the ligand in the protein binding site. In practical application, we may need to define a starting point with the StartFrom mover or to manually place the ligand into an approximate region using Pymol.
4. Once you close Pymol, make sure Rosetta has these four necessary input structure/parameter files in the docking directory. If you are missing any of these, copy them from ../answers/docking/
  1. 3PBL_A.pdb: a single chain of the protein receptor structure
  2. ETQ.pdb: a default starting conformation for eticlopride
  3. ETQ_conformers.pdb: A pdb file containing all conformers generated from the eticlopride library
  4. ETQ.params: a Rosetta parameter file that provides the necessary properties for Rosetta to treat eticlopride
5. Next we need to make sure we have the proper RosettaScripts XML file, input options file, and crystal complex (correct answer) in our directory. These files are provided to you as dock.xml, options.txt, and crystal_complex.pdb
  1. dock.xml - This is the RosettaScripts XML file that tells Rosetta the type of sampling and scoring to do. It defines the scoring function and provides parameters for both low-resolution coarse sampling and high-resolution Monte Carlo sampling.
  2. options.txt - This is the options file that tells Rosetta where to locate our input PDB structures and ligand parameters. It also directs Rosetta to the proper XML file.
  3. crystal_complex.pdb - This is the D3-eticlopride complex from the PDB. It will serve as the correct answer in our case allowing us to make comparisons between our models and actual structures.

Run the docking study (This should take a few minutes at most, as we're using a reduced number of output structures):

~/rosetta_workshop/Rosetta/main/source/bin/rosetta_scripts.linuxgccrelease \
@options.txt -nstruct 5 -database ~/rosetta_workshop/Rosetta/main/database/

The Rosetta models are saved with the prefix 3PBL_A_ETQ_aligned followed by a four digit identifier. Each model PDB contains the coordinates and Rosetta score corresponding to that model. In addition, the model scores are summarized in table format in the score.sc file. The two main scoring terms to consider are:
1. total_score: the total score is reflective of the entire protein-ligand complex and is good as an overall model assessment
2. interface_delta_X: the interface score is the difference between the bound protein-ligand complex and the unbound protein-ligand. Interface score is useful for analyzing ligand effects and for comparing different complexes.
One other metric to keep an eye on is the Transform_accept_ratio. This is the fraction of Monte Carlo moves that were accepted during the low resolution Transform grid search. If this number is zero or very low, the search space may be too restrictive to allow for proper sampling.
In benchmarking examples when we have a correct crystal structure, ligand_rms_no_super_X will give us the RMSD difference between our model ligand and the crystal structure ligand given in crystal_complex.pdb. This is an important metric when benchmarking how well your models correlate to reality. When the crystal structure is unknown, we can also calculate model RMSDs using the best scoring structure as the "true answer".
Use pymol to visually compare your best-scoring model and worse-scoring model with the crystal structure provided in crystal_complex.pdb. The "All->Action->preset->ligand sites->cartoon" setting in Pymol is ideal for visualizing interfaces. What interactions were successfully predicted by Rosetta?
Since we generated such a small number of structures, it is unlikely to capture all the possible binding modes that you would expect to encounter in an actual docking run. In the "out" directory, there are 500 models pre-generated using the exact same protocol. We will look at an example of how we can analyze this dataset.
1. cd into the out directory
```
cd out
```
2. In addition to the 500 structures here, you will find the score.sc, a score_vs_rmsd.csv file, and several .png image files.
  1. score.sc: summary score file for the 500 structures as outputted by Rosetta
  2. score_vs_rmsd.csv: a comma separated file with the structure ID in the first column, total_score for the complex in the second column, the interface score in the third column, ligand RMSD to the native structure in the fourth column, and ligand RMSD to the best scoring model in the fifth column. This file was tabulated using bash scripting from the values listed at the end of each model, but you are free to use your own favorite language for processing. If you have any in-depth questions about how we can get this information out, feel free to ask.
  3. PNG files: plots made from the csv file mentioned above. Python was used here but you are free to use any plotting language you prefer.
3. In this case, we have the correct answer based on the crystal structure so we can examine a score vs rmsd plot to see if the better scoring models are indeed closer to the native ligand binding mode. Open up the plot with the following command:
```
gthumb score_vs_crystal_rmsd_plot.png 
```
  On the X-axis you will see the ligand RMSD to the ligand in the crystal structure. On the Y-axis you will see the interface delta score in Rosetta Energy Units. Notice the general correlation between RMSD and Rosetta Score, with a large cluster of highly accurate and good scoring models in the lower left hand corner.
4. In practical applications, we would not have the crystal structure for comparison. We can treat the best scoring model as the correct model and see if we generate a similar funnel. To identify the best scoring model for this example, I selected the top 200 models based on the best overall score and then identified the best model by interface score. The best model for these plots is 3PBL_A_ETQ_0347.pdb. Open up the first plot with:
```
gthumb score_vs_low_rmsd_plot.png 
```
  Again, we see a cluster of good scoring models near the best scoring model with a general downward trend further away. We can zoom in on the cluster in the lower left hand corner to get an even better picture.
```
gthumb score_vs_low_rmsd_zoom_plot.png 
```
  We see the same overall trend in this cluster, suggesting that the top scoring models in this run are likely to be good predictors of the true ligand binding position.
5. Finally let's look at some structures. To sort the CSV file by interface score and take the top twenty, type:
```
sort -t, -nk3 score_vs_rmsd.csv | head -n 20 
```
  These should all be very low RMSD models. To compare a certain structure to the native in Pymol, use:
```
pymol 3PBL_A_ETQ_0211.pdb ../crystal_complex.pdb 
```
  I used 3PBL_A_ETQ_0211.pdb as the sample structure because it is one of the best scoring models, but feel free to examine any model you like. Don't forget the ligand site preset mode for visualizing interfaces. If you like, we can also look at some of the poor scoring models to see exactly what went wrong. To find the top 20 worse models by interface score:
```
sort -t, -nk3 score_vs_rmsd.csv | tail -n 20 
```
  3PBL_A_ETQ_0424.pdb should come up as a poor scoring, high RMSD structure. When we open it up in Pymol, we can see that the ligand binding direction is flipped 180 degrees compared to the native position. This can happen when there is an extended binding pocket but in this case, the Rosetta score was able to discern the difference between these models.
```
pymol 3PBL_A_ETQ_0424.pdb ../crystal_complex.pdb
```

Congratulations, you have performed RosettaLigand docking study! Now use your docked models to generate hypotheses and test them in the wet lab!