2021 AMBER tutorial 3 with PDBID 1S19
In this tutorial, we will be modeling ligand binding to our receptor using AMBER 16, a molecular dynamics simulation software package created in part by our very own Carlos Simmerling.
We will be saving all of our initial structures in a directory called 01.structure.
For the initial protein structure, we will be using the PDB we downloaded from the Protein Data Bank Website (see here). Load this PDB into chimera and delete the ligand and any nonstandard residues (ex. waters). Save this as a PDB file, 1s19_fresh.pdb, and transfer it to your 01.structure folder.
For the ligand, we will again load the 1s19 pdb file from the Protein Data Bank in Chimera. We will delete everything except for the ligand. For this example, the ligand is under the name MC9. To delete everything:
Select -> Residue -> MC9 Select -> Invert (all models) Actions -> Atoms/Bonds -> Delete
With the ligand isolated, we will add hydrogens and charge. To do this:
Tools -> Structure Editing -> Add H Tools -> Structure Editing -> Add Charge -> (have Amber ff14SB and AM1-BCC selected) -> Ok
Save this as a mol2 file in Chimera and save under the name 1s19_ligand_dockprep.mol2.
NOTE It is VERY important to make sure that Chimera adds hydrogens correctly. For this particular ligand the net charge was zero and Chimera was able to model the hydrogens correctly. Many times however, Chimera will add extra hydrogens. If it does, just select the extra hydrogen and go to:
Actions -> Atoms/Bonds -> Delete
Your ligand will then be protonated and charged correctly.
Generating Parameters for the Simulation
Create a new directory called 02.parameters
In order to utilize Amber for molecular dynamics, parameters for the bio molecules will be needed. Luckily, there have been years of parameter development so parameters for the protein do not have to worried about. However, the small ligand does not have parameters in the standard protein force field. Consequently, we will need to generate a fcmod file specific for the ligand.
To do this, we are going to run the following command:
antechamber -i ./../01.structure/1s19_ligand_dockprep.mol2 -fi mol2 -o1s19_ligand_antechamber.mol2 -fo mol2 -at gaff2 -c bcc -rn LIG -nc 0
Gaff2 stands for General Amber Force Field 2, which allows us to generate the parameters for the ligand. The flag at the end -nc stands for net charge. In our case the net charge is zero so we put a zero there, but change it accordingly for your ligand.
Next, we are going to check these parameters and generate a frcmod file with the following command:
parmchk2 -i 1s19_ligand_antechamber.mol2 -f mol2 -o 1s19_ligand.am1bcc.frcmod
This command will generate our 1s19_ligand.am1bcc.frcmod file. It is important to keep checking your output files to ensure that everything looks okay. Once checked, we can move onto the next step.