GROMACS Tutorial

Step One: Prepare the Protein Topology

We must download the protein structure file we will be working with. For this tutorial, we will utilize T4 lysozyme L99A/M102Q (PDB code 3HTB). Go to the RCSB website and download the PDB text for the crystal structure.

Once you have downloaded the structure, you can visualize it using a viewing program such as VMD, Chimera, PyMOL, etc. Once you've had a look at the molecule, you are going to want to strip out the crystal waters, PO4, and BME. Note that such a procedure is not universally appropriate (i.e., the case of a bound active site water molecule). For our intentions here, we do not need crystal water or other ligands. We will instead focus on the ligand called "JZ4."

If you want a cleaned version of the .pdb file to check your work, you can download it here. The problem we now face is that the JZ4 ligand is not a recognized entity in any of the force fields provided with GROMACS, so pdb2gmx will give a fatal error if you were try to pass this file through it. Topologies can only be assembled automatically if an entry for a building block is present in the .rtp file for the force field. Since this is not the case, we will prepare our system topology in two steps:

  1. Prepare the protein topology with pdb2gmx
  2. Prepare the ligand topology using external tools

Since we will be preparing these two topologies separately, we must move the protein and JZ4 into separate coordinate files. Save the JZ4 coordinates like so:

grep JZ4 3HTB_clean.pdb > JZ4.pdb

Then simply delete the JZ4 lines from 3HTB_clean.pdb. At this point, preparing the protein topology is trivial. There are no missing atoms or residues in the 3HTB structure, so simply run pdb2gmx:

gmx pdb2gmx -f 3HTB_clean.pdb -o 3HTB_processed.gro -water spc

The structure will be processed by pdb2gmx, and you will be prompted to choose a force field:

Select the Force Field:
From '/usr/local/gromacs/share/gromacs/top':
 1: AMBER03 protein, nucleic AMBER94 (Duan et al., J. Comp. Chem. 24, 1999-2012, 2003)
 2: AMBER94 force field (Cornell et al., JACS 117, 5179-5197, 1995)
 3: AMBER96 protein, nucleic AMBER94 (Kollman et al., Acc. Chem. Res. 29, 461-469, 1996)
 4: AMBER99 protein, nucleic AMBER94 (Wang et al., J. Comp. Chem. 21, 1049-1074, 2000)
 5: AMBER99SB protein, nucleic AMBER94 (Hornak et al., Proteins 65, 712-725, 2006)
 6: AMBER99SB-ILDN protein, nucleic AMBER94 (Lindorff-Larsen et al., Proteins 78, 1950-58, 2010)
 7: AMBERGS force field (Garcia & Sanbonmatsu, PNAS 99, 2782-2787, 2002)
 8: CHARMM27 all-atom force field (CHARM22 plus CMAP for proteins)
 9: GROMOS96 43a1 force field
10: GROMOS96 43a2 force field (improved alkane dihedrals)
11: GROMOS96 45a3 force field (Schuler JCC 2001 22 1205)
12: GROMOS96 53a5 force field (JCC 2004 vol 25 pag 1656)
13: GROMOS96 53a6 force field (JCC 2004 vol 25 pag 1656)
14: GROMOS96 54a7 force field (Eur. Biophys. J. (2011), 40,, 843-856, DOI: 10.1007/s00249-011-0700-9)
15: OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)

For this tutorial, we will use the united-atom GROMOS96 43A1 force field, so type 9 at the command prompt, followed by 'Enter'.

Back: Introduction Next: The Topology

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