Now it is is time to run your own simulation! We’ll begin with just a small protein, nothing else --- in other words, it is as if the protein is suspended in outer space (but warmer). The first step is to select an appropriate system. Here is a list of candidates from RCSB.
- Proteins we have seen before such as 1pgb (immunoglobulin-binding domain of streptoccocal protein G - 460 atoms) or 103d (human centromere DNA - 772 atoms)
- helix-turn-helix motifs (use this as a keyword search at RCSB) such as 2k10 (antibacterial peptide rantuerin-2csa - 479 atoms) or 1q8h (porcine osteocalcin - 378 atoms)
- larger proteins such as 1adv or 1adu (adenovirus 5 - 4000 atoms) or 1ab3 (ribosomal protein S15 from thermus thermophilus - 1507 atoms)
You will notice that there is not a lot of *new* material this week — I expect that you will spend much of the early part of the week simply being able to construct your PSF/PDB pair. If you are able to do that quickly, so much the better! Once that is done, we will focus on the nuts and bolts of the simulation (using NAMD), and the hows and whys. You will probably need to do a lot of checking and comparing your new simulation files with your simple working case from Week 2 (alanin?). There are tools in VMD which can help — most notably the NAMD GUI.
There is one important thing to check: NAMD needs to refer to a parameter file that matches the topology file the PSF was constructed from. If these files are mismatched, you will get an error running NAMD. To be safe, download the file top_all27_prot_na.rtf and par_all27_prot.na.prm, and save them someplace convenient (your Chem 181 class directory?). Then when you enter the ‘topology’ command, specify the path to this file. For example, if the current working directory is Chem 181\Protein (DOS) or Chem 181/Protein (OS X/Unix), then the first two psfgen commands might be
package require psfgen
topology ../top_all27_prot_na.rtf
Next, we run through the steps needed to prepare a vacuum simulation, using tkcon for 1dfe (find it HERE), another small protein. Commands are in red; output is in blue. Histine is either protonated or deprotonated; simulations must specify this: protonated: HSE; deprotonated HSD. Also, the atom type ‘CD1’ is not used in NAMD, so this must be aliased to ‘CD’.
mol load pdb 1dfe_prot.pdb
12
pdbalias atom ILE CD1 CD
aliasing residue ILE atom CD1 to CD
pdbalias residue HIS HSE
aliasing residue HIS to HSE
segment MAIN {pdb 1dfe_prot.pdb}
building segment MAIN
reading residues from pdb file 1dfe_prot.pdb
extracted 37 residues from pdb file
Info: generating structure...
Info: skipping improper N-C-CA-HN at beginning of segment.
Info: skipping cross-term C-N-CA-C-N-CA-C-N at beginning of segment.
Info: skipping conformation C-N-CA-C at beginning of segment.
Info: skipping conformation C-CA-N-HN at beginning of segment.
Info: skipping bond C-N at end of segment.
Info: skipping improper C-CA-N-O at end of segment.
Info: skipping cross-term C-N-CA-C-N-CA-C-N at end of segment.
Info: skipping conformation CA-C-N-CA at end of segment.
Info: skipping conformation N-CA-C-O at end of segment.
Info: skipping conformation N-CA-C-N at end of segment.
Info: segment complete.
coordpdb 1dfe_prot.pdb MAIN
reading coordinates from pdb file 1dfe_prot.pdb for segment MAIN
Warning: failed to set coordinate for atom H2 MET:1 MAIN
Warning: failed to set coordinate for atom HB3 MET:1 MAIN
Warning: failed to set coordinate for atom HG3 MET:1 MAIN
.
. (lines removed)
.
Warning: failed to set coordinate for atom OXT GLY:37 MAIN
Warning: failed to set coordinate for atom H GLY:37 MAIN
Warning: failed to set coordinate for atom HA3 GLY:37 MAIN
guesscoord
Info: guessing coordinates for 121 atoms (2 non-hydrogen)
Warning: poorly guessed coordinates for 67 atoms (2 non-hydrogen):
Warning: poorly guessed coordinate for atom HT1 MET:1 MAIN
Warning: poorly guessed coordinate for atom HT2 MET:1 MAIN
.
. (lines removed)
.
Warning: poorly guessed coordinate for atom HG1 GLN:36 MAIN
Warning: poorly guessed coordinate for atom OT1 GLY:37 MAIN
Warning: poorly guessed coordinate for atom OT2 GLY:37 MAIN
writepdb 1dfe_sim.pdb
Info: writing pdb file 1dfe_sim.pdb
Info: Atoms with guessed coordinates will have occupancy of 0.0.
Info: pdb file complete.
writepsf 1dfe_sim.psf
Info: writing psf file 1dfe_sim.psf
total of 645 atoms
total of 648 bonds
total of 1175 angles
total of 1705 dihedrals
total of 105 impropers
total of 35 cross-terms
Info: psf file complete.
mol delete all
mol load psf 1dfe_sim.psf pdb 1dfe_sim.pdb
At this point, you can start up the NAMD GUI to create the control file. When you are done, the specified parameter file path is probably incorrect, and must be fixed. Find the line similar to
parameters /usr/local//toppar/par_all27_prot_na.prm
and change the line to specifiy the par_all27_prot_na.prm file you downloaded at the top of the page. Again, using the example from the top of the page, if you the current working directory is ‘Chem 181\Protein’ (or ‘Chem 181/Protein’), you would change this line to
parameters /Users/mccallum/Chem181/Protein/par_all27_prot_na.prm
Also remember to fix the parameter line, and add IMD commands if you wish to watch your simulation run via VMD.
Be sure to ask Prof. McCallum for help with these commands if you need it!