[Chimera-users] Linking two chains in a PDB file

Elaine Meng meng at cgl.ucsf.edu
Mon Aug 12 11:44:03 PDT 2013

Hi Gerald,
You don't need to do any linker-building, you can use Modeller to fill it in for you, keeping the ends (ligands) in their input positions.  There are probably a few alternative routes, but what I would do is:

(1) Save PDB containing the peptide ends (the two ligands) in the positions that you want.  What you want at the end is just these two ligands with the same chain ID, and no other atoms.  That can be achieved either by text-editing the PDB after writing it out, or by changing chain IDs if necessary (Tools… Structure Editing… Change Chain IDs) of the ligands before writing it out.  The PDB save dialog gives you the option of saving only selected atoms, so you could select the ligands only and use that option.

 ** Here's probably the biggest caveat of the approach, which I realized after I sent the last message:  via the Chimera-Modeller interface it is not possible to model one chain while taking other additional chains into account.  In other words, it won't help to include the protein in the file because it will be ignored.  Modeller itself is capable of doing that but the Chimera-Modeller interface only accesses a subset of Modeller's many capabilities… you'd have to run Modeller yourself independent of Chimera, which is much more difficult.  However, all may not be lost: the ensemble of possible solutions (linker conformations) may include some that happen to avoid intersecting with the protein.

(2) Create plain text sequence file (FASTA, named something.fa) which contains the sequence of entire peptide: the ligand ends and the missing linker portion.  I would just create it in a text editor.  FASTA format is very simple, basically one line starting with > followed by a comment or description (or nothing, whatever you want) followed by one or more lines containing the amino acid sequence.

(3) In Chimera, open the new PDB file containing the two ligands, and open the FASTA sequence file.  Then from the sequence window menu, choose Structure… Modeller (loops/refinement).  In the resulting dialog, you will want to model "non-terminal missing structure" or "all missing structure" (equivalent in this case).  You will probably aso want to change "Allow this many residues adjacent to missing regions to move" to 0 and increase the number of models to generate … I'd try something much higher like 20 or 50 in hopes of getting something that doesn't intersect with the protein.

I hope this helps,
Elaine C. Meng, Ph.D. 
UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco

On Aug 9, 2013, at 7:19 PM, Maxwell Cherf <maxcherf at gmail.com> wrote:

> Hi Elaine,
> Using modeller sounds like the exact solution I'm looking for. Both the ligands and the linker I will connect them with are peptides. 
> I tried using modeller, but I haven't gotten it to work yet. Here's what I've done:
> 1. built peptide linker using the "build structure" function
> 2. attached the linker to one ligand1 using the "join" function
> 3. combined the ligand1-linker with the second ligand into one model using the "combine" function
> 4. added an unrealistic ~40 angstrom bond between the linker and second ligand (now we have ligand1 - linker - 40A bond - ligand 2, all in one model).
> 5. I then tried using the "model/refine loop" function to find acceptable orientations for the linker (including the 40A bond) I incorporated, but I was unable to do this because the two ligands were still separate chains, and the "model/refine loop" function requires them to be in the same chain. If I use the "join" command instead of the "combine" command to attach the ligand1-linker to ligand2, I get one chain and then the model/refine loop function works perfectly to remodel the linker. However, using the join function changes the orientation of the ligands relative to the receptor, and I need the ligands to remain docked properly to the receptor. 
> Is there a way to join both ligands into the same chain without changing their orientation? Or, do you know of an alternate approach for adding a linker between the two ligands without changing their orientation using modeller? 
> Thanks again,
> Gerald

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