[chimerax-users] Modeling post-translational modifications

[Elaine Meng]

Hi Guillaume,
In general, rotamer libraries do not include nonstandard residues or even standard residues with PTMs. So, there is no built-in single step way to replace a residue with one of those in ChimeraX.   Some possibilities with more steps are:

(A) start with lysine at that position, and then build on the N-carboxylate (a few steps of modifying and adding individual atoms) e.g. using Build Structure tool, the Modify Structure section.
<https://rbvi.ucsf.edu/chimerax/docs/user/tools/buildstructure.html>

(B) with the "align" command, match some of the KCX atoms onto the corresponding lysine atoms (backbone or part of sidechain, whatever works in your situation), save PDB of the KCX atoms "relative to" the model onto which it was just matched, then using a text-editor on these two resulting PDB files, manually replace the lysine sidechain atoms from the original structure with the KCX sidechain atoms.  You would also want to change the residue name to KCX in this text-editing process.
<https://rbvi.ucsf.edu/chimerax/docs/user/commands/align.html>
<https://rbvi.ucsf.edu/chimerax/docs/user/formats/pdbintro.html>

(C) sometimes depending on the specific modification, the unnatural amino acid will be available in the SwissSidechain database.  In that case, you can use the SwissSidechain plugin available for Chimera (not ChimeraX).  
<https://swisssidechain.ch/visualization/chimera.php>

However, I looked in that database in the lysine derivative section and it does not have lysine N-carboxylic acid (KCX)
<http://ligand-expo.rcsb.org/pyapps/ldHandler.py?formid=cc-index-search&target=kcx&operation=ccid>
<https://swisssidechain.ch/browse/family/table.php?family=lysine>

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Apr 11, 2023, at 7:26 AM, Guillaume Gaullier via ChimeraX-users <chimerax-users at cgl.ucsf.edu> wrote:
> 
> Hello chimerax-users,
> 
> I am trying to model a PTM, and the most sensible way to me seems to use the swapaa command and specify the sensible monomer from the PDB Chemical Component Dictionary as the target residue.
> 
> Here is what I tried (residue #1/a:9 is a Lys):
> 
> open 1kx5
> swapaa #1/a:9 KCX
> 
> And the error I got: Dynameomics rotamer library does not support KCX
> 
> I tried all the rotamer libraries listed in the swapaa help page ( https://www.cgl.ucsf.edu/chimerax/docs/user/commands/swapaa.html#rotLib ), but none of them can provide stereochemical info for KCX.
> 
> I can open the monomer I want (carbamylated Lys) with: open ccd:KCX
> But of course in this case it’s floating by itself as a new model, not incorporated in the protein sequence and structure I am interested in.
> 
> Is this the correct way to model a PTM? If it is, where can I find a rotamer library that will contain the stereochemical info for KCX, and how do I tell ChimeraX to use it? If it is not how modelling a PTM is done, then how else should I do this?
> 
> Thank you in advance,
> 
> Guillaume