[chimerax-users] Choosing solvent exposed residues automatically

[Elaine Meng]

Hi Yaikhomba,
Depends what you mean by quick, but there are certainly better ways than choosing them one by one! 

I would use "measure sasa" to define an "area" attribute of residues, then assign residue hydrophobicities from a file with the "defattr" command, and then use a single "select" command to intersect these two attributes.  

<http://rbvi.ucsf.edu/chimerax/docs/user/commands/measure.html#sasa>
<http://rbvi.ucsf.edu/chimerax/docs/user/commands/defattr.html>
<http://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html#attributes>

In the "defattr" example files I already happened to include a file kdHydrophobicity.txt to assign Kyte-Doolittle hydrophobity, as well as for several other hydrophobicity scales.  Just download whichever one(s) you want to try from the links here:
<http://rbvi.ucsf.edu/chimerax/docs/user/commands/defattr.html#examples>

The commands could be something like:

open 6g2j
measure sasa protein
defattr ~/Desktop/kdHydrophobicity.txt
select ::area>25 & ::kdHydrophobicity>0.0
surface protein enclose protein
color sel magenta

... which looks like this ... 


... in pretty good agreement with MLP coloring, e.g.

mlp surfaces #1.3


In the select command you may want to experiment with the surface-area and hydrophobicity cutoffs, as well as which hydrophobicity scale you use.  I didn't try a lot of values for surface-area cutoff, but here is with all residues with values >25 colored red, reasonable at least as a first approximation:



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

> On Sep 6, 2020, at 3:26 PM, Mutum Yaikhomba <mutum.yaikhomba at mrc-mbu.cam.ac.uk> wrote:
> 
> Hi
> I am working with a very big pdb model: pdb - 6g2j. I was wondering if there is a quick way to select all residues (surface residues) that are exposed to the solvent (including those facing the membrane)? This is instead of going over each residue one by one and selecting them.
> 
> Additionally, say you have selected these solvent-exposed residues and now you want to filter them based on Hydrophobicity - is there a way to filter based on the standard Kyte-Doolittle Hydrophobicity index for these residues?
> 
> If there is, it would be of great help and very much appreciated.
> Thanks
> Yaikhomba

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