[Chimera-users] Displaying and calculating protein surface enclosed pockets

[Elaine Meng]

Dear Mathias,
Chimera automatically excludes water from the molecular surface calculation, so in general (unless something is weird about your input file) you wouldn’t have to worry about that.

One approach is to only show the surface of the protein for the atoms lining the pocket.  In most cases, the pocket is not completely enclosed, however, so there will be an opening and somewhat rough edges.  For example, see this figure in the “structure analysis and comparison” tutorial:
<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/tutorials/squalene.html#surfaces>

Another difficulty is that of figuring out which atoms line the pocket and thus which surface patches to show.  If you use the CASTp interface, that is taken care of; CASTp calculates pocket areas/volumes and figures out which sets of atoms define each pocket and tunnel.  You can fetch directly from the CASTp database (if your structure of interest is in the PDB, its results may already be there) or upload a structure to the CASTp server and have the results emailed to you.  In the latter case, you can still use the Chimera interface to view the results; this lists pocket volumes and areas calculated with both molecular surface and solvent-accessible surface, number of openings, etc.  More about the Chimera-CASTp interface:
<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/castp.html>

As for display, all of the above are just showing parts of the surface of the protein.  A different approach is to calculate a separate surface of the void space.  There are several programs, mainly separate from Chimera. (For example, CAVER doesn’t have a Chimera plugin.)

(A) Chimera does have a Surfnet tool (in menu under Tools… Surface/Binding Analysis) that you may want to take a look at.  Personally, I have had trouble getting a nice display of pocket or tunnel volume with this tool.  I usually get a bunch of smaller blobs instead of the desired one filling the whole pocket.
<http://www.rbvi.ucsf.edu/chimera/docs/ContributedSoftware/surfnet/surfnet.html>

(B) Voss Volume Voxelator <http://3vee.molmovdb.org/> calculates “density” maps filling void spaces, and you can open and view these maps in Chimera’s Volume Viewer.  Just like other density maps, you could adjust the isosurface level, do coloring, etc. 

There are probably others.  WIth A or B, you can measure volume enclosed in surfaces with Chimera’s Measure and Color Blobs tool (in menu under Tools… Surface/Binding Analysis).
<http://www.rbvi.ucsf.edu/chimera/docs/ContributedSoftware/pickblobs/pickblobs.html>

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

On Apr 30, 2015, at 8:34 PM, Matthias Fellner <mfellner at chemistry.otago.ac.nz> wrote:

> Good day
>  
> I am a PhD student at the University of Otago New Zealand. First I would like to thank you for this very useful software, I use it for all my protein crystallography pictures and a lot of analysis as well, I always cite your citation for sure when I publish using your software.
>  
> Normally I get very far with your tutorials but this time I failed to achieve my goal and I wanted to write a direct question.
>  
> I have a protein with an active site pocket. I can display the surface of the protein and then I see the pocket inside but I am really interested in the enclosed surface = the active site pocket.
>  
> How can I display the “negative” (enclosed) surface area and possibly calculate its volume. Can this be done with the pdb file or is such an analysis only possible based on the electron density. I learned how to manipulate the density map in chimera, so if this is the only way I should also be able to do it if you explain the details.
>  
> If I get this done I wanted to ask if there is a way to ignore water molecules from the displaying of surfaces and later calculations or if I should simply work with a pdb which has the waters removed.
>  
> Thank you
> Matthias Fellner
>