[Chimera-users] Advice on SASA

[Elaine Meng]

Hi George,
It doesn’t matter if the structure is from MD or an individual PDB… as long as you can show a molecular surface, then the SASA is automatically calculated and reported in the Reply Log.  You can see from looking at the surface whether it included the ligand or not.  If the ligand is outside of the surface, it is not included in the reported values in the Reply Log either.  My guess is that it would not be lumped into the surface and so you would not need to strip it, but I suppose it depends on the details of your data.

But it sounds like you don’t want the SASA anyway, but the area that is buried between the chains.  In that case skip down to part 2 of my answer below.

(1) After you show the surfaces of interest, solvent-excluded and solvent-accessible areas per atom and residue are assigned as attributes named areaSES and areaSAS, respectively.  You can add up the per-atom or per-residue SASA for any sets of atoms or residues that you want (Tools… Structure Analysis… Attribute Calculator), but that will be in the context of the surface(s) already calculated.  E.g. if you sum the surface areas of residues 12 and 13,  but they are not exposed in the context of the surfaces that you are currently showing, the answer will be zero. 
<http://www.rbvi.ucsf.edu/chimera/docs/ContributedSoftware/calculator/calculator.html>

At least in some cases Chimera will not make separate surfaces for the monomers but instead group all the protein atoms into one surface. If you wanted separate surfaces, you could use “surfcat” to specify the specific sets of atoms you want in separate surfaces.  For example:

surfcat one :.a&protein
surfcat two :.b&protein
surface one
surface two

<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/msms.html>

(2) If you just want the interchain buried surface area, regardless of how the surface is displayed, you should instead use the “measure buriedArea” command and specify the two sets of atoms, e.g. if they are protein chains A and B:

measure buriedArea protein&:.A protein&:.B

<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/measure.html#buriedArea>

If your MD data doesn’t have two different chain IDs but the monomers have different residue numbers, you could just give two residue-number ranges instead.
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 5, 2016, at 7:39 AM, George Tzotzos <gtzotzos at me.com> wrote:
> 
> I have a couple of questions on Chimera’s implementation of SASA. First, I’ll try to explain briefly my problem and then ask the questions.
> 
> My protein is a homodimer. There exist 3 X-ray structures of the protein in complex with serendipitous ligand as well as in complex with biological ones. The ligands bind at the interface between the two chains of the homodimer. It is not clear whether dimer formation is a crystallographic artefact. Biochemical experiments are likewise inconclusive. It has been argued that the biological ligands may stabilise the formation of the dimer. Furthermore, it has been argued that upon ligand binding some of the interface residues become buried. 
> 
> I’ve conducted extensive MD simulations with and without the biological ligands. I would like to test whether ligand binding has an effect on the SASA of the interface residues. 
> 
> 1. Can I restrict measurement of SASA to selected residues only? In other words, can I use a mask specifying the residues of interest?
> 2. From the documentation, I understand that the Chimera implementation of SASA is applied on single pdb structures and not on trajectories. If I use representative structures from MD clusters, should I strip the ligand before measuring SASAs? 
> 
> Thank you in advance for your suggestions and apologies for the long-winded explanation. 
> 
> Best regards
> 
> George