# [Chimera-users] Find pseudo 2-fold axes relating heavy and light chain

Elaine Meng meng at cgl.ucsf.edu
Fri Dec 27 11:48:17 PST 2013

```Hi Jian,
Most important is to make sure you have superimposed the correct (pseudoequivalent) parts. My earlier answer was for calculating one pseudosymmetry axis.  From what you say, and also from this descriptive page I found, you need to calculate two of those, and then measure the angle between them.
<http://proteinmodel.org/AS2TS/RBOW/calculation.htm>

I was able to do it in Chimera and reproduce the result for the example structure 1bbd in that page.  My calculation gave 53 degrees (because the axes in the calculation don't have a direction), whereas the real answer is 180 - 53 = 127 degrees as stated in that page. You can see by just looking at the structure whether it is 53 or 127.

(1) open two copies, superimpose V-H of one copy onto V-L of the other copy, use "measure rotation" to show that pseudo 2-fold axis.  The tricky part is only using the V (variable) parts of the chains instead of the whole H and L chains when superimposing the structures.  More detail on that below.

(2) next superimpose the C-H of one copy onto C-L of the other copy, use "measure rotation" to show that pseudo 2-fold axis.

(3) now you have two axes, but they are defined as marker sets by the "measure" command and there is no direct way to get the angle between them... so it is necessary to use Define Axes to copy them as axes that can be used in measurements.  My Fab copies were models #0,1 and the axes from measure were models #2,3.  I selected #2 (for example, command: select #2) and then used define axis for the selected atoms (markers are fake atoms), then selected #3 and used define axis again for the selected atoms, making sure to turn OFF the option to replace the existing define-axis.

(4) then in the table of Axes/Planes/Centroids, two axes are listed.  Use the mouse to choose those two rows in the table, which automatically reports the axes' angle of intersection near the bottom of that dialog.  Image of what I had then is attached below.

OK, now the detail about how to only use parts of the chains in the matches.  I opened 1bba two times, so it was models #0 and #1.  Each model has chains H (heavy) and L (light).  I just used Ctrl-drag in the Chimera window to drag a selection box only around the variable domains.  Then in MatchMaker I chose "Chain pairing: Specific chain(s) in reference structure with specific chain(s) in match structure" with reference chain #0 chain L and chain to match #1 chain H, and also checked the box "Further restrict matching to current selection."  Thus I could superimpose #0 and #1 using only V-H and V-L.  After using "measure rotation" to generate the first axis, I chose from the main Chimera menu "Select... Invert (selected models)" so now only the constant parts are selected, and used Matchmaker again without changing any of the settings in that dialog.  Then used "measure rotation" ... etc. The main thing is to draw the first selection box carefully.  It doesn't matter if you draw it around variable or constant, because the other one will be calculated after you invert selection.

I don't have any ideas about what to do if the angle is near zero, but if you really superimposed the structures correctly, my guess is it should not be near zero.

Well, now I've learned a lot (too much?) about Fab structures!
Elaine

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On Dec 27, 2013, at 10:40 AM, Jian Guan wrote:

> Hi Elaine,
> Thank you so much for your reply.
> I did what you said in the letter. However the return is "Rotation angle is
> near zero". Then no axis came out.
> What I want to do is to get the elbow angle of the Fab. The elbow angle is
> the angle between pseudo 2-fold axes relating heavy and light chains in the
> variable domain and heavy and light chains in the constant domain. So I have
> to make two axes for variable and constant domain respectively. Then measure
> the angles.
> Thanks,
> Jian

```