[Chimera-users] creating a peptide bond between two fragments

[Elaine Meng]

Hi Glenn,
It may be tricky because Join Models requires some overlap of the input structures -- not spatially, as it will move the models for you, but an extra atom from each model that will be eliminated as it is replaced by the other model.  It is done that way so you there are fewer geometrical parameters for the user to specify -- only one torsion and one bond length.

Here's what I did to attach 1zik chain A to the end of 1gcn chain A. First some setup commands (same stuff could be done with menus and dialogs):

open 1gcn
open 1zik
# display all atoms
display
# get rid of waters and 1zik chain B just to simplify system
delete solvent
delete :.b
# hide ribbon
~ribbon
rlabel

For replacement by Join Models, we need an -OXT at the C-term of 1gcn and at least one hydrogen on the N-term of 1zik. Generally if these are missing, they can be added with AddH, or the corresponding command:

addh

1gcn already had an OXT on Thr29, but addh was needed to put hydrogens on the terminal N of 1zik Arg1.  For Join Models, I select one of those N-term hydrogens of 1zik and the OXT of 1gcn (could also be done with the mouse, Ctrl-click on one and Shift-Ctrl-click on the other):

select #1:1.a at h1#0:29.a at oxt

Then start Build Structure (under Tools... Structure Editing) and go to Join Models tab. It already has 180 as the torsion angle for the peptide bond that will be created, CA-C-N-CA, so I  go ahead and click Apply.

Now the whole thing is a joined single model, but there is still an extra hydrogen on the former N-terminal nitrogen of 1zik (since that nitrogen used to be tetrahedral but should now be a planar amide).  Remove those incorrectly placed hydrogens.  You could select them with the mouse and use command "del sel" or delete by name as in this command:

delete :1.a at h2,h3

If you run addh again, it will correctly place a single amide hydrogen on that nitrogen:

addh

Verify that you get a single continuous ribbon:

ribbon
# refresh residue labels
~rlabel
rlabel

I see the numbering may be somewhat unexpected: the whole 1gcn part got negative numbers.  We hope to make peptide joining less tricky in the future. 

Yet another complication is that if you are lacking an OXT, but addh does not add it, it is because Chimera knows that residue is not the "real" C-terminus because it is looking at the SEQRES information in the input file.  In that case, you could try removing the SEQRES information from the PDB file before opening it in Chimera and running addh.

I hope this helps (rather than discourages!),
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 Jun 7, 2011, at 2:30 PM, Glenn Millhauser wrote:

> Hi Folks,
>    I am trying to add a four amino acid segment to the N-terminus of a protein in the pdb.  I can generate the four residue stretch and move its C-term close to the N-term of the structured protein.  I can then select the C' and N atoms, which are about 1.0 Angstrom apart.  At that point, I am stuck.  Not sure how to add a bond and then create a proper structure.  I've played around with Join Models, but no luck.  Without a peptide bond recognized by Chimera, ribbons are discontinuous.  Any ideas?
> Many thanks,
> glenn
>