[Chimera-users] Attaching a Probe to Modify an Existing Amino Acid

[Eric Pettersen]

Hi Shana,
	If you were starting from scratch, the procedure to add the CN would probably be simpler by using chimera.molEdit.addDihedralAtom() to add the C and then the N at the appropriate bond length, angle and dihedral and then rename the residue with: r.type = "MSCN".  Nonetheless, you have come a long way down the road with your script and probably would not prefer to start over from scratch!  So let's suppose you have your two relevant Residue instances in variables named 'cys' and 'cn'.  The code to do what you want would be:

for a in cn.atoms:
	cn.removeAtom(a)
	cys.addAtom(a)
cys.type = "MSCN"
# empty residues are problematic in various parts of Chimera, so...
cn.molecule.deleteResidue(cn)

	If you don't know how to get the 'cys' and 'cn' variables but you know an atom specifier that selects each, you can use this code to set the variables:

runCommand("sel atom-spec-for-cys")
cys = chimera.selection.currentResidues()[0]
runCommand("sel atom-spec-for-cn")
cn = chimera.selection.currentResidues()[0]

	There's a couple of further relevant things here.  Strictly speaking, 4-letter residue names are not legal in PDB format (while there is a column where a 4th character could go, according to the format definition that column is supposed to be blank).  Therefore if you export your modified structure to other software programs, they may or may not be able to handle the 4-character residue name without problems.  Therefore I recommend you use the same residue name that the PDB uses for this moiety, 4CY (which can be found in the 3or0 entry).
	Along the same lines, 4CY can actually be found in the SwissSidechain rotamer library plugin for Chimera (SwissSidechain - A database of non-natural side chains) and therefore if you install that plugin then the entire substitution process becomes a single command:  swapnaa 4CY cysteine-atom-spec.

--Eric

                        Eric Pettersen
                        UCSF Computer Graphics Lab
                        http://www.cgl.ucsf.edu

On Jun 18, 2014, at 6:50 AM, Shana Burstein <sburstei at haverford.edu> wrote:

> Hello,
> I am an undergraduate student at Haverford College and I am attempting to attach thiocyanate probes onto different sites along a protein by first replacing the existing amino acid with a cysteine and then by adding a C and N onto the S- of the Cys residue. I know it is possible to carry this out manually using Tools>Structure Editing>Modify Structure, but I am hoping to automate this process by calling an external .py script that will primarily contain a list of preexisting Chimera commands. 
> 
> I have been successful at converting the targeted amino acid to Cys using "swapaa," building a CN residue called MSCN (using the createMolecule.py as a guide), combining the two models into one ("combine selected close True") and forming a bond between the Cys S and CN C atoms ("bond").
> 
> At this point, I am seeking help in automating the renaming of the modified residue to MSCN. Currently, Chimera sees this residue as two different separate residues called CYS and MSCN. Is there a way to combine these two structures so that the label corresponds to MSCN and the residue number corresponds to CYS? 
> 
> Sincerely,
> 
> Shana Burstein
> Haverford College '15
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