• ## Outreach

From: Ashar Malik (asharjm_at_gmail.com)
Date: Thu Dec 01 2016 - 02:42:56 CST

Hi,

If you have used superpose to compare your two structures, as an output
Superpose gives a transformation matrix (4x4). You can load both structures
into VMD. You can then load the transformation matrix into vmd. You can
then move one structure by applying the transformation matrix on it. You
will then be able to see the two aligned structures.

If you don't want to view superpose's output you can calculate your own. To
this -- do the following.

Open structure 1 in VMD.
Look for sections of structure that form an alpha helix. If you see a
section in the display that makes an alpha helix press 1 on the keyboard
and using your mouse click on one of the residues in the helix. When you
click VMD will generate a label of the residue number. Note that residue
number. Say that number is 77. Probably 2-3 residues after residue 77 and
before residue 77 are also involved in the helix. Which means residue 73 to
80 from structure 1 are in the helix. Note these numbers.

Delete the structure and load structure 2 in VMD.
Repeat the same exercise as you did with structure 1. If you know what
should align across the two structure good, if you don't take note of an
alpha helix forming set of residues like in the previous case. Say you
identify residue 384. For equivalence purpose if you selected 7 residues in
the previous case [73 to 80] select seven residues from this structure. So
if residue 384 is of interest, select 380 to 387. Take a note of these.

Now you have two selections. The residues in these selections i.e. 73 to 80
in structure 1 and 380 to 387 may or may not be the same. This means that
you still need to find something common --- this is your alpha carbons or
backbone atoms which would be common between the two selections.

Close VMD
Open vmd
load structure 1 (across which you liked residues 73 to 80)
load structure 2 (across which you liked 380 to 387)
--------------------------------------------------------------------------------------
use the following commands:

set sel0 [atomselect 0 "resid 73 to 80 and name CA"]
set sel0_all [atomselect 0 all]
set sel1 [atomselect 1 "resid 380 to 387 and name CA"]
set mx_ [measure fit \$sel0 \$sel1]
\$sel0_all move \$mx

--------------------------------------------------------------------------------------
Now your VMD display window will show the structure 1's residue 73 to 80
aligned with structure 2's residue 380 to 387.
This way you can choose what ever par from structure 1 to align with
whatever part from structure 2.

If you really want to do it your way and edit the PDB so that the two
structures are similar size-- you can do it manually delete lines for
residue 830 till 1304 or do this :

close VMD if its open.
Open VMD
Load the larger structure into VMD:

use the following command:

set sel0 [atomselect 0 "resid 1 to 829"]
\$sel0 writepdb reduced_structure.pdb

The above will create a new pdb file called reduced_structure.pdb which
will only have residues 1 to 829 of the larger structure. Assuming there
are no missing residues across the two structures, they should now be
compareable (as long as you stick to the backbone atoms like entire
backbone or CA etc). Including the side chains would still result in a
different number of atoms. However this is a bad way to do it. You should
know before hand which part to align, if you don't know you should use
programs like Superpose to decide how to do this. Simply truncating a
portion of the structure to fit your need is not the correct way. As a
result of this truncation you may end up with a very poor match.

Summary:
If you don't know which parts align - use Superpose to align and use the
transformation matrix in its output to move 1 structure onto the other.
If you know which sections across the two structures align, use the method
I stated above.
If you want to cutoff a part of protein - do it by proper consideration
instead of just editing a structure.

Hope this helps.

Best,
/A

p.s. when I say run the commands - you have to use the command line window
(in windows this would be a dos like screen in the background), in
linux/mac it would be the shell. Hit enter to get the prompt and type away.
If you are not comfortable with that - goto extensions menu in VMD main
and select TKconsole and enter commands there.

On Thu, Dec 1, 2016 at 6:53 PM, Wong Li Zhe <Wong.LiZhe_at_student.imu.edu.my>
wrote:

> Dear VMD users,
>
>
> I would like to superimpose 2 protein structures.
>
> It is however, they are of different size, 1304 and 829 respectively.
>
> I wonder how can I edit the PDB file manually in order to view them in VMD?
>
> I have tired using SuperPose version 1.0 to superimpose the structure, but
> I would like to view it in VMD.
>
> Any suggestions on how I should edit the protein sizes?
>
>
>
>
> Best regards,
>
> Li Zhe
>
>
>
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