On 12-02-21 10:11 AM, Björn Kauppi wrote:
Hi all,
I recently encountered a modified surface cysteine residue in one of my
structures. The protein is expressed in E.coli and my data is to 1.7Å so I am
positive of the number of extra atoms. It really look like one of the tails
(the Propanoic acid) of a TCEP molecule. TCEP was added during purification at
2 mM. I tried to look in pdb (or google) but I could not find any reference of
that TCEP could do this. The modified Cystein is involved in a nice crystal
contact (salt bridge) to a Lysine, and thereby stabilizing it further. Looking
back at older structures of the same protein, I see the same modification of
this Cys, but not as pronounced since they were done at lower resolution.
Does anyone recognize this behavior of TCEP? Or, any other ideas?
Björn Kauppi
Structure and Design
Karo Bio AB
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Did you use cacodylate as buffer. TCEP may have reduced some of the
dimethylarsenate, which is really what cacodylate is, to
dimethylarsenite which can react with cysteine residues. It works best
if the cysteine has a depressed pKa, as it is reactive in the thiolate
form, and the proximity to the lysine you mention might do just that.
There are several structures with arsenylated cysteine, the one we came
across was a poxviral glutaredoxin (2HZE, Bacik & Hazes)
Bart
