Kirk,
This is almost certainly S-hydroxycysteine. If the distance to the
center of the density to the S-atom is about 1.6 A, that would be
additional supporting information for an S-O bond. Under oxidizing
conditions (exposure to air) it is possible for cysteines to become
oxidized to the sulfenate. A recent structure of ours (3UAO) has a
cysteine sulfenate in the active site which is almost certainly an
artifact of storage in the absence of reducing agents. When we solve the
structure of the enzyme from protein that has been protected from
oxygen, this feature is not present and a normal Cys residue is observed.
Cheers,
_______________________________________
Roger S. Rowlett
Gordon & Dorothy Kline Professor
Department of Chemistry
Colgate University
13 Oak Drive
Hamilton, NY 13346
tel: (315)-228-7245
ofc: (315)-228-7395
fax: (315)-228-7935
email: [email protected]
On 12/12/2012 4:23 PM, Shahila Mehboob wrote:
Dear Users,
I am currently refining a structure of a bacterial oxido-reductase
enzyme and I’m trying to fit extra density seen on a cysteine residue.
Would appreciate some recommendations re. what this may be (here is a
link to the map- http://sdrv.ms/XTNTFs ). There is clearly one extra
atom here (seen in both chains very clearly), but unsure what it could
be … methylation? -SOH? The cysteine residue is not in the active site
and is solvent exposed. The protein sequence was confirmed.
Thanks for any help.
Kirk Hevener
ps: the map was contoured at 1.2sigma and the difference map at 3sigma
