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Single Cys is all it takes. Single Met or His sometimes, but an -SH will go for Hg like a chubby kid after a tub of Nutella.

In your case you have a twofold disordered mercury with approximately 50% occupancy at each site.

You should fit Hg first, then figure out how the Cys fits into the picture. Generally speaking the S may occupy a point exactly halfway between the two Hg half-sites, assuming the distance permits it. If the distance between the sites is too large then there are two half-disordered S atom sites as well.

Artem

www.harkerbio.com
"Ah, Mercury. Sweetest of the transition metals!"
(Sealab 2021, season 1, episode 8)



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On Sat, Apr 16, 2016 at 6:46 PM, khaja faisal tarique <[log in to unmask]> wrote:
Hello everyone

I have used ethyl mercuric phosphate as one of the components in my crystallisation buffer (native crystals ). After solving the structure  at (3Angstrom ) I was able to see strong positive peak around one of the Cysteine residues. My question is  how to link  mercuric ion (here CH3CH2Hg+ ion)  to sulfhydryl group of cysteine as I cannot see other residues  close enough to link them to Hg ion. Is one cysteine is enough to link to Hg or it has some specific pattern or geometry ? I have shown the possibilities which may arise in case two ion binds to Cys residue (as it appears in my case). I have attached the screen shot from the Coot. My second question is about the optimum distance of Hg with respect to sulfhydryl group of cysteine as i could see variation in the various entries of protein data bank (ranging from 2.05 A to even 2.5A). At this resolution of 3A if i bring it closer to SH group then ethyl moiety can easily fit  into the electron density other wise it is only mercury which can be placed inside the density blob while the ethyl group remains flanking outside.

Faisal