Dear Tina,
my guess would be minor oxidation of the cysteine. The green positive
difference density suggests a bound atom in three different positions; this
could however just be the first atom of something bigger like
beta-mercaptoethanol, with the rest of that molecule too disordered to see.
2cystein.jpeg looks just like oxidation of cysteine to
S-hydroxy-cysteine (a.k.a. cysteine sulfenic acid). I have seen this
repeatedly in one of my structures (E. coli aminopeptidase P, see 1WL9
for an example). We discuss this a bit in Graham et al (2005)
Biochemistry 44: 13820-36 - see Figure 2
Rather difficult to decide. It depends on whether BME was present during
all protein purification steps. Also, BME is somewhat short lived. You
have to add new BME every few days or so. At least one looks like it
could be oxidized, a process that cannot be reversed by BME
J.
Artem Evdoki
Hi,
They're likely both BME adducts, just in the first case the CH2CH2OH portion
is way more disordered.
Artem
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