[Protein-analysis] Re: equilibrium const for disulfide bond formation

hanson via proteins%40net.bio.net (by hanson At quick.net)
Thu Jan 11 22:43:01 EST 2007


"Bob" <bbx107.XYZ At excite.XYZ.com> wrote in message 
news:5srdq2haa7beq0d40s2o9ipn1eobfki0br At 4ax.com...
> "hanson" <hanson At quick.net> wrote:
>
>>"Bob" <bbx107.XYZ At excite.XYZ.com> wrote in message
>>news:a49bq2lve6k3skf7thuqronoo2csh4hq3a At 4ax.com...
>>>
>>[Bob]
>>> If you have it at fairly high concentration in an oxidizing
>>> environment (air), and the Cys are exposed, one would expect that.
>>> Solutions include keeping the conc low, avoiding oxidation -- and
>>> reducing the dimers back to monomers, say with mercaptoethanol.
>>>
>>[hanson]
>>Interesting. Are you basically telling here that the oxidized/dimer of
>>Cysteine, now Cystine: HOOC(NH2)C-CH2-S-S-CH2-C(NH2)COOH
>>together with 2 HS-CH2-CH2-OH disproportionates/redoxes into
>> 2 HS-CH2-C(NH2)COOH  and HO-CH2-CH2-S-S-CH2-CH2-OH.
>>
[Bob]
> or heterodimers
>
> Note that the OP is talking about Cys in proteins, not free cystine.
> The oxidation is dimerizing his protein (60 subunits to 120, in his
> case). In other cases, it can create unwanted crosslinks within a
> peptide chain. Major structural effects.
>
[hanson]
>>Does HSEtOH work as a reducer for all -S-S- molecules in biologial
>>systems?
>>
[Bob]
> Yes, sort of.
>
> But a key point is that enzymes are large and BME
> (beta-mercaptoethanol) is small. So one can easily have a vast molar
> excess of BME. (You may recall that, typically, in enzyme reactions
> the conc of substrate is a few order of magnitude higher than the
> enzyme conc. Same idea.) Using it with small molecules, at similar
> conc, would be more problematic.
>
> Bill Penrose has usefully elaborated. BME is simply one commonly
> reducing agent.
>
> (But BME is doubly effective as a reducing agent. Not only does it
> reduce disulfide bonds in the proteins, but it reduces the
> concentration of people around you, thus making it easier for you to
> concentrate on your work. The second effect is sometimes
> irreversible.)
> bob
>
[hanson]
Bob, you are a good man. Thanks.
I remember the smell faintly (though not being as bad a
Et-SH), so, since I have left the lab long time ago, when
I come back home from my stints in the field (deserts,
mtns. or jungles, after a few weeks and 18 hrs flights),
BME smells like roses when compared to me... ahaha...
Thanks for the laughs, Bob,
ahahaha... ahahanson 




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