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I am interested to know if cysteine can form disulphide bridges in proteins within organelles. Typically cysteine will not form disulphide bonds in the reducing environment of the cytosol, but will in nonreducing environments such as the extracellular space.

From the Handbook of Biologically Active Peptides:

The ER is a vastly more common site than the cytosol (which is very rarely the site of disulfide bond formation) because the ER intralumenal environment is more oxidizing than that of the cytosol.

This is fairly intuitive since the ER is where the disulphide bonds will be formed before translocation to the extracellular space. But what about other organelles?

Does this apply to all lumenal spaces as implied in the below picture, or is the ER a special case as a result of its necessity to fold proteins destined for the extracellular space?

enter image description here

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    $\begingroup$ I can't speak generally, but I study a lysosomal protein that has many disulfide bonds so, at a minimum, the lysosome isn't reducing and I'd wager the entire endo-lysosomal pathway as well. $\endgroup$ – yp66t89 Jun 8 at 22:09

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