The spontaneous reaction looks like this.
R-CH2-SH + R'-CH2 +02 = R-CH2-S-S-CH2-R' + H202
The formation of disulfide bonds can be reversed by reducing conditions. These conditions may include agents with free sulfhydryl groups
2 HO-CH2-CH2-SH + R-S-S-R' = R-SH + HS-R' + HO-CH2-CH2-S-S-CH2-CH2-OH
The formation of disulfide bonds occurs during the folding of proteins in the endoplasmic reticulum of eukaryotes and the periplasmic space of prokaryotes. Thiol disulfide exchange and disulfide bond formation are catalyzed by thiol-disulfide oxidoreductases. In eukaryotes this process is catalyzed by protein disulfide isomerase and in procaryotes, by dsbA. The active sites of both PDI and dsbA share homology with the active site of thioredoxin. For a disulfide bond to form, the redox environment must be oxidizing. Eukaryotic ER is more oxidizing than the surrounding cytosol. This is maintained by the high ratio of oxidized glutathione to reduced glutathione.
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