Disulfide Bonding


Nature of Disulfide Bonds

Disulfide bonds are covalent bonds formed by the oxidation of two cysteine residues

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

Function of Disulfide Bonds

Disulfide bonds function to stabilize the tertiary and/or quaternary structures of proteins and may be intra-protein (i.e., stabilizing the folding of a single polypeptide chain) or inter-protein (i.e., multi-subunit proteins such as antibodies or the A and B chains of insulin).

Formation of Disulfide Bonds in Cells

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.

Distribution of Disulfide Bonds

In eukaryotes, most secreted proteins are disulfide bonded whereas most cytosolic proteins are not. The greater the number of disulfide bonds the less susceptible the protein to denaturation by other forces such as detergents, heat, etc.

Send comments to Darren Fast, last modified May 23, 1995