Disulfide Bonds


A disulfide bond is a covalent bond between two sulfur atoms. Disulfide bonds are prevalent in biology and found in a range of biological molecules, for example between the sulfur atoms of protein cysteine residues. Protein disulfide bonds can fulfil a wide range of functions including promoting protein stability and regulating protein activity or function. Dedicated disulfide generating and transferring machineries that promote the introduction of disulfide bonds during oxidative protein folding can be found in the endoplasmic reticulum, mitochondrial intermembrane space and bacterial periplasm. Regulatory disulfide bonds are formed by different pathways to the structural disulfides. For example, hydrogen peroxide (H2O2) may regulate protein activity by inducing the formation of disulfide bonds. In a few cases, H2O2 may directly oxidise cysteine residues, but evidence is accumulating that thiol peroxidases may first react with H2O2 and subsequently transfer their disulfide to specific target proteins.

Key Concepts

  • Disulfide bonds have multiple functions in proteins including instability and regulation of protein activity.
  • Dedicated machineries to generate and transfer disulfide bonds to newly folding proteins are found in the endoplasmic reticulum, the mitochondrial intermembrane space and the bacterial periplasm.
  • Cysteine residues in some proteins can react with H2O2 leading to the formation of a disulfide bond that can regulate protein activity.
  • Proteins that react efficiently with H2O2 harbour specific mechanisms to increase the reaction efficiency.
  • Thiol peroxidases can react extremely sensitively with H2O2 leading to the formation of a disulfide bond.
  • Thiol peroxidases can transfer their disulfide bond to specific target proteins. These peroxidase‐based redox relays may represent an important mechanism of H2O2‐mediated protein oxidation.

Keywords: oxidative protein folding; disulfide bond; redox regulation; cysteine; peroxiredoxin; thiol disulfide exchange; oxidoreductase; sulfhydryl oxidase; hydrogen peroxide

Figure 1. The structure of a disulfide bond. The covalent bond between two sulfur atoms is known as a disulfide bond. (a) A Bohr model of the electron configuration in a disulfide bond. (b, c) Disulfide bonds can be classified as ‘hooks’, ‘staples’ or ‘spirals’ on the basis of their geometry relative to neighbouring bonds. Five different bond angles can be taken into consideration, denoted as χ1–χ5. Bond being denoted as right‐handed (RH) or left‐handed (LH) on the basis of whether the χ3 angle is positive or negative. Further subdivision is possible, depending on the angle of the χ1 and χ5 bonds, leading bond being labelled +, − or ±. Sulfur atoms are represented in yellow, carbon atoms in green, nitrogen in blue and oxygen in red.
Figure 2. Oxidative protein folding in the bacterial periplasm. The periplasm of Escherichia coli employs two thioredoxin‐based systems for the oxidation and the reduction of proteins, respectively. DsbA is a member of the thioredoxin family that introduces disulfide bonds into proteins that reach the periplasm after translocation through the Sec translocase of the inner membrane (IM). DsbB is a membrane protein that maintains DsbA in an oxidised, active conformation. DsbB serves as a ‘disulfide generator’ that transfers electrons via ubiquinone and the respiratory chain to oxygen or, under anaerobic conditions, via menaquinone to nitrate and fumarate. The second thioredoxin‐like protein in the periplasm, DsbC, reduces proteins in order to allow their isomerisation. The membrane protein DsbD reduces DsbC, thereby receiving its electrons from the cytosolic thioredoxin system and NADPH.
Figure 3. Oxidative protein folding in the endoplasmic reticulum. Members of the protein disulfide isomerase (PDI) family play a central role in the oxidation and isomerisation of ER proteins. PDI is oxidised by the FAD‐binding protein Ero1. While Ero1 is essential for PDI oxidation in yeast, several alternative oxidation mechanisms exist in mammalian cells including the peroxiredoxin PRDX4 and the vitamin K oxidoreductase VKOR (the latter is not shown here for simplicity). PDI is also essential for protein isomerisation, so that the ER machinery needs to accurately balance the redox state of PDI. To this end, Ero1 contains regulatory cysteine residues (shown in red) which, once engaged in disulfide bonds, shut off Ero1 activity as soon as PDI is overoxidised.
Figure 4. The mitochondrial disulfide relay. Many proteins of the intermembrane space (IMS) are imported in a Mia40‐dependent reaction. Mia40 serves as a trans‐site receptor, which binds incoming polypeptides by hydrophobic interactions and promotes their translocation across the outer membrane. Moreover, Mia40 serves as oxidoreductase and introduces disulfide bonds into its substrates. Mia40 is maintained in an oxidised, active conformation by Erv1, an FAD‐bound sulfhydryl oxidase. Erv1 passes its electrons on via the respiratory chain to molecular oxygen giving rise to the production of water. It is not clear whether the IMS also contains an isomerase activity. It was proposed that proteins can be reduced by glutathione (GSH); however, the levels of glutaredoxins might be too low to catalyse this reaction efficiently.
Figure 5. The mechanism of H2O2‐mediated disulfide bond formation. Thiolate anions can perform a nucleophilic attack on H2O2 leading to the formation of a sulfenic acid group and the release of a water molecule. This is a redox reaction resulting in a change in the oxidation number of the sulfur atom of 2. The sulfenic acid group is susceptible to attack by a second thiolate anion leading to the formation of a disulfide bond. These reactions are illustrated in (a) with Bohr models showing the electron configurations during the reaction. (b) Most cysteine residues react poorly with H2O2. Enzymes with highly H2O2‐reactive cysteine residues typically contain additional features that promote cysteine reactivity. For example, the H2O2‐binding site of peroxiredoxins is lined by residues that act as hydrogen bond donors and acceptors, which serve to stabilise the reaction transition state and promote leaving group departure.


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Further Reading

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Morgan, Bruce, and Herrmann, Johannes(Jan 2017) Disulfide Bonds. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003013]