Protein Disulfide Isomerases

Abstract

Protein disulfide isomerase (PDI) is a multifunctional protein that facilitates the formation of correct disulfide crosslinks between cysteine residues during the early stages of protein folding and secretion in the endoplasmic reticulum. It is a member of a large family of oxidoreductases that catalyse exchange reactions between thiols and disulfides. PDI is a multidomain protein, consisting of four tandem thioredoxin domains. The N‐ and C‐terminal thioredoxin domains have catalytic disulfide/dithiol centers while the two internal domains are structural and provide additional interactions with the protein substrates. The catalysis of disulfide formation relies almost entirely on the high reactivity of PDI's active site disulfides. However, the ability to catalyse disulfide isomerisation requires multiple domains. In the cell, PDI's essential activity is the formation of disulfide bonds, but it does catalyse isomerisation.

Key Concepts:

  • PDI structure consists of four tandem thiredoxin domains.

  • The ability to form disulfides between cysteines in substrate proteins results from highly reactive active site disulfides.

  • PDI can correct (isomerise) misformed disulfides by reducing incorrect disulfides and reoxidising in a different configuration.

  • The multidomain structure is needed for catalysis of isomerisation but a single catalytic domain is sufficient to catalyse substrate oxidation.

  • In the cell, the essential activity of PDI is its ability to form substrate disulfides.

Keywords: PDI; disulfide; protein folding; chaperone; disulfide isomerisation

Figure 1.

Three‐dimensional structures of the active site of the a domain of PDI. Arrows point to the more N‐terminal cysteine residue of each active site (nucleophilic cysteine, CGHC). The structure was drawn using the Swiss‐Pdb Viewer (Guex and Peitsch, ) using the coordinates in PDB file 2B5E (Tian et al., ).

Figure 2.

The domain structure of PDI. The active site cysteines in the a and a′ domains are shown as space‐filling models. The structure was drawn using the Swiss‐Pdb Viewer (Guex and Peitsch, ) using the coordinates in PDB file 2B5E (Tian et al., ).

Figure 3.

Catalysis of disulfide isomerisation. Disulfide isomerisation is initiated by attack of the nucleophilic cysteine of the PDI active site (CGHC) on a substrate disulfide. The substrate‐PDI covalent complex can be resolved by two possible isomerisation mechanisms. Which pathway occurs is determined by the action of the resolving cysteine (CGHC) at the PDI active site. If an intramolecular rearrangement of the substrate disulfides occurs slowly, the resolving cysteine will displace the substrate mixed disulfide from the nucleophilic cysteine (CGHC) reducing it and forming an oxidised PDI active site. Further cycles of reduction/reoxidation in different disulfide configurations would result in overall isomerisation. Adapted from Wilkinson and Gilbert , with permission from Elsevier.

Figure 4.

Redox balance in the endoplasmic reticulum. Oxidising equivalents are provided to the eukaryotic ER by the oxidase Ero1, which oxidises the PDI active site. Reducing equivalents, to help balance the redox state and allow disulfide isomerisation are introduced from the cytoplasm by translocation of the newly synthesises substrate and by a glutathione (GSH)‐dependent mechanism.

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Further Reading

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Mamathambika BS and Bardwell JC (2008) Disulfide‐linked protein folding pathways. Annual Review of Cell and Developmental Biology 24: 211–235.

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Gilbert, Hiram F(Apr 2011) Protein Disulfide Isomerases. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003021.pub2]