Oxidative Protein Folding

Abstract

Oxidative protein folding is the process by which a protein recovers both its native structure and its native disulphide bonds. Disulphide bonds are vital for the correct folding of many secreted proteins, such as insulin, albumin, tissue plasminogen activator and antibodies. The formation of a disulphide bond between two cysteine residues is a rate‐limiting step of the folding process. Therefore, living cells contain proteins that catalyse this reaction (DsbA, protein disulphide isomerase (PDI), Mia40). Pathways that form disulphide bonds have now been unraveled in the bacterial periplasm, the endoplasmic reticulum and the mitochondrial intermembrane space. These pathways have in common to form a relay that channels the electrons released upon cysteine oxidation to a final electron acceptor.

Key Concepts:

  • Disulphide bond formation is required for the correct folding of many secreted proteins.

  • Disulphide bond formation is a catalysed process in vivo.

  • Pathways of disulphide bond formation are found both in prokaryotes and in eukaryotes.

  • Pathways of disulphide bond formation form a relay that channels the electrons away from the substrate protein to a final electron acceptor.

  • Pathways of disulphide bond formation involve a direct disulphide donor and a disulphide generator.

  • Formation of native disulphides in proteins with multiple cysteine residues involves a disulphide isomerase.

  • A disulphide formation pathway and a disulphide isomerisation pathway co‐exist in bacteria such as Escherichia coli.

  • Protein disulphide isomerase (PDI) catalyses both disulphide bond formation and disulphide bond isomerisation in the endoplasmic reticulum.

Keywords: DsbA; DsbC; PDI; Ero1; Mia40; Erv1; DsbD; disulfide; electrons; cysteine

Figure 1.

Disulphide bond formation in the bacterial periplasm. The cell envelope proteins are synthesised in the cytoplasm and translocated to the periplasm as reduced and unfolded precursors. In order to reach their native structure, they need to acquire one or more disulphide bonds. These disulphides are introduced by DsbA, which is then re‐oxidised by the inner membrane protein DsbB. DsbB generates disulphides de novo from quinone reduction (UQ, ubiquinone; MQ, menaquinone). The electrons are then transferred to the respiratory chain. The final electron acceptor is molecular oxygen under aerobic conditions and nitrate or fumarate under anaerobic conditions. The direction of the electron flow is shown by dotted arrows.

Figure 2.

Disulphide bond isomerisation in the periplasm. When disulphide bonds need to be formed between nonconsecutive cysteines, DsbA, which does not have a proofreading activity, may incorrectly pair cysteines. Nonnative disulphides can either be reshuffled (isomerised) (1) or reduced (2) by the soluble homodimeric protein DsbC. In the first case, DsbC remains reduced and active, but, in the second case, DsbC is left oxidised. Reduced DsbC is regenerated by the membrane‐embedded protein DsbD. DsbD provides reducing power to the periplasm by channeling electrons from the cytoplasmic thioredoxin system. The latter system consists of two proteins, thioredoxin (Trx) and thioredoxin reductase (TrxR). TrxR uses reduced nicotine adenine dinucleotide phosphate (NADPH) as a cofactor. The direction of the electron flow is shown by dotted arrows.

Figure 3.

Disulphide bond formation in the endoplasmic reticulum. In the ER, disulphide bonds are predominantly introduced into proteins by the U‐shaped protein disulphide isomerase (PDI). PDI is then reoxidised by the membrane‐anchored flavoprotein Ero1. Ero1 transfers the electrons to molecular oxygen via its FAD cofactor, which results in the generation of hydrogen peroxide. When the ER becomes too oxidising, Ero1 activity is decreased by the formation of regulatory disulphides, which prevents the reoxidation of PDI. When the catalytic site of PDI is reduced, this enzyme can function as an isomerase/reductase (not shown). The direction of the electron flow is shown by dotted arrows.

Figure 4.

Disulphide bond formation in the mitochondrial intermembrane space. Proteins destined to the mitochondrial intermembrane space acquire their native disulphides and hairpin structure by interacting with Mia40. Mia40 is reoxidised by the soluble homodimeric Erv1 flavoprotein. Erv1 provides oxidative power to Mia40 in conjunction with its FAD cofactor by successively transferring electrons to cytochrome c (Cyt c) and then to cyclooxygenase (COX), which results in the reduction of molecular oxygen to water. The direction of the electron flow is shown by dotted arrows.

Figure 5.

Overview of the basic machinery for disulphide bond formation. (Left panel): The basic machinery for disulfide bond formation is shown. This machinery is formed of a disulphide bond donor that gives disulfide bonds to folding substrates. The formation of a disulphide bond leads to the release of two electrons. The second component of the machinery, the disulphide bond generator, channels those electrons onto a final electron acceptor (O2 in aerobic conditions), and therefore provides the disulfide donor with oxidative power. Both components hence cooperate in the formation of disulphide bonds in oxidatively folding proteins by channeling electrons from the substrate to the final acceptor. (Right panel): This panel illustrates the disulphide bond formation machineries present in each of the three compartments where disulphide bond formation has been described. In the Gram‐negative bacterial envelope, periplasmic proteins acquire their disulphide bonds from DsbA, the latter drawing its oxidative power from DsbB. In the eukaryotic endoplasmic reticulum, secretory proteins receive their disulphides from PDI, which draws oxidative power from Ero1 a. However, additional disulphide bond generation systems have recently been described in that compartment (not shown). In the mitochondrial intermembrane space (IMS), a subset of IMS proteins gain their disulphide bonds from Mia40, the latter getting oxidative power from Erv1. In all compartments, the final electron acceptor is O2 (under aerobic conditions).

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Gennaris, Alexandra, and Collet, Jean‐François(Jan 2012) Oxidative Protein Folding. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023480]