Catalysis of Disulfide Bond Formation by the Quiescin Sulfhydryl Oxidases


Quiescin sulfhydryl oxidase (QSOX), an enzyme that catalyses disulfide bond formation in the late secretory pathway and extracellular environment, is emerging as an important player in extracellular matrix assembly, with apparent physiological and pathological functions in development and cancer, respectively. High‐resolution views into the QSOX catalytic machinery are providing information not only on the QSOX enzymes themselves but also on the larger protein disulfide isomerase family of oxidoreductases and on thioredoxin‐fold superfamily proteins more generally. The two redox‐active sites of QSOX are present within domains flexibly tethered to one another, but the sites cooperate tightly and productively to generate disulfide bonds de novo and transfer them to substrate proteins. Backbone and side‐chain dynamics in the vicinity of the redox‐active disulfides of QSOX enzymes appear to modulate the local electrostatics during catalysis, facilitating electron transfer in dithiol/disulfide exchange reactions.

Key Concepts:

  • QSOX enzymes accomplish both the generation of disulfides de novo and the delivery of disulfides to substrate proteins using a redox relay.

  • The amino (N)‐terminal portion of QSOX is structurally and functionally similar to domains of protein disulfide isomerase family proteins.

  • The carboxy (C)‐terminal portion of QSOX appears to have arisen by domain duplication and fusion from an Erv‐family sulfhydryl oxidase precursor.

  • QSOX enzymes have structural features and appropriately positioned functional groups to enhance electron transfer between the two modules.

Keywords: enzyme; disulfide bond formation; flavin adenine dinucleotide; oxidative protein folding; cysteine; thioredoxin; protein disulfide isomerase

Figure 1.

Schematic diagram of the QSOX reaction cycle. Structural domains are represented by boxes, with the redox‐active domains in colour. The fused hexagons represent FAD (orange) and FADH2 (white). Redox‐active cysteine residues are indicated by SH for the reduced state and S‐S for a disulfide bond.

Figure 2.

Ribbon representations of the structures of (a) the thioredoxin‐fold oxidoreductase module and (b) the sulfhydryl oxidase module of QSOX enzymes determined by X‐ray crystallography, with various functional features highlighted and labelled. The sulfur atoms of a subset of the cysteines in each module are shown as yellow balls. A conserved cis‐proline (cis‐P) in the oxidoreductase module is purple. A buried glutamic acid (buried E) in human QSOX is red. The FAD in the sulfhydryl oxidase module is orange.

Figure 3.

Structures of QSOX cysteine mutants designed to mimic the intermediate state in disulfide transfer between the oxidoreductase and sulfhydryl oxidase modules. (a) The structures of trypanosome and mouse QSOX mutants reveal the nearly end‐on approach of the two redox‐active helices and demonstrate how the flexible linkers (grey dots) between modules accommodate the domain orientation necessary to form the inter‐domain disulfide. The sulfur atoms of a subset of the cysteines are shown as yellow balls. (b) A surface representation of the ψErv/Erv domains shows how the Trx1 domain orientation during domain–domain docking may be restricted by an outcrop of structured loops, labelled ‘Backstop’, to one side of the Erv active site. (c) Docking of the conserved tryptophan immediately upstream of the Trx1 redox‐active cysteines into a pocket in the Erv domain allows formation of a buried hydrogen bond with a tyrosine from the ψErv domain.

Figure 4.

Electrostatic interactions between the QSOX oxidoreductase and sulfhydryl oxidase modules. (a) In trypanosome QSOX, an arginine side chain (R74) from the Trx1 domain is in position to make a hydrogen bond to the O2 atom of the FAD in the Erv domain. This interaction may contribute to the electron‐withdrawing potential of the FAD during resolution of the inter‐domain disulfide. (b) Two superposed structures of mouse QSOX show differences inside chain rotamers, indicated by double‐headed arrows, in the active‐site region. The histidine in the Cys‐Gly‐His‐Cys motif (H75) may stabilise negative charge on active‐site cysteine side chains during the thiol/disulfide exchange reactions. White arrows indicate the direction of electron transfer during resolution of the inter‐domain disulfide bond by attack of the resolving cysteine, which was mutated to alanine in the crystallised construct.



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Further Reading

Bulleid NJ and Ellgaard L (2011) Multiple ways to make disulfides. Trends in Biochemical Sciences 36: 485–492.

Kodali VK and Thorpe C (2010) Oxidative protein folding and the Quiescin‐sulfhydryl oxidase family of flavoproteins. Antioxidants & Redox Signaling 13: 1217–1230.

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Alon, Assaf, and Fass, Deborah(Sep 2012) Catalysis of Disulfide Bond Formation by the Quiescin Sulfhydryl Oxidases. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0024168]