Oxidative Protein Folding and the Quiescin–Sulfhydryl Oxidase Family of Flavoproteins

Abstract

Flavin-linked sulfhydryl oxidases participate in the net generation of disulfide bonds during oxidative protein folding in the endoplasmic reticulum. Members of the Quiescin-sulfhydryl oxidase (QSOX) family catalyze the facile direct introduction of disulfide bonds into unfolded reduced proteins with the reduction of molecular oxygen to generate hydrogen peroxide. Current progress in dissecting the mechanism of QSOX enzymes is reviewed, with emphasis on the CxxC motifs in the thioredoxin and Erv/ALR domains and the involvement of the flavin prosthetic group. The tissue distribution and intra- and extracellular location of QSOX enzymes are discussed, and suggestions for the physiological role of these enzymes are presented. The review compares the substrate specificity and catalytic efficiency of the QSOX enzymes with members of the Ero1 family of flavin-dependent sulfhydryl oxidases: enzymes believed to play key roles in disulfide generation in yeast and higher eukaryotes. Finally, limitations of our current understanding of disulfide generation in metazoans are identified and questions posed for the future.

The disease-associated mutation of the mitochondrial thiol oxidase Erv1 impairs cofactor binding during its catalytic reaction

“…Previous studies have shown that Erv1 and ALR use similar catalytic mechanisms and go through several intermediate states (e.g. the S-FAD charge-transfer complex, FADH 2 -Erv1) during their catalytic cycle [16,[31][32][33], so it would be interesting to see if the ALR R194H mutant also loses its cofactor during its catalytic cycle. We cannot conclude at which stage of the catalytic cycle the cofactor is released and the R182H mutation may affect more than one Erv1 intermediate.…”

Structure of a Baculovirus Sulfhydryl Oxidase, a Highly Divergent Member of the Erv Flavoenzyme Family

“…In particular, the first trx domain of some QSOX orthologs has recognizable homology to the redox-active trx-fold domains of protein disulfide isomerase (PDI), an oxidoreductase involved in disulfide formation and rearrangement in the endoplasmic reticulum. Within QSOX, the Erv and PDI-like domains cooperate in the generation and transfer of disulfides to substrate proteins (24). Although Erv sulfhydryl oxidases encoded by viruses generally are small, compact proteins like most cellular Erv enzymes, exceptions are found, for example, in ascoviruses, mimivirus, and baculoviruses.…”

Enzymatic control of cysteinyl thiol switches in proteins

“…Such catalytic cysteines are, e.g., found in many proteins of the thioredoxin superfamily (Deponte, 2013). In contrast, structural disulfides are formed under partially oxidizing conditions in the periplasm of Gramnegative bacteria or in the endoplasmic reticulum and the mitochondrial intermembrane space of eukaryotes (Deponte and Hell, 2009;Endo et al, 2010;Kodali and Thorpe, 2010;Codding et al, 2012;Oka and Bulleid, 2013;Hatahet et al, 2014;Kojer and Riemer, 2014). Once formed, such structural disulfides usually stabilize a protein in a defined tertiary or quaternary structure until the protein is degraded.…”

“…However, experimental evidence for this hypothesis outside the bacterial periplasm and the eukaryotic endoplasmic reticulum is sparse. Thiol-disulfide exchange reactions with analogous mechanisms are furthermore catalyzed by enzymes with alternative protein folds, i.e., (i) GR and TrxR, (ii) Mia40, which catalyzes the oxidative protein folding in the intermembrane space of mitochondria, or (iii) enzymes of the Erv/QSOX family as reviewed (Fass, 2008;Deponte and Hell, 2009;Endo et al, 2010;Kodali and Thorpe, 2010;Kojer and Riemer, 2014).…”