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Query: KEGG:D02011 (FAD)
 5,530 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Eukaryotic flavin-dependent sulfhydryl oxidases catalyze oxidative protein folding with the generation of disulfides and the reduction of oxygen to hydrogen peroxide. This review deals principally with the Quiescinsulfhydryl oxidases (QSOX) that are found in multiple forms in multicellular organisms and singly in a number of protozoan parasites. QSOX is an ancient fusion of thioredoxin domains and an FAD-binding module, ERV1/ALR. Interdomain disulfide exchanges transmit reducing equivalents from substrates to the flavin cofactor and thence to molecular oxygen. The in vitro substrate specificity of avian QSOX1 and the likely substrates of QSOXs in vivo are discussed. The location of QSOX immunoreactivity and mRNA expression levels in human cells and tissues is reviewed. Generally, there is a marked association of QSOX1 expression with cell types that have a high secretory load of disulfide-containing peptides and proteins. The abundance of sulfhydryl oxidases in the islets of Langerhans suggests that oxidative protein folding may directly contribute to the oxidative stress believed to be a factor in the progression to type II diabetes. Finally, the structure and mechanism of QSOX proteins is compared to their smaller stand-alone cousins: yeast ERV1p and ERV2p, the mammalian augmenter of liver regeneration (ALR), and the viral ALR homologs.
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PMID:Multidomain flavin-dependent sulfhydryl oxidases. 1667 76

The QSOX1 protein, belonging to a new class of FAD-linked Quiescin/Sulfhydryl oxidase, catalyzes disulfide bond formation. To give new insight into the biological function of QSOX1, we studied its involvement in oxidative stress-induced apoptosis and cell recovery of PC12 cells. By real time RT-PCR and flow cytometric analysis, we show that the QSOX1 mRNA and protein levels increased late after the beginning of oxidative treatment and were sustained for 72 h. These levels were still high when the PC12 cells were not dying but had resumed proliferation. The kinetics of QSOX1 expression suggest a more protective effect of QSOX1 rather than an involvement of this protein in apoptosis. Human breast cancer MCF-7 cell lines overexpressing the guinea pig QSOX1 protein submitted to the same treatments appeared less sensitive to cell death than the MCF-7 control cells. The protective effect is partly due to a preservation of the mitochondrial polarization generally lost after an oxidative stress. These results strengthen our hypothesis of a protective role of QSOX1 against apoptosis.
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PMID:Involvement of sulfhydryl oxidase QSOX1 in the protection of cells against oxidative stress-induced apoptosis. 1792 79

Both metal and flavin-dependent sulfhydryl oxidases catalyze the net generation of disulfide bonds with the reduction of oxygen to hydrogen peroxide. The first mammalian sulfhydryl oxidase to be described was an iron-dependent enzyme isolated from bovine milk whey (Janolino, V.G., and Swaisgood, H.E. (1975) J. Biol. Chem. 250, 2532-2537). This protein was reported to contain 0.5 atoms of iron per 89 kDa subunit and to be completely inhibited by ethylenediaminetetraacetate (EDTA). However the present work shows that a soluble 62 kDa FAD-linked and EDTA-insensitive sulfhydryl oxidase apparently constitutes the dominant disulfide bond-generating activity in skim milk. Unlike the metalloenzyme, the flavoprotein is not associated tightly with skim milk membranes. Sequencing of the purified bovine enzyme (>70% coverage) showed it to be a member of the Quiescin-sulfhydryl oxidase (QSOX) family. Consistent with its solubility, this bovine QSOX1 paralogue lacks the C-terminal transmembrane span of the long form of these proteins. Bovine milk QSOX1 is highly active toward reduced RNase and with the model substrate dithiothreitol. The significance of these new findings is discussed in relation to the earlier reports of metal-dependent sulfhydryl oxidases.
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PMID:A flavin-dependent sulfhydryl oxidase in bovine milk. 1794 90

The flavoprotein quiescin-sulfhydryl oxidase (QSOX) rapidly inserts disulfide bonds into unfolded, reduced proteins with the concomitant reduction of oxygen to hydrogen peroxide. This study reports the first heterologous expression and enzymological characterization of a human QSOX1 isoform. Like QSOX isolated from avian egg white, recombinant HsQSOX1 is highly active toward reduced ribonuclease A (RNase) and dithiothreitol but shows a >100-fold lower k cat/ K m for reduced glutathione. Previous studies on avian QSOX led to a model in which reducing equivalents were proposed to relay through the enzyme from the first thioredoxin domain (C70-C73) to a distal disulfide (C509-C512), then across the dimer interface to the FAD-proximal disulfide (C449-C452), and finally to the FAD. The present work shows that, unlike the native avian enzyme, HsQSOX1 is monomeric. The recombinant expression system enabled construction of the first cysteine mutants for mechanistic dissection of this enzyme family. Activity assays with mutant HsQSOX1 indicated that the conserved distal C509-C512 disulfide is dispensable for the oxidation of reduced RNase or dithiothreitol. The four other cysteine residues chosen for mutagenesis, C70, C73, C449, and C452, are all crucial for efficient oxidation of reduced RNase. C452, of the proximal disulfide, is shown to be the charge-transfer donor to the flavin ring of QSOX, and its partner, C449, is expected to be the interchange thiol, forming a mixed disulfide with C70 in the thioredoxin domain. These data demonstrate that all the internal redox steps occur within the same polypeptide chain of mammalian QSOX and commence with a direct interaction between the reduced thioredoxin domain and the proximal disulfide of the Erv/ALR domain.
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PMID:Human quiescin-sulfhydryl oxidase, QSOX1: probing internal redox steps by mutagenesis. 1839 49

Quiescin sulfhydryl oxidase (QSOX) flavoenzymes catalyze the direct, facile, insertion of disulfide bonds into reduced unfolded proteins with the reduction of oxygen to hydrogen peroxide. To date, only QSOXs from vertebrates have been characterized enzymatically. These metazoan sulfhydryl oxidases have four recognizable domains: a redox-active thioredoxin (Trx) domain containing the first of three CxxC motifs (C(I)-C(II)), a second Trx domain with no obvious redox-active disulfide, a helix-rich domain, and then an Erv/ALR domain. This last domain contains the FAD moiety, a proximal C(III)-C(IV) disulfide, and a third CxxC of unknown function (C(V)-C(VI)). Plant and protist QSOXs lack the second Trx domain but otherwise appear to contain the same complement of redox centers. This work presents the first characterization of a single-Trx QSOX. Trypanosoma brucei QSOX was expressed in Escherichia coli using a synthetic gene and found to be a stable, monomeric, FAD-containing protein. Although evidently lacking an entire domain, TbQSOX shows catalytic activity and substrate specificity similar to the vertebrate QSOXs examined previously. Unfolded reduced proteins are more than 200-fold more effective substrates on a per thiol basis than glutathione and some 10-fold better than the parasite bisglutathione analogue, trypanothione. These data are consistent with a role for the protist QSOX in oxidative protein folding. Site-directed mutagenesis of each of the six cysteine residues (to serines) shows that the CxxC motif in the single-Trx domain is crucial for efficient catalysis of the oxidation of both reduced RNase and the model substrate dithiothreitol. As expected, the proximal disulfide C(III)-C(IV), which interacts with the flavin, is catalytically crucial. However, as observed with human QSOX1, the third CxxC motif shows no obvious catalytic role during the in vitro oxidation of reduced RNase or dithiothreitol. Pre-steady-state kinetics demonstrates that turnover in TbQSOX is limited by an internal redox step leading to 2-electron reduction of the FAD cofactor. In sum, the single-Trx domain QSOX studied here shows a striking similarity in enzymatic behavior to its double-Trx metazoan counterparts.
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PMID:Quiescin sulfhydryl oxidase from Trypanosoma brucei: catalytic activity and mechanism of a QSOX family member with a single thioredoxin domain. 2012 Dec 44