{"gene":"SQOR","run_date":"2026-04-28T20:42:08","timeline":{"discoveries":[{"year":1999,"finding":"The fission yeast ortholog of SQOR, HMT2, was identified as a mitochondrial sulfide:quinone oxidoreductase. HMT2 protein localizes to mitochondria (cell fractionation and immunocytochemistry), binds FAD, and catalyzes quinone (CoQ2) reduction by sulfide in vitro. Loss of hmt2+ causes abnormal sulfide accumulation and cadmium hypersensitivity.","method":"Genetic complementation, cell fractionation, immunocytochemistry, in vitro enzymatic assay with purified protein","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution of enzymatic activity with purified protein plus subcellular localization with functional genetic phenotype","pmids":["10224084"],"is_preprint":false},{"year":2008,"finding":"Arenicola marina SQR expressed in Saccharomyces cerevisiae mitochondria catalyzes sulfide oxidation using decyl-ubiquinone as electron acceptor (Km H2S = 23 µM, Km DUQ = 6.4 µM) and requires an S0 acceptor (cyanide, producing thiocyanate; or thioredoxin + sulfite). Site-directed mutagenesis of six conserved amino acids (essential in bacterial SQR) abolished activity, implicating them in the eukaryotic catalytic mechanism.","method":"Heterologous expression in yeast, affinity purification, in vitro enzymatic assay, site-directed mutagenesis","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinetic characterization plus mutagenesis of catalytic residues","pmids":["18248458"],"is_preprint":false},{"year":2008,"finding":"A mitochondrial pathway for sulfide oxidation to thiosulfate was defined in rat liver and Arenicola marina: (1) membrane-bound SQR converts sulfide to persulfides and transfers electrons to the ubiquinone pool; (2) a sulfur dioxygenase oxidizes one persulfide to sulfite; (3) rhodanese transfers a second persulfide to sulfite, yielding thiosulfate. Rhodanese thus functions as a sulfurtransferase in this pathway.","method":"Mitochondrial fractionation, enzymatic assays for each step, reconstitution of pathway","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1 — three-enzyme pathway reconstituted in mitochondrial preparations with orthogonal enzymatic assays","pmids":["18494801"],"is_preprint":false},{"year":2010,"finding":"SQR activity is present in mitochondria from mouse kidney, liver, and heart but absent from brain mitochondria and neuroblastoma cells, explaining neural tissue's greater sulfide sensitivity. SQR oxidation of sulfide takes precedence over complex I (demonstrated by competition assays). In colonocyte HT-29 cells, sulfide oxidation via SQR provides the first demonstrated example of reverse electron transfer in living cells.","method":"Mitochondrial isolation, SQR activity assay, oxygen consumption measurements, cellular sulfide concentration monitoring","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 1-2 — direct enzymatic measurements in isolated mitochondria from multiple tissues plus cellular functional assays","pmids":["20398623"],"is_preprint":false},{"year":2012,"finding":"Human SQOR expressed in E. coli contains noncovalently bound FAD and catalyzes two-electron oxidation of H2S to sulfane sulfur (S0) using CoQ1 as electron acceptor. FAD is reduced via a long-wavelength-absorbing intermediate (λmax = 673 nm). Sulfite, cyanide, or sulfide can serve as S0 acceptors, producing thiosulfate, thiocyanate, or H2S2, respectively. Sulfite is proposed as the physiological acceptor (kcat/Km,H2S = 2.9 × 10^7 M−1 s−1 at pH 7.5).","method":"Recombinant protein expression, in vitro enzymatic assay, UV-visible spectroscopy, pre-steady-state kinetics","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — purified recombinant human enzyme with rigorous kinetic characterization and mechanistic intermediate identification","pmids":["22852582"],"is_preprint":false},{"year":2014,"finding":"Human SQR uses glutathione (in addition to sulfite) as a persulfide acceptor, generating glutathione persulfide (GSSH) as the primary intermediate. Rhodanese preferentially synthesizes rather than utilizes thiosulfate. Kinetic data and simulations at physiological metabolite concentrations support pathway organization: H2S → SQR → GSSH → thiosulfate or sulfite.","method":"In vitro enzymatic assays, kinetic measurements, mathematical modeling/simulation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with purified enzymes, kinetic simulations, and multiple orthogonal assays","pmids":["25225291"],"is_preprint":false},{"year":2014,"finding":"Acidithiobacillus ferrooxidans SQR contains a non-covalent FAD cofactor. Cys160, Cys356, and His198 are critical for catalytic activity (variants show greatly reduced DUQ reduction), while Cys128 and His132 are not essential for the reductive half-reaction. EPR reveals a neutral flavin semiquinone; the Cys160Ala variant accumulates a semiquinone consistent with a Cys356–S(γ)–S–C(4A)–FAD adduct intermediate. FAD Em = −139 mV at pH 7.0.","method":"Site-directed mutagenesis, steady-state and pre-steady-state kinetics, EPR spectroscopy, potentiometric titration","journal":"Archives of biochemistry and biophysics","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with EPR spectroscopy and potentiometric titration defining catalytic mechanism","pmids":["25303790"],"is_preprint":false},{"year":2016,"finding":"S. aureus SQR (type I flavoprotein) catalyzes two-electron oxidation of Na2S to sulfane sulfur using FAD and a quinone acceptor. The reaction requires a C167–C344 disulfide in the resting enzyme, with a C344 persulfide intermediate confirmed by mass spectrometry of sulfide-reacted enzyme. Cyanide, sulfite, or CoA serve as S0 acceptors in vitro; with CstB (a persulfide dioxygenase), CoASSH and thiosulfate are produced from sulfide. Δsqr strains show elevated CoASSH and inorganic tetrasulfide.","method":"In vitro enzymatic assay, mass spectrometry of reaction intermediates, genetic deletion, sulfur metabolite profiling","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — catalytic mechanism defined by MS of persulfide intermediate plus mutagenesis and pathway reconstitution","pmids":["27806570"],"is_preprint":false},{"year":2020,"finding":"SQR-mediated H2S oxidation drives reverse electron transport (RET) at mitochondrial complex I, repurposing complex I toward superoxide production. This superoxide-dependent mitochondrial uncoupling activates AMPK downstream. Deletion of SQR, complex I, or AMPK abolishes therapeutic effects of H2S after intracerebral hemorrhage. SQR-induced uncoupling is mechanistically separable from complex IV inhibition by H2S.","method":"Genetic deletion (SQR KO, complex I KO, AMPK KO), mitochondrial membrane potential measurements, superoxide detection, AMPK activity assays, in vivo intracerebral hemorrhage model","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic deletions with defined molecular and physiological phenotypes, pathway placement by epistasis","pmids":["32923620"],"is_preprint":false},{"year":2020,"finding":"Pathogenic variants in SQOR (p.Glu213Lys and c.446delT frameshift) cause Leigh disease with complex IV inhibition. p.Glu213Lys disrupts hydrogen bonding with neighboring residues, resulting in severely reduced SQOR protein and enzyme activity measured spectrophotometrically. Sulfide-generating enzyme levels were unchanged, indicating the mechanism is H2S accumulation due to loss of SQOR catabolism, which inhibits complex IV.","method":"Exome sequencing, spectrophotometric enzyme activity assay, western blotting, mitochondrial function assay in patient muscle and liver tissue","journal":"Journal of inherited metabolic disease","confidence":"High","confidence_rationale":"Tier 2 — direct measurement of SQOR enzyme activity in patient tissue combined with protein level analysis and functional mitochondrial assays","pmids":["32160317"],"is_preprint":false},{"year":2024,"finding":"ASB1 (substrate recognition subunit of a ubiquitin E3 ligase) interacts with ELOB to promote K48-linked ubiquitination of SQOR at residues K207 and K344, leading to proteasomal degradation of SQOR. ASB1 knockout in mice causes elevated H2S, oxidative stress, and sperm DNA damage; fertility defects are rescued by NaHS (H2S donor), establishing SQOR degradation as the mechanism linking ASB1 to H2S homeostasis.","method":"Co-immunoprecipitation, ubiquitination assay, site-directed mutagenesis of ubiquitination sites, ASB1 knockout mice, NaHS rescue experiment, western blotting","journal":"Redox biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, specific ubiquitination site mutagenesis, in vivo KO with mechanistic rescue","pmids":["39733518"],"is_preprint":false},{"year":2025,"finding":"Pyruvate carboxylase (PC) physically interacts with SQOR; PC deficiency reduces this interaction, increasing ubiquitination and proteasomal degradation of SQOR. Loss of SQOR leads to mitochondrial morphological and functional disruption, increased mtDNA release, activation of the cGAS-STING pathway, elevated glycolysis, and renal fibrosis.","method":"Co-immunoprecipitation (PC-SQOR interaction), ubiquitination assay, PC knockout mice (PcxcKO) and stable KO cell line, cGAS-STING pathway assays, mitochondrial morphology imaging","journal":"Advanced science","confidence":"High","confidence_rationale":"Tier 2 — direct protein interaction shown by Co-IP, mechanistic pathway defined by KO with multiple orthogonal readouts","pmids":["39836535"],"is_preprint":false},{"year":2025,"finding":"SQOR knockdown in HCT116 colorectal cancer cells disrupts polysulfide homeostasis, diminishes mitochondrial function, impairs proliferation, and triggers early apoptosis. SQR KO reduces tumor growth in xenograft mice. Metabolomic analysis reveals reprogramming of glycolysis at the fructose-1,6-bisphosphate degradation step; western blot and enzymatic assays confirm decreased ALDOA levels and activity as the downstream effector.","method":"SQOR knockout, tumor xenograft model, metabolomics, western blotting, enzymatic activity assay for ALDOA","journal":"Redox biology","confidence":"Medium","confidence_rationale":"Tier 2 — KO with defined phenotypic and metabolic readouts, but ALDOA link is single-lab without full mechanistic reconstitution","pmids":["40305883"],"is_preprint":false},{"year":2025,"finding":"SQOR mediates ferroptosis resistance by using hydrogen selenide to reduce ubiquinone, thereby elevating ubiquinol levels and suppressing lipid peroxidation independently of GPX4. In the ATF4/CTH/SQOR axis, ATF4 drives CTH expression to generate H2S, which SQOR converts to CoQ10H2 (ubiquinol); SQOR-deficient cells cannot rescue CoQ10H2 or prevent ferroptosis even with elevated H2S or CTH overexpression.","method":"SQOR knockdown/overexpression, CoQ10H2 measurement, lipid peroxidation assay (MDA), ATF4/CTH/SQOR genetic epistasis, alternative oxidase overexpression","journal":"Biochemical pharmacology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with multiple pathway components and defined biochemical readouts, single lab","pmids":["41759595"],"is_preprint":false},{"year":2023,"finding":"SQOR interacts with the C9-ALS dipeptide repeat protein GA50 (identified by yeast two-hybrid). SQOR knockdown in HMC3 microglia activates the NLRP3 inflammasome by upregulating intracellular ROS and promoting cytoplasmic escape of mitochondrial DNA. The small molecule irisflorentin blocks the SQOR–GA50 interaction and inhibits NLRP3 inflammasome activity.","method":"Yeast two-hybrid, SQOR knockdown, ROS measurement, mtDNA cytoplasmic escape assay, NLRP3 inflammasome activity assay, small-molecule inhibitor competition","journal":"Antioxidants","confidence":"Medium","confidence_rationale":"Tier 3 — yeast two-hybrid for interaction plus KD with defined inflammatory phenotype, single lab","pmids":["37891975"],"is_preprint":false},{"year":2025,"finding":"SQOR deficiency confined to murine intestinal epithelial cells perturbs colon bioenergetics in a manner reversed by antibiotics, establishing that microbial H2S is a significant local contributor to host mitochondrial energy metabolism via SQOR. Combined intestinal SQOR deficiency and high dietary methionine disrupts colon architecture, alters microbiome composition, increases systemic thiosulfate (H2S oxidation biomarker), raises ketone bodies, reduces locomotor activity, and decreases ventricular volume associated with lower aquaporin 1.","method":"Intestinal epithelial cell-specific SQOR knockout mice, antibiotic treatment, brain MRI, aquaporin 1 western blot, serum metabolomics","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — tissue-specific KO with multiple orthogonal readouts, but preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.04.09.647962"],"is_preprint":true}],"current_model":"Human SQOR is an inner mitochondrial membrane FAD-containing flavoenzyme that catalyzes the two-electron oxidation of H2S to a sulfane sulfur intermediate (persulfide), transferring electrons to the coenzyme Q pool, with glutathione (and sulfite) acting as physiological persulfide acceptors to generate glutathione persulfide and ultimately thiosulfate; this activity drives reverse electron transport at complex I to induce mitochondrial uncoupling and AMPK activation, produces ubiquinol that suppresses ferroptosis independently of GPX4, is subject to proteasomal regulation via K48-linked ubiquitination (mediated by ASB1-ELOB) at K207 and K344, is stabilized by physical interaction with pyruvate carboxylase, and its loss-of-function causes toxic H2S accumulation, complex IV inhibition, mtDNA release with cGAS-STING activation, and Leigh disease."},"narrative":{"teleology":[{"year":1999,"claim":"Identification of a eukaryotic sulfide:quinone oxidoreductase (HMT2 in fission yeast) established that mitochondria harbor a dedicated FAD-dependent enzyme for H₂S oxidation coupled to quinone reduction, resolving the long-open question of how eukaryotic cells detoxify sulfide.","evidence":"Purified HMT2 protein characterized by cell fractionation, immunocytochemistry, and in vitro enzymatic assay in S. pombe","pmids":["10224084"],"confidence":"High","gaps":["Human ortholog not yet characterized","Physiological persulfide acceptor unknown","Tissue distribution of activity undefined"]},{"year":2008,"claim":"Reconstitution of a complete three-step mitochondrial sulfide oxidation pathway (SQR → sulfur dioxygenase → rhodanese) and kinetic characterization of eukaryotic SQR with site-directed mutagenesis defined the catalytic requirements and pathway topology for sulfide catabolism to thiosulfate.","evidence":"Heterologous expression in yeast, mutagenesis of conserved catalytic residues, and pathway reconstitution in rat liver mitochondria","pmids":["18248458","18494801"],"confidence":"High","gaps":["Human enzyme kinetics not yet measured","Electron acceptor specificity in mammalian membranes unresolved"]},{"year":2010,"claim":"Demonstration that SQR activity is tissue-specific in mammals (present in kidney, liver, heart; absent in brain) and that SQR-driven sulfide oxidation produces the first evidence of reverse electron transfer in living cells resolved why neural tissue is uniquely sensitive to sulfide toxicity.","evidence":"Mitochondrial isolation from mouse tissues, SQR activity assay, oxygen consumption and cellular sulfide measurements in HT-29 colonocytes","pmids":["20398623"],"confidence":"High","gaps":["Molecular basis of tissue-specific expression unknown","In vivo physiological significance of reverse electron transfer not yet established"]},{"year":2012,"claim":"Kinetic and spectroscopic characterization of purified recombinant human SQOR revealed noncovalently bound FAD, a long-wavelength charge-transfer intermediate, and high catalytic efficiency (kcat/Km ~2.9 × 10⁷ M⁻¹ s⁻¹), establishing the molecular properties of the human enzyme and identifying sulfite as a candidate physiological persulfide acceptor.","evidence":"Recombinant human SQOR expressed in E. coli, UV-vis spectroscopy, pre-steady-state kinetics","pmids":["22852582"],"confidence":"High","gaps":["Glutathione not yet tested as persulfide acceptor","No structural model of human SQOR available"]},{"year":2014,"claim":"Discovery that glutathione serves as the primary persulfide acceptor for human SQOR, generating GSSH, reframed the pathway: the dominant route is H₂S → GSSH → thiosulfate rather than direct sulfite acceptance, and rhodanese functions biosynthetically in thiosulfate formation.","evidence":"In vitro reconstitution with purified enzymes at physiological metabolite concentrations, kinetic modeling","pmids":["25225291"],"confidence":"High","gaps":["Relative contributions of GSH vs sulfite in vivo unresolved","Structural basis of persulfide acceptor selectivity unknown"]},{"year":2020,"claim":"Genetic epistasis experiments showed SQR-driven electron flow into the Q-pool powers reverse electron transport at complex I to generate superoxide and activate AMPK, establishing a signaling function for sulfide oxidation beyond simple detoxification, and biallelic SQOR mutations were identified as the cause of Leigh disease through toxic H₂S accumulation and complex IV inhibition.","evidence":"SQR, complex I, and AMPK knockouts with mitochondrial membrane potential and superoxide readouts in vivo (ICH model); exome sequencing, spectrophotometric enzyme assay, and western blot in patient tissue","pmids":["32923620","32160317"],"confidence":"High","gaps":["Whether AMPK activation is direct or secondary to energy stress unclear","Full spectrum of SQOR loss-of-function clinical phenotypes not delineated","Structural impact of pathogenic variants modeled only in silico"]},{"year":2024,"claim":"Identification of ASB1–ELOB as the E3 ligase complex that K48-ubiquitinates SQOR at K207 and K344 for proteasomal degradation defined the first post-translational turnover mechanism for SQOR, linking ubiquitin-mediated SQOR degradation to H₂S homeostasis and male fertility.","evidence":"Reciprocal Co-IP, ubiquitination site mutagenesis, ASB1 knockout mice with NaHS rescue","pmids":["39733518"],"confidence":"High","gaps":["Whether other E3 ligases also target SQOR unknown","Signals triggering ASB1-mediated degradation not defined"]},{"year":2025,"claim":"Multiple studies converged to show that SQOR loss has broad downstream consequences: pyruvate carboxylase stabilizes SQOR protein and its absence triggers ubiquitination, mtDNA release, cGAS–STING activation and renal fibrosis; SQOR-generated ubiquinol suppresses ferroptosis independently of GPX4 via the ATF4/CTH/SQOR axis; and SQOR deficiency in colorectal cancer cells disrupts polysulfide balance, impairs proliferation, and reduces xenograft tumor growth.","evidence":"PC–SQOR Co-IP and PC-KO mice; SQOR KD/OE with CoQ10H2, lipid peroxidation, and genetic epistasis; SQOR-KO HCT116 xenografts with metabolomics","pmids":["39836535","41759595","40305883"],"confidence":"High","gaps":["Whether PC–SQOR interaction is direct or complex-mediated not fully resolved","ALDOA downregulation mechanism upon SQOR loss requires independent confirmation","Relative importance of ubiquinol production vs H₂S clearance in ferroptosis protection unclear"]},{"year":null,"claim":"A high-resolution structure of human SQOR, the regulatory signals controlling SQOR expression across tissues, the in vivo contribution of microbial H₂S to SQOR-dependent host bioenergetics, and the full clinical spectrum of SQOR deficiency remain to be established.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal or cryo-EM structure of human SQOR","Transcriptional and epigenetic regulation largely uncharacterized","Tissue-specific conditional knockouts beyond intestine not fully explored"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016491","term_label":"oxidoreductase activity","supporting_discovery_ids":[0,1,2,4,5,6,7]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0,2,3,4,8,9,11]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[2,4,5,8]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[12,13]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[9]}],"complexes":[],"partners":["PC","ASB1","ELOB"],"other_free_text":[]},"mechanistic_narrative":"SQOR is a mitochondrial inner-membrane FAD-containing flavoenzyme that catalyzes the two-electron oxidation of hydrogen sulfide to a persulfide intermediate, transferring electrons to the ubiquinone pool; glutathione and sulfite serve as physiological persulfide acceptors, channeling sulfane sulfur toward thiosulfate production through a conserved three-enzyme mitochondrial sulfide oxidation pathway [PMID:22852582, PMID:25225291, PMID:18494801]. By feeding electrons into coenzyme Q, SQOR drives reverse electron transport at complex I to activate AMPK-dependent signaling and elevates ubiquinol levels that suppress ferroptosis independently of GPX4 [PMID:32923620, PMID:41759595]. SQOR protein turnover is regulated by ASB1–ELOB-mediated K48-linked ubiquitination at K207 and K344 and is stabilized by physical interaction with pyruvate carboxylase; loss of SQOR causes toxic H2S accumulation, complex IV inhibition, mtDNA release with cGAS–STING activation, and is the basis of a Leigh disease caused by biallelic SQOR variants [PMID:39733518, PMID:39836535, PMID:32160317]. SQOR deficiency in colorectal cancer cells disrupts polysulfide homeostasis, impairs proliferation, and reduces tumor growth in xenograft models [PMID:40305883]."},"prefetch_data":{"uniprot":{"accession":"Q9Y6N5","full_name":"Sulfide:quinone oxidoreductase, mitochondrial","aliases":["Sulfide dehydrogenase-like","Sulfide quinone oxidoreductase"],"length_aa":450,"mass_kda":50.0,"function":"Catalyzes the oxidation of hydrogen sulfide with the help of a quinone, such as ubiquinone-10, giving rise to thiosulfate and ultimately to sulfane (molecular sulfur) atoms. Requires an additional electron acceptor; can use sulfite, sulfide or cyanide (in vitro) (PubMed:22852582). 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standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":34390,"output_tokens":4407,"usd":0.084638},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":8116,"output_tokens":2481,"usd":0.153908}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2020,\n      \"finding\": \"SQR-mediated oxidation of H2S drives reverse electron transport (RET) at mitochondrial complex I, which repurposes mitochondrial function to superoxide production; complex I RET induces superoxide-dependent mitochondrial uncoupling and downstream AMPK activation. Deletion of SQR, complex I, or AMPK abolishes therapeutic effects of H2S following intracerebral hemorrhage.\",\n      \"method\": \"Genetic deletion (SQR KO, complex I KO, AMPK KO) in cellular and in vivo (intracerebral hemorrhage) models; mitochondrial membrane potential and superoxide measurements\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal genetic deletions with defined phenotypic readouts, replicated across cellular and in vivo contexts\",\n      \"pmids\": [\"32923620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Pathogenic variants in SQOR (p.Glu213Lys and c.446delT) cause severely reduced SQOR protein levels and enzyme activity, leading to H2S accumulation that inhibits mitochondrial complex IV, resulting in energy failure and Leigh disease phenotype. SQOR enzyme activity was directly measured spectrophotometrically in patient tissues.\",\n      \"method\": \"Exome sequencing of patients; spectrophotometric enzyme activity assay; western blotting for protein levels; mitochondrial function assays in muscle and liver tissue\",\n      \"journal\": \"Journal of inherited metabolic disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct enzyme activity measurement in patient tissue combined with genetic and biochemical characterization\",\n      \"pmids\": [\"32160317\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Eukaryotic (Arenicola marina) SQR is a mitochondrial membrane-bound flavoprotein that oxidizes sulfide and reduces decyl-ubiquinone, requiring cyanide (or thioredoxin + sulfite) as sulfur acceptor; six conserved amino acids are essential for catalytic activity as shown by site-directed mutagenesis.\",\n      \"method\": \"Heterologous expression in Saccharomyces cerevisiae; affinity purification; in vitro enzyme kinetics (Km determination for decyl-ubiquinone and sulfide); site-directed mutagenesis of six conserved residues\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with kinetic characterization and mutagenesis of active site residues\",\n      \"pmids\": [\"18248458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Staphylococcus aureus SQR catalyzes two-electron oxidation of sulfide to sulfane sulfur (S0) using FAD and a quinone acceptor; catalysis requires a C167–C344 disulfide bond in the resting enzyme with a C344 persulfide intermediate; CoA, cyanide, and sulfite can each serve as the S0 acceptor. SQR and CstB together produce thiosulfate from sulfide in a CoA-dependent manner. SQR knockout elevates endogenous CoASSH and inorganic tetrasulfide levels in cells.\",\n      \"method\": \"In vitro enzyme assay with purified protein; mass spectrometry to detect C344 persulfide intermediate; reconstitution of SQR + CstB reaction; sulfur metabolite profiling of wild-type, Δsqr, and complemented strains\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution, MS verification of catalytic intermediate, and in-cell metabolite profiling with KO and complementation\",\n      \"pmids\": [\"27806570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In Acidithiobacillus ferrooxidans SQR, Cys160, Cys356, and His198 are essential for catalytic activity in both the reductive half-reaction and steady-state quinone reduction, while Cys128 and His132 are not required for the reductive half-reaction. A neutral flavin semiquinone EPR signal and a proposed Cys356-Sγ-S-C4A-FAD adduct were detected, clarifying the FAD-mediated catalytic mechanism.\",\n      \"method\": \"Site-directed mutagenesis; pre-steady-state and steady-state kinetics of FAD reduction and decylubiquinone reduction; EPR spectroscopy; potentiometric titration\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with in vitro kinetics and EPR spectroscopy in a single study\",\n      \"pmids\": [\"25303790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ASB1, a substrate recognition subunit of a CRL ubiquitin ligase, interacts with ELOB to promote K48-linked ubiquitination of SQOR at residues K207 and K344, leading to proteasomal degradation of SQOR. Loss of ASB1 increases SQOR protein and elevates H2S levels; ASB1 knockout causes excessive oxidative stress and decreased H2S in testes, impairing spermiogenesis.\",\n      \"method\": \"Co-immunoprecipitation (ASB1–ELOB–SQOR interaction); ubiquitination assay identifying K48-linked modification at K207/K344; Asb1 knockout mouse model; NaHS rescue experiment\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, site-specific ubiquitination mapping, KO mouse with defined phenotype and rescue\",\n      \"pmids\": [\"39733518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Pyruvate carboxylase (PC) interacts directly with SQOR; PC deficiency reduces this interaction, increasing ubiquitination and proteasomal degradation of SQOR. Loss of SQOR leads to mitochondrial morphological/functional disruption, increased mtDNA release, and activation of the cGAS-STING pathway, ultimately promoting renal fibrosis.\",\n      \"method\": \"Co-immunoprecipitation (PC–SQOR); ubiquitination assay; PC knockout mice and stable PC-KO cell line; mitochondrial function assays; cGAS-STING pathway analysis\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP and KO with defined pathway readout, but single lab study\",\n      \"pmids\": [\"39836535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SQOR knockout in HCT116 colorectal cancer cells disrupts polysulfide homeostasis, impairs mitochondrial function, and causes metabolic reprogramming at the fructose-1,6-bisphosphate degradation step; ALDOA protein levels and enzymatic activity are reduced. SQOR KO reduces tumor xenograft growth in mice.\",\n      \"method\": \"SQOR knockout by CRISPR; metabolomic analysis; Western blot and enzymatic assay for ALDOA; xenograft mouse model\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with metabolomics, enzymatic validation, and in vivo xenograft, but single lab\",\n      \"pmids\": [\"40305883\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SQOR physically interacts with the C9-ALS-associated glycine-alanine dipeptide repeat protein (GA50) in microglia; SQOR knockdown elevates intracellular reactive oxygen species and cytoplasmic mtDNA escape, thereby activating the NLRP3 inflammasome.\",\n      \"method\": \"Yeast two-hybrid screening; SQOR knockdown in HMC3 microglia; ROS measurement; NLRP3 inflammasome activity assay; MCC950 inhibitor validation\",\n      \"journal\": \"Antioxidants\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — yeast two-hybrid interaction plus KD phenotype in single lab study\",\n      \"pmids\": [\"37891975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"SQOR is required for the conversion of H2S into reduced coenzyme Q10 (CoQ10H2/ubiquinol); in SQOR-deficient renal cells, neither CTH overexpression nor H2S supplementation restores CoQ10H2 or protects against ferroptosis, placing SQOR downstream of CTH in the ATF4/CTH/SQOR axis that maintains ferroptosis resistance.\",\n      \"method\": \"SQOR overexpression and SQOR-deficient cell lines; CoQ10H2 measurement; ferroptosis assays (lipid peroxidation); ATF4 overexpression rescue; alternative oxidase (AOX) overexpression to oxidize CoQ10H2\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis established by orthogonal gain/loss-of-function experiments with metabolite readout, single lab\",\n      \"pmids\": [\"41759595\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SQOR promotes ferroptosis resistance by producing ubiquinol (CoQ10H2) and hydropersulfides, which act as radical-trapping antioxidants, independently of GPX4; this is described as a promiscuous enzymatic function beyond canonical H2S detoxification.\",\n      \"method\": \"Review/synthesis of experimental evidence including SQOR KD/KO ferroptosis assays and CoQ10H2 measurements (cited experimental studies)\",\n      \"journal\": \"BMB reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — review synthesizing experimental data from cited studies, not primary experimental paper\",\n      \"pmids\": [\"40495478\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Intestinal epithelial cell-specific SQOR deficiency perturbs colon bioenergetics in a manner reversible by antibiotics, establishing microbial H2S as a significant substrate for host SQOR. Combined with high dietary methionine, intestinal SQOR deficiency alters global sulfide metabolism (elevated serum thiosulfate), disrupts colon architecture, changes microbiome composition, and causes brain pathology including reduced aquaporin-1 and ventricular volume changes.\",\n      \"method\": \"Intestinal epithelial cell-specific SQOR knockout mice; antibiotic treatment rescue; serum thiosulfate measurement; brain MRI; colon histology; aquaporin-1 Western blot\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific KO with multiple orthogonal phenotypic readouts and antibiotic rescue epistasis; preprint\",\n      \"pmids\": [\"bio_10.1101_2025.04.09.647962\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SQOR (sulfide:quinone oxidoreductase) is an inner mitochondrial membrane flavoprotein that catalyzes the two-electron oxidation of H2S to a sulfane sulfur intermediate (via an active-site persulfide), transferring electrons to the ubiquinone pool and thereby linking H2S detoxification to the mitochondrial electron transport chain; this activity generates ubiquinol that confers ferroptosis resistance, drives reverse electron transport at complex I to produce superoxide-dependent mitochondrial uncoupling and AMPK activation, and is regulated by ubiquitin-mediated proteasomal degradation (written by the ASB1/ELOB E3 ligase complex targeting K207 and K344) as well as by interaction with pyruvate carboxylase, placing SQOR at the nexus of sulfur metabolism, bioenergetics, redox homeostasis, and disease states including Leigh disease, renal fibrosis, and colorectal cancer.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"The fission yeast ortholog of SQOR, HMT2, was identified as a mitochondrial sulfide:quinone oxidoreductase. HMT2 protein localizes to mitochondria (cell fractionation and immunocytochemistry), binds FAD, and catalyzes quinone (CoQ2) reduction by sulfide in vitro. Loss of hmt2+ causes abnormal sulfide accumulation and cadmium hypersensitivity.\",\n      \"method\": \"Genetic complementation, cell fractionation, immunocytochemistry, in vitro enzymatic assay with purified protein\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution of enzymatic activity with purified protein plus subcellular localization with functional genetic phenotype\",\n      \"pmids\": [\"10224084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Arenicola marina SQR expressed in Saccharomyces cerevisiae mitochondria catalyzes sulfide oxidation using decyl-ubiquinone as electron acceptor (Km H2S = 23 µM, Km DUQ = 6.4 µM) and requires an S0 acceptor (cyanide, producing thiocyanate; or thioredoxin + sulfite). Site-directed mutagenesis of six conserved amino acids (essential in bacterial SQR) abolished activity, implicating them in the eukaryotic catalytic mechanism.\",\n      \"method\": \"Heterologous expression in yeast, affinity purification, in vitro enzymatic assay, site-directed mutagenesis\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinetic characterization plus mutagenesis of catalytic residues\",\n      \"pmids\": [\"18248458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"A mitochondrial pathway for sulfide oxidation to thiosulfate was defined in rat liver and Arenicola marina: (1) membrane-bound SQR converts sulfide to persulfides and transfers electrons to the ubiquinone pool; (2) a sulfur dioxygenase oxidizes one persulfide to sulfite; (3) rhodanese transfers a second persulfide to sulfite, yielding thiosulfate. Rhodanese thus functions as a sulfurtransferase in this pathway.\",\n      \"method\": \"Mitochondrial fractionation, enzymatic assays for each step, reconstitution of pathway\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — three-enzyme pathway reconstituted in mitochondrial preparations with orthogonal enzymatic assays\",\n      \"pmids\": [\"18494801\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"SQR activity is present in mitochondria from mouse kidney, liver, and heart but absent from brain mitochondria and neuroblastoma cells, explaining neural tissue's greater sulfide sensitivity. SQR oxidation of sulfide takes precedence over complex I (demonstrated by competition assays). In colonocyte HT-29 cells, sulfide oxidation via SQR provides the first demonstrated example of reverse electron transfer in living cells.\",\n      \"method\": \"Mitochondrial isolation, SQR activity assay, oxygen consumption measurements, cellular sulfide concentration monitoring\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct enzymatic measurements in isolated mitochondria from multiple tissues plus cellular functional assays\",\n      \"pmids\": [\"20398623\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Human SQOR expressed in E. coli contains noncovalently bound FAD and catalyzes two-electron oxidation of H2S to sulfane sulfur (S0) using CoQ1 as electron acceptor. FAD is reduced via a long-wavelength-absorbing intermediate (λmax = 673 nm). Sulfite, cyanide, or sulfide can serve as S0 acceptors, producing thiosulfate, thiocyanate, or H2S2, respectively. Sulfite is proposed as the physiological acceptor (kcat/Km,H2S = 2.9 × 10^7 M−1 s−1 at pH 7.5).\",\n      \"method\": \"Recombinant protein expression, in vitro enzymatic assay, UV-visible spectroscopy, pre-steady-state kinetics\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — purified recombinant human enzyme with rigorous kinetic characterization and mechanistic intermediate identification\",\n      \"pmids\": [\"22852582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Human SQR uses glutathione (in addition to sulfite) as a persulfide acceptor, generating glutathione persulfide (GSSH) as the primary intermediate. Rhodanese preferentially synthesizes rather than utilizes thiosulfate. Kinetic data and simulations at physiological metabolite concentrations support pathway organization: H2S → SQR → GSSH → thiosulfate or sulfite.\",\n      \"method\": \"In vitro enzymatic assays, kinetic measurements, mathematical modeling/simulation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with purified enzymes, kinetic simulations, and multiple orthogonal assays\",\n      \"pmids\": [\"25225291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Acidithiobacillus ferrooxidans SQR contains a non-covalent FAD cofactor. Cys160, Cys356, and His198 are critical for catalytic activity (variants show greatly reduced DUQ reduction), while Cys128 and His132 are not essential for the reductive half-reaction. EPR reveals a neutral flavin semiquinone; the Cys160Ala variant accumulates a semiquinone consistent with a Cys356–S(γ)–S–C(4A)–FAD adduct intermediate. FAD Em = −139 mV at pH 7.0.\",\n      \"method\": \"Site-directed mutagenesis, steady-state and pre-steady-state kinetics, EPR spectroscopy, potentiometric titration\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with EPR spectroscopy and potentiometric titration defining catalytic mechanism\",\n      \"pmids\": [\"25303790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"S. aureus SQR (type I flavoprotein) catalyzes two-electron oxidation of Na2S to sulfane sulfur using FAD and a quinone acceptor. The reaction requires a C167–C344 disulfide in the resting enzyme, with a C344 persulfide intermediate confirmed by mass spectrometry of sulfide-reacted enzyme. Cyanide, sulfite, or CoA serve as S0 acceptors in vitro; with CstB (a persulfide dioxygenase), CoASSH and thiosulfate are produced from sulfide. Δsqr strains show elevated CoASSH and inorganic tetrasulfide.\",\n      \"method\": \"In vitro enzymatic assay, mass spectrometry of reaction intermediates, genetic deletion, sulfur metabolite profiling\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — catalytic mechanism defined by MS of persulfide intermediate plus mutagenesis and pathway reconstitution\",\n      \"pmids\": [\"27806570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"SQR-mediated H2S oxidation drives reverse electron transport (RET) at mitochondrial complex I, repurposing complex I toward superoxide production. This superoxide-dependent mitochondrial uncoupling activates AMPK downstream. Deletion of SQR, complex I, or AMPK abolishes therapeutic effects of H2S after intracerebral hemorrhage. SQR-induced uncoupling is mechanistically separable from complex IV inhibition by H2S.\",\n      \"method\": \"Genetic deletion (SQR KO, complex I KO, AMPK KO), mitochondrial membrane potential measurements, superoxide detection, AMPK activity assays, in vivo intracerebral hemorrhage model\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic deletions with defined molecular and physiological phenotypes, pathway placement by epistasis\",\n      \"pmids\": [\"32923620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Pathogenic variants in SQOR (p.Glu213Lys and c.446delT frameshift) cause Leigh disease with complex IV inhibition. p.Glu213Lys disrupts hydrogen bonding with neighboring residues, resulting in severely reduced SQOR protein and enzyme activity measured spectrophotometrically. Sulfide-generating enzyme levels were unchanged, indicating the mechanism is H2S accumulation due to loss of SQOR catabolism, which inhibits complex IV.\",\n      \"method\": \"Exome sequencing, spectrophotometric enzyme activity assay, western blotting, mitochondrial function assay in patient muscle and liver tissue\",\n      \"journal\": \"Journal of inherited metabolic disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct measurement of SQOR enzyme activity in patient tissue combined with protein level analysis and functional mitochondrial assays\",\n      \"pmids\": [\"32160317\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ASB1 (substrate recognition subunit of a ubiquitin E3 ligase) interacts with ELOB to promote K48-linked ubiquitination of SQOR at residues K207 and K344, leading to proteasomal degradation of SQOR. ASB1 knockout in mice causes elevated H2S, oxidative stress, and sperm DNA damage; fertility defects are rescued by NaHS (H2S donor), establishing SQOR degradation as the mechanism linking ASB1 to H2S homeostasis.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, site-directed mutagenesis of ubiquitination sites, ASB1 knockout mice, NaHS rescue experiment, western blotting\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, specific ubiquitination site mutagenesis, in vivo KO with mechanistic rescue\",\n      \"pmids\": [\"39733518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Pyruvate carboxylase (PC) physically interacts with SQOR; PC deficiency reduces this interaction, increasing ubiquitination and proteasomal degradation of SQOR. Loss of SQOR leads to mitochondrial morphological and functional disruption, increased mtDNA release, activation of the cGAS-STING pathway, elevated glycolysis, and renal fibrosis.\",\n      \"method\": \"Co-immunoprecipitation (PC-SQOR interaction), ubiquitination assay, PC knockout mice (PcxcKO) and stable KO cell line, cGAS-STING pathway assays, mitochondrial morphology imaging\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct protein interaction shown by Co-IP, mechanistic pathway defined by KO with multiple orthogonal readouts\",\n      \"pmids\": [\"39836535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SQOR knockdown in HCT116 colorectal cancer cells disrupts polysulfide homeostasis, diminishes mitochondrial function, impairs proliferation, and triggers early apoptosis. SQR KO reduces tumor growth in xenograft mice. Metabolomic analysis reveals reprogramming of glycolysis at the fructose-1,6-bisphosphate degradation step; western blot and enzymatic assays confirm decreased ALDOA levels and activity as the downstream effector.\",\n      \"method\": \"SQOR knockout, tumor xenograft model, metabolomics, western blotting, enzymatic activity assay for ALDOA\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined phenotypic and metabolic readouts, but ALDOA link is single-lab without full mechanistic reconstitution\",\n      \"pmids\": [\"40305883\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SQOR mediates ferroptosis resistance by using hydrogen selenide to reduce ubiquinone, thereby elevating ubiquinol levels and suppressing lipid peroxidation independently of GPX4. In the ATF4/CTH/SQOR axis, ATF4 drives CTH expression to generate H2S, which SQOR converts to CoQ10H2 (ubiquinol); SQOR-deficient cells cannot rescue CoQ10H2 or prevent ferroptosis even with elevated H2S or CTH overexpression.\",\n      \"method\": \"SQOR knockdown/overexpression, CoQ10H2 measurement, lipid peroxidation assay (MDA), ATF4/CTH/SQOR genetic epistasis, alternative oxidase overexpression\",\n      \"journal\": \"Biochemical pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple pathway components and defined biochemical readouts, single lab\",\n      \"pmids\": [\"41759595\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"SQOR interacts with the C9-ALS dipeptide repeat protein GA50 (identified by yeast two-hybrid). SQOR knockdown in HMC3 microglia activates the NLRP3 inflammasome by upregulating intracellular ROS and promoting cytoplasmic escape of mitochondrial DNA. The small molecule irisflorentin blocks the SQOR–GA50 interaction and inhibits NLRP3 inflammasome activity.\",\n      \"method\": \"Yeast two-hybrid, SQOR knockdown, ROS measurement, mtDNA cytoplasmic escape assay, NLRP3 inflammasome activity assay, small-molecule inhibitor competition\",\n      \"journal\": \"Antioxidants\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — yeast two-hybrid for interaction plus KD with defined inflammatory phenotype, single lab\",\n      \"pmids\": [\"37891975\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SQOR deficiency confined to murine intestinal epithelial cells perturbs colon bioenergetics in a manner reversed by antibiotics, establishing that microbial H2S is a significant local contributor to host mitochondrial energy metabolism via SQOR. Combined intestinal SQOR deficiency and high dietary methionine disrupts colon architecture, alters microbiome composition, increases systemic thiosulfate (H2S oxidation biomarker), raises ketone bodies, reduces locomotor activity, and decreases ventricular volume associated with lower aquaporin 1.\",\n      \"method\": \"Intestinal epithelial cell-specific SQOR knockout mice, antibiotic treatment, brain MRI, aquaporin 1 western blot, serum metabolomics\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific KO with multiple orthogonal readouts, but preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.04.09.647962\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"Human SQOR is an inner mitochondrial membrane FAD-containing flavoenzyme that catalyzes the two-electron oxidation of H2S to a sulfane sulfur intermediate (persulfide), transferring electrons to the coenzyme Q pool, with glutathione (and sulfite) acting as physiological persulfide acceptors to generate glutathione persulfide and ultimately thiosulfate; this activity drives reverse electron transport at complex I to induce mitochondrial uncoupling and AMPK activation, produces ubiquinol that suppresses ferroptosis independently of GPX4, is subject to proteasomal regulation via K48-linked ubiquitination (mediated by ASB1-ELOB) at K207 and K344, is stabilized by physical interaction with pyruvate carboxylase, and its loss-of-function causes toxic H2S accumulation, complex IV inhibition, mtDNA release with cGAS-STING activation, and Leigh disease.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SQOR (sulfide:quinone oxidoreductase) is a mitochondrial inner membrane flavoprotein that serves as the primary entry point for hydrogen sulfide catabolism, coupling the two-electron oxidation of H2S—via an FAD cofactor and active-site cysteine persulfide intermediate—to the reduction of ubiquinone, thereby linking sulfide detoxification to the mitochondrial electron transport chain [PMID:18248458, PMID:27806570, PMID:25303790]. The ubiquinol produced by SQOR confers ferroptosis resistance independently of GPX4 by acting as a radical-trapping antioxidant within the ATF4/CTH/SQOR axis [PMID:41759595], and SQOR-driven electron flux can induce reverse electron transport at complex I to generate superoxide, triggering mitochondrial uncoupling and AMPK activation [PMID:32923620]. SQOR protein levels are regulated by ASB1/ELOB-dependent K48-linked ubiquitination at K207 and K344, leading to proteasomal degradation, and are stabilized by interaction with pyruvate carboxylase; loss of this regulation causes mitochondrial dysfunction, mtDNA release, and cGAS-STING-driven inflammation [PMID:39733518, PMID:39836535]. Loss-of-function SQOR variants cause Leigh disease through toxic H2S accumulation and secondary complex IV inhibition [PMID:32160317].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Establishing that eukaryotic SQR is a mitochondrial membrane-bound flavoprotein that oxidizes sulfide and reduces ubiquinone resolved the identity of the enzyme linking H2S catabolism to the respiratory chain in eukaryotes.\",\n      \"evidence\": \"Heterologous expression in S. cerevisiae, purification, in vitro kinetics (Km for sulfide and decyl-ubiquinone), and mutagenesis of six conserved residues\",\n      \"pmids\": [\"18248458\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human SQOR had not yet been biochemically characterized\", \"Identity of physiological sulfur acceptor in eukaryotes unknown\", \"No structural model available\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification of essential active-site cysteines and a Cys-S-FAD adduct intermediate defined the catalytic mechanism of the reductive half-reaction.\",\n      \"evidence\": \"Site-directed mutagenesis, pre-steady-state kinetics of FAD reduction, and EPR spectroscopy in A. ferrooxidans SQR\",\n      \"pmids\": [\"25303790\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic details not yet confirmed for the mammalian ortholog\", \"Oxidative half-reaction mechanism with quinone not fully resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Demonstrating that SQR catalyzes two-electron oxidation of sulfide to a persulfide intermediate on C344, with CoA, cyanide, or sulfite as sulfane sulfur acceptors, established the product specificity and metabolic output of the enzyme.\",\n      \"evidence\": \"Purified S. aureus SQR enzyme assay, mass spectrometry detection of C344 persulfide, reconstitution of SQR + CstB thiosulfate pathway, and metabolite profiling in Δsqr strains\",\n      \"pmids\": [\"27806570\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether CoA-dependent persulfide transfer operates similarly in mammalian mitochondria\", \"Relative flux through different sulfur acceptor routes in vivo unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Showing that SQOR-dependent H2S oxidation drives reverse electron transport at complex I to produce superoxide, mitochondrial uncoupling, and AMPK activation revealed a signaling function beyond simple detoxification.\",\n      \"evidence\": \"Genetic deletion of SQR, complex I subunits, and AMPK in cellular and intracerebral hemorrhage mouse models with mitochondrial membrane potential and superoxide measurements\",\n      \"pmids\": [\"32923620\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether RET-mediated signaling is a general physiological function or context-dependent\", \"Quantitative contribution of SQOR-derived ubiquinol to total mitochondrial Q pool unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identification of pathogenic SQOR variants (p.Glu213Lys, c.446delT) that abolish enzyme activity and cause Leigh disease through toxic H2S accumulation and secondary complex IV inhibition established SQOR deficiency as a Mendelian metabolic disorder.\",\n      \"evidence\": \"Patient exome sequencing, spectrophotometric enzyme activity in muscle/liver tissue, Western blot for SQOR protein\",\n      \"pmids\": [\"32160317\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Number of confirmed families remains small\", \"Genotype-phenotype spectrum not fully defined\", \"No therapeutic intervention demonstrated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Detection of a physical interaction between SQOR and the C9orf72-ALS-associated GA dipeptide repeat protein, and the finding that SQOR knockdown elevates ROS and activates NLRP3 inflammasome, linked SQOR to neuroinflammatory innate immune signaling.\",\n      \"evidence\": \"Yeast two-hybrid screen; SQOR knockdown in HMC3 microglia; ROS and NLRP3 activity assays\",\n      \"pmids\": [\"37891975\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"GA50–SQOR interaction not confirmed by reciprocal Co-IP or in vivo\", \"Whether SQOR loss directly activates NLRP3 or acts through general mitochondrial dysfunction is unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Discovery that the ASB1/ELOB E3 ubiquitin ligase targets SQOR for K48-linked ubiquitination at K207 and K344, leading to proteasomal degradation, identified the first regulatory mechanism controlling SQOR protein turnover.\",\n      \"evidence\": \"Reciprocal Co-IP of ASB1–ELOB–SQOR; ubiquitination site mapping; Asb1 knockout mouse with spermiogenesis defect rescued by NaHS\",\n      \"pmids\": [\"39733518\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether additional E3 ligases target SQOR\", \"Signals that modulate ASB1-mediated SQOR degradation unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstration that pyruvate carboxylase physically interacts with and stabilizes SQOR against ubiquitin-dependent degradation, and that SQOR loss triggers mtDNA release and cGAS-STING-driven renal fibrosis, connected SQOR regulation to innate immune activation in kidney disease.\",\n      \"evidence\": \"Co-IP of PC–SQOR; PC knockout mice and cell lines; mitochondrial morphology, mtDNA release, and cGAS-STING pathway assays\",\n      \"pmids\": [\"39836535\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which PC stabilizes SQOR not defined at the structural level\", \"Whether PC-SQOR interaction is tissue-specific is unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"SQOR knockout in colorectal cancer cells disrupted polysulfide homeostasis, impaired ALDOA activity and fructose-1,6-bisphosphate metabolism, and reduced xenograft tumor growth, revealing a tumor-promoting metabolic role for SQOR.\",\n      \"evidence\": \"CRISPR SQOR KO in HCT116; metabolomics; ALDOA enzymatic assay; xenograft mouse model\",\n      \"pmids\": [\"40305883\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether ALDOA impairment is a direct or indirect consequence of polysulfide perturbation\", \"Generalizability beyond HCT116 not tested\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Epistasis experiments placed SQOR downstream of CTH in the ATF4/CTH/SQOR axis and showed that SQOR is required to convert H2S into ubiquinol (CoQ10H2) to confer ferroptosis resistance, independently of GPX4.\",\n      \"evidence\": \"SQOR-deficient renal cells; CTH and ATF4 overexpression; CoQ10H2 measurement; ferroptosis and lipid peroxidation assays; alternative oxidase expression to consume CoQ10H2\",\n      \"pmids\": [\"41759595\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Quantitative contribution of SQOR-derived CoQ10H2 versus other reductases in ferroptosis suppression not resolved\", \"Whether hydropersulfides contribute independently of ubiquinol requires further deconvolution\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of human SQOR catalysis, the tissue-specific hierarchy of sulfane sulfur acceptors in vivo, and the full spectrum of post-translational regulation remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of mammalian SQOR\", \"Relative in vivo contributions of CoA, glutathione, and sulfite as sulfur acceptors are not quantified\", \"Whether SQOR-derived RET signaling operates in tissues beyond brain is unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [0, 1, 2, 3, 4, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 1, 2, 6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 2, 3, 7, 9]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [0, 5, 6, 9]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ASB1\",\n      \"ELOB\",\n      \"PC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"SQOR is a mitochondrial inner-membrane FAD-containing flavoenzyme that catalyzes the two-electron oxidation of hydrogen sulfide to a persulfide intermediate, transferring electrons to the ubiquinone pool; glutathione and sulfite serve as physiological persulfide acceptors, channeling sulfane sulfur toward thiosulfate production through a conserved three-enzyme mitochondrial sulfide oxidation pathway [PMID:22852582, PMID:25225291, PMID:18494801]. By feeding electrons into coenzyme Q, SQOR drives reverse electron transport at complex I to activate AMPK-dependent signaling and elevates ubiquinol levels that suppress ferroptosis independently of GPX4 [PMID:32923620, PMID:41759595]. SQOR protein turnover is regulated by ASB1–ELOB-mediated K48-linked ubiquitination at K207 and K344 and is stabilized by physical interaction with pyruvate carboxylase; loss of SQOR causes toxic H2S accumulation, complex IV inhibition, mtDNA release with cGAS–STING activation, and is the basis of a Leigh disease caused by biallelic SQOR variants [PMID:39733518, PMID:39836535, PMID:32160317]. SQOR deficiency in colorectal cancer cells disrupts polysulfide homeostasis, impairs proliferation, and reduces tumor growth in xenograft models [PMID:40305883].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Identification of a eukaryotic sulfide:quinone oxidoreductase (HMT2 in fission yeast) established that mitochondria harbor a dedicated FAD-dependent enzyme for H₂S oxidation coupled to quinone reduction, resolving the long-open question of how eukaryotic cells detoxify sulfide.\",\n      \"evidence\": \"Purified HMT2 protein characterized by cell fractionation, immunocytochemistry, and in vitro enzymatic assay in S. pombe\",\n      \"pmids\": [\"10224084\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human ortholog not yet characterized\", \"Physiological persulfide acceptor unknown\", \"Tissue distribution of activity undefined\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Reconstitution of a complete three-step mitochondrial sulfide oxidation pathway (SQR → sulfur dioxygenase → rhodanese) and kinetic characterization of eukaryotic SQR with site-directed mutagenesis defined the catalytic requirements and pathway topology for sulfide catabolism to thiosulfate.\",\n      \"evidence\": \"Heterologous expression in yeast, mutagenesis of conserved catalytic residues, and pathway reconstitution in rat liver mitochondria\",\n      \"pmids\": [\"18248458\", \"18494801\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human enzyme kinetics not yet measured\", \"Electron acceptor specificity in mammalian membranes unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstration that SQR activity is tissue-specific in mammals (present in kidney, liver, heart; absent in brain) and that SQR-driven sulfide oxidation produces the first evidence of reverse electron transfer in living cells resolved why neural tissue is uniquely sensitive to sulfide toxicity.\",\n      \"evidence\": \"Mitochondrial isolation from mouse tissues, SQR activity assay, oxygen consumption and cellular sulfide measurements in HT-29 colonocytes\",\n      \"pmids\": [\"20398623\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of tissue-specific expression unknown\", \"In vivo physiological significance of reverse electron transfer not yet established\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Kinetic and spectroscopic characterization of purified recombinant human SQOR revealed noncovalently bound FAD, a long-wavelength charge-transfer intermediate, and high catalytic efficiency (kcat/Km ~2.9 × 10⁷ M⁻¹ s⁻¹), establishing the molecular properties of the human enzyme and identifying sulfite as a candidate physiological persulfide acceptor.\",\n      \"evidence\": \"Recombinant human SQOR expressed in E. coli, UV-vis spectroscopy, pre-steady-state kinetics\",\n      \"pmids\": [\"22852582\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Glutathione not yet tested as persulfide acceptor\", \"No structural model of human SQOR available\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovery that glutathione serves as the primary persulfide acceptor for human SQOR, generating GSSH, reframed the pathway: the dominant route is H₂S → GSSH → thiosulfate rather than direct sulfite acceptance, and rhodanese functions biosynthetically in thiosulfate formation.\",\n      \"evidence\": \"In vitro reconstitution with purified enzymes at physiological metabolite concentrations, kinetic modeling\",\n      \"pmids\": [\"25225291\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of GSH vs sulfite in vivo unresolved\", \"Structural basis of persulfide acceptor selectivity unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Genetic epistasis experiments showed SQR-driven electron flow into the Q-pool powers reverse electron transport at complex I to generate superoxide and activate AMPK, establishing a signaling function for sulfide oxidation beyond simple detoxification, and biallelic SQOR mutations were identified as the cause of Leigh disease through toxic H₂S accumulation and complex IV inhibition.\",\n      \"evidence\": \"SQR, complex I, and AMPK knockouts with mitochondrial membrane potential and superoxide readouts in vivo (ICH model); exome sequencing, spectrophotometric enzyme assay, and western blot in patient tissue\",\n      \"pmids\": [\"32923620\", \"32160317\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AMPK activation is direct or secondary to energy stress unclear\", \"Full spectrum of SQOR loss-of-function clinical phenotypes not delineated\", \"Structural impact of pathogenic variants modeled only in silico\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identification of ASB1–ELOB as the E3 ligase complex that K48-ubiquitinates SQOR at K207 and K344 for proteasomal degradation defined the first post-translational turnover mechanism for SQOR, linking ubiquitin-mediated SQOR degradation to H₂S homeostasis and male fertility.\",\n      \"evidence\": \"Reciprocal Co-IP, ubiquitination site mutagenesis, ASB1 knockout mice with NaHS rescue\",\n      \"pmids\": [\"39733518\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other E3 ligases also target SQOR unknown\", \"Signals triggering ASB1-mediated degradation not defined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Multiple studies converged to show that SQOR loss has broad downstream consequences: pyruvate carboxylase stabilizes SQOR protein and its absence triggers ubiquitination, mtDNA release, cGAS–STING activation and renal fibrosis; SQOR-generated ubiquinol suppresses ferroptosis independently of GPX4 via the ATF4/CTH/SQOR axis; and SQOR deficiency in colorectal cancer cells disrupts polysulfide balance, impairs proliferation, and reduces xenograft tumor growth.\",\n      \"evidence\": \"PC–SQOR Co-IP and PC-KO mice; SQOR KD/OE with CoQ10H2, lipid peroxidation, and genetic epistasis; SQOR-KO HCT116 xenografts with metabolomics\",\n      \"pmids\": [\"39836535\", \"41759595\", \"40305883\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PC–SQOR interaction is direct or complex-mediated not fully resolved\", \"ALDOA downregulation mechanism upon SQOR loss requires independent confirmation\", \"Relative importance of ubiquinol production vs H₂S clearance in ferroptosis protection unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A high-resolution structure of human SQOR, the regulatory signals controlling SQOR expression across tissues, the in vivo contribution of microbial H₂S to SQOR-dependent host bioenergetics, and the full clinical spectrum of SQOR deficiency remain to be established.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of human SQOR\", \"Transcriptional and epigenetic regulation largely uncharacterized\", \"Tissue-specific conditional knockouts beyond intestine not fully explored\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [0, 1, 2, 4, 5, 6, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 2, 3, 4, 8, 9, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [2, 4, 5, 8]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [12, 13]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"PC\",\n      \"ASB1\",\n      \"ELOB\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}