1.8.5.8: eukaryotic sulfide quinone oxidoreductase
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For detailed information about eukaryotic sulfide quinone oxidoreductase, go to the full flat file.
Reaction
Synonyms
glutathione:CoQ reductase, HMT2, ScSQR, SQOR, SQR, Sqrdl, sulfide : quinone oxidoreductase, sulfide quinone oxidoreductase, sulfide:quinone oxidoreductase
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General Information
General Information on EC 1.8.5.8 - eukaryotic sulfide quinone oxidoreductase
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evolution
malfunction
metabolism
physiological function
additional information
human SQOR belongs to a family of flavoprotein disulfide reductases (FDR), members of which contain a pair of redox-active cysteine residues that typically form a disulfide in the resting enzyme
evolution
SQOR is a member of the diverse and extensive flavin disulfide reductase (FDR) superfamily. Group 4 FDRs, which includes SQOR, are the most diverse both structurally and functionally. SQORs contain two redox active cysteines that are widely separated in the primary sequence but are spatially proximal. A hallmark of this subgroup is that they can utilize diverse substrates. Many members, including SQOR and flavocytochrome c sulfide dehydrogenase (FCSD), do not use pyridine nucleotides, but instead, substitute CoQ (in SQOR), or cytochrome c (in FCSD) as an electron acceptor. In both of these cases, the enzymes oxidize sulfide. SQOR utilizes several of the chemical principles common to the FDR superfamily, as well as some unique sulfurbased chemistry during the catalytic cycle
evolution
sulfide quinone oxidoreductase (SQOR) is a member of the flavin disulfide reductase superfamily
cyanolysis leads to reversible loss of SQOR activity that is restored in the presence of sulfide. Inherited deficiency of SQOR presents as Leigh disease. Cyanolysis of the bridging sulfur decreases enzyme SQOR protein stability
malfunction
inherited deficiency of SQOR presents as a cause of Leigh disease with decreased complex IV activity. The symptoms include lactic acidosis, multi-organ failure, neurological disorders, and Leigh-like brain lesions. Two patients who are siblings are homozygous for the Glu213Lys mutation, affecting a residue that is remote from the active site, but predicted to disrupt hydrogen bonding with neighboring arginine residues. The third patient is homozygous for a single base pair deletion (c446delT) in the SQOR gene, predicted to lead to mRNA degradation or production of nonfunctional enzyme due to the resulting frameshift. The mutations lead to greatly diminished SQOR levels in tissues expressing the Glu213Lys mutation while SQOR is not detected in fibroblasts carrying the deletion mutation. Complex IV activity but not its assembly is adversely affected by SQOR deficiency. The gastrointestinal defects and accumulation of acylcarnitines reported for ETHE1 deficiency are not seen in SQOR deficiency although elevated H2S is reported for both conditions
malfunction
STI1 dramatically improves survival, preserves cardiac function, and prevents the progression to HFrEF by impeding the transition from compensated to decompensated left ventricle hypertrophy
malfunction
treatment of transverse aortic constriction (TAC) mice with enzyme SQOR inhibitor STI1 mitigates the development of cardiomegaly, pulmonary congestion, dilatation of the left ventricle, and cardiac fibrosis and decreases the pressure gradient across the aortic constriction. STI1 dramatically improves survival, preserves cardiac function, and prevents the progression to HFrEF by impeding the transition from compensated to decompensated left ventricle hypertrophy
malfunction
treatment of transverse aortic constriction (TAC) mice with enzyme SQOR inhibitor STI1 mitigates the development of cardiomegaly, pulmonary congestion, dilatation of the left ventricle, and cardiac fibrosis and decreases the pressure gradient across the aortic constriction. STI1 mitigates the pressure gradient across the aortic constriction created by TAC
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Heart failure patients with reduced ejection fraction (HFrEF) exhibit low levels of H2S. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
metabolism
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
metabolism
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
metabolism
sulfide quinone oxidoreductase (SQOR) catalyzes the first and committing step in the mitochondrial sulfide oxidation pathway, overview. Hydrogen sulfide (H2S) is an environmental toxin and a heritage of ancient microbial metabolism, acting as a neuromodulator. While many physiological responses have been attributed to low H2S levels, higher levels inhibit complex IV in the electron transport chain. To prevent respiratory poisoning, a dedicated set of enzymes that make up the mitochondrial sulfide oxidation pathway exists to clear H2S. The committed step in this pathway is catalyzed by sulfide quinone oxidoreductase (SQOR), which couples sulfide oxidation to coenzyme Q10 reduction in the electron transport chain. The SQOR reaction prevents H2S accumulation and generates highly reactive persulfide species as products. These can be further oxidized or can modify cysteine residues in proteins by persulfidation. Alternatively, sulfite is oxidized to sulfate by sulfite oxidase, which resides in the intermembrane space. Electrons from the sulfide oxidation pathway enter the electron transfer chain at the level of Complex III (from SQOR) and cytochrome c/Complex IV (from sulfite oxidase). Interplay of sulfide and butyrate oxidation. The activities of SQOR and ACADS both drive electrons into the mitochondrial Q pool, which restricts the capacity for sulfide oxidation during acute H2S exposure. As a countermeasure, SQOR can catalyze the formation of CoA-SSH, a tightbinding inhibitor of ACADs. Inhibition of ACADS by CoA-SSH relieves competition for the Q pool to prioritize sulfide oxidation. Under certain pathological conditions, such as sulfite oxidase deficiency that is marked by elevated sulfite, oxidative stress conditions that lead to GSH depletion, or periodontitis marked by elevated methanethiol, adventitious nucleophilic additions into the active site trisulfide in SQOR can become physiologically relevant and lead to impaired H2S clearance. Sulfide oxidation by SQOR can exert a direct influence on bioenergetics via the mitochondrial CoQ pool, which represents a major redox nexus. Coupling of SQOR activity to the CoQ pool creates intersections between sulfide metabolism and: 1. complex I, which oxidizes NADH, 2. complex II, which oxidizes succinate and FADH2, 3. dihydroorotate dehydrogenase, which is involved in de novo pyrimidine biosynthesis, 4. glycerol-3-phosphate dehydrogenase, which links to both carbohydrate and lipid metabolism, and 5. the electron-transferring flavoprotein dehydrogenase, which is involved in fatty acid and branched chain amino acid metabolism. Acute exposure to high H2S levels in the colon, with a consequent decrease in the CoQ/CoQH2 ratio could limit other activities that rely on an oxidized CoQ pool. Thus, in addition to respiratory inhibition, H2S oxidation can also inhibit other oxidative metabolic pathways
metabolism
sulfide quinone oxidoreductase (SQOR) catalyzes the first step in sulfide clearance, coupling H2S oxidation to coenzyme Q reduction
metabolism
sulfide:quinone oxidoreductase (SQOR) catalyzes the first irreversible step in the mitochondrial metabolism of hydrogen sulfide (H2S). SQOR plays a major role in controlling physiological levels of H2S and sits at a key pharmacological intervention point
due to the little exchange of seawater and to anoxic conditions, Sinonovacula constricta is exposed to considerable amounts of sulfide during low tide, but exhibits strong sulfide tolerance. Mitochondrial sulfide oxidation is a particular defense strategy against sulfide toxicity of sulfide-tolerant organisms, for which sulfide:quinone oxidoreductase (SQR) is the first key enzyme. In order to investigate the mechanism of sulfide tolerance including ScSQR. Th enzyme has an important role in protecting cells from sulfide stress by participating in mitochondrial sulfide detoxification, it catalyzes electron transfer from sulfide to ubiquinone through the FAD cofactor
physiological function
higher H2S levels inhibit complex IV in the electron transport chain. To prevent respiratory poisoning, a dedicated set of enzymes that make up the mitochondrial sulfide oxidation pathway exists to clear H2S. The committed step in this pathway is catalyzed by sulfide quinone oxidoreductase (SQOR), which couples sulfide oxidation to coenzyme Q10 reduction in the electron transport chain. The SQOR reaction prevents H2S accumulation and generates highly reactive persulfide species as products. These can be further oxidized or can modify cysteine residues in proteins by persulfidation. The human SQOR shows unconventional redox cofactor configuration and substrate promiscuity leading to sulfide clearance and potentially expand the signaling potential of H2S. SQOR is a mitochondrial inner membrane-anchored flavoenzyme that is poised to play a critical role in H2S-based signaling while also serving as a guardian of the electron transfer chain against H2S poisoning. Interplay of sulfide and butyrate oxidation. The activities of SQOR and ACADS both drive electrons into the mitochondrial Q pool, which restricts the capacity for sulfide oxidation during acute H2S exposure. As a countermeasure, SQOR can catalyze the formation of CoA-SSH, a tightbinding inhibitor of ACADs. Inhibition of ACADS by CoA-SSH relieves competition for the Q pool to prioritize sulfide oxidation. The promiscuity of SQOR can be traced to the large electropositive entrance to the active site that accommodates a range of substrates and has the potential to generate a variety of low-molecular-weight persulfides. The production of CoA-SSH, long known as a tight binding inhibitor of ACADS, has been traced to the relaxed substrate specificity of SQOR and can prioritize sulfide over butyrate oxidation during acute colonic exposure to H2S. It is yet not known whether other persulfides are produced via SQOR in a tissue-specific manner and in response to intra- or extracellular triggers. SQOR was proposed to play a role in proteostasis overview
physiological function
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Heart failure patients with reduced ejection fraction (HFrEF) exhibit low levels of H2S. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
physiological function
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
physiological function
hydrogen sulfide (H2S) is a potent signalling molecule that activates diverse cardioprotective pathways by posttranslational modification (persulfidation) of cysteine residues in upstream protein targets. Sulfide:quinone oxidoreductase (SQOR) catalyses the first irreversible step in the metabolism of H2S and plays a key role in regulating H2S-mediated signalling
physiological function
hydrogen sulfide (H2S) is a signaling molecule that exerts physiological effects in the cardiovascular, central nervous, and gastrointestinal systems. At higher concentrations, H2S can act as a respiratory poison that blocks the electron transport chain by inhibiting complex IV. Due to the dual effects of H2S, its levels must be strictly regulated. The accumulation of toxic concentrations of H2S is prevented by its oxidation to thiosulfate and sulfate via the mitochondrial sulfide oxidation pathway. The first and committed step in this pathway is catalyzed by sulfide quinone oxidoreductase (SQOR), an inner mitochondrial membrane-anchored flavoprotein. SQOR catalyzes coupling of H2S oxidation to coenzyme Q reduction. It transfers the oxidized sulfane sulfur to a small molecule acceptor, which is predicted to be glutathione (GSH) under physiological conditions
physiological function
SQOR plays a major role in controlling physiological levels of H2S. SQOR catalyzes a two-electron oxidation of H2S to sulfane sulfur (S0). It uses coenzyme Q (CoQ) as electron acceptor and sulfite or glutathione as sulfane sulfur acceptor in reactions that produce thiosulfate or glutathione persulfide (GSS-), respectively
physiological function
the catalytic promiscuity of human sulfide quinone oxidoreductase supports formation of CoA-persulfide, a known inhibitor of short-chain fatty acid oxidation in colonocytes, analysis of several enzyme crystal structures shows a different mechanism for sulfide oxidation, which is catalyzed by an active site cysteine trisulfide. Identification of CoA as an additional sulfur acceptor for human SQR, its generation in situ leads to formation of the inhibitory CoA-SS- to FAD charge-transfer complex in human ACADS (short-chain specific acyl-CoA dehydrogenase)
molecular basis for the enzyme's ability to catalyze sulfane sulfur transfer reactions with structurally diverse acceptors, molecular basis for the enzyme's ability to catalyze sulfane sulfur transfer reactions with structurally diverse acceptors, overview. Human SQOR contains unique features: an electro-positive surface depression implicated as a binding site for sulfane sulfur acceptors and postulated to funnel negatively charged substrates to a hydrophilic H2S-oxidizing active site, which is connected to a hydrophobic internal tunnel that binds coenzyme Q. The enzyme has a unique binding site for sulfane sulfur acceptors. The two active-site cysteine residues (Cys201, Cys379) lie just above the re-face of the flavin ring. In both the SeMet-substituted and native enzyme structures, these cysteine residues are linked by a bridging sulfur to form thiocystine (Cys-S-S-S-Cys). Structure-analysis relationship, overview. The two C-terminal helices lie in the plane of the mitochondrial membrane, with their hydrophobic faces communicating with the membrane interior. This monotopic mode of association allows the enzyme to gain access to its hydrophobic electron acceptor, CoQ. The position of the C-terminal helix is likely perturbed by a crystal contact with the penultimate helix in a symmetry-related molecule. Thus, the last residue in the C-terminal helix, Glu452, forms hydrogen bonds with residues (Arg411, Leu412, Ser413) in the penultimate helix of the adjacent chain
additional information
structural basis for catalytic promiscuity in SQOR, overview. Electrostatic surface potential map of the SQOR monomer, revealing a large electropositive cavity containing the exposed 379Cys-SSH persulfide. GSH is docked in the cavity, binding structure analysis
additional information
structures of human SQOR reveal a sulfur atom bridging the SQOR active site cysteines in a trisulfide configuration. Computational modeling and molecular dynamics simulations revealed an about 105fold rate enhancement for nucleophilic addition of sulfide into the trisulfide versus a disulfide cofactor. The cysteine trisulfide in SQOR is thus critical for activity and provides a significant catalytic advantage over a cysteine disulfide. Descriptors of intrinsic reactivity derived from the electronic structure of Cys379 and Cys201 are calculated at the QM/MM level in the framework of a conceptual DFT. DFT-PCM modeling of reaction mechanisms and barriers for sulfide nucleophilic attack, detailed overview
additional information
the deduced ScSQR protein contains conserved FAD-binding domains, two cysteine residues (C190 and C371), two histidines (H68 and H282), and one glutamic acid (E147), which are the essential elements for the catalytic mechanism of SQR
additional information
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the deduced ScSQR protein contains conserved FAD-binding domains, two cysteine residues (C190 and C371), two histidines (H68 and H282), and one glutamic acid (E147), which are the essential elements for the catalytic mechanism of SQR
additional information
the large cavity extending from the matrix face to the active site exposes Cys379 and explains the vulnerability of solubilized SQR to errant attack by nucleophiles (sulfite, GSH, homocysteine, cysteine and methanethiol). Docking models reveal that either GSH or CoA can be accommodated at the mouth of the cavity with their thiol moieties oriented to interact with the 379Cys-SSH intermediate. The significantly greater intracellular concentration of GSH than sulfite or methanethiol, except perhaps under pathological conditions, is predicted to favor GSSH formation. Overall structure and active site architecture of SQR, overview. The sulfur substrate entry site is on the matrix side, CoQ entry from the mitochondrial membrane side