GO:0045273 respiratory chain complex II (succinate dehydrogenase): Components, Assembly and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0045273 describes respiratory chain complex II (succinate dehydrogenase, SDH), the only enzyme shared between the TCA cycle and the mitochondrial electron transport chain.
• Complex II contains four canonical polypeptide subunits (SDHA, SDHB, SDHC, SDHD) plus FAD and iron-sulfur clusters, and catalyzes succinate oxidation by ubiquinone.
• Beyond bioenergetics, complex II acts as a general sensor for apoptosis and a source of reactive oxygen species signals.
• Germline and somatic mutations in SDH subunits drive hereditary paragangliomas, pheochromocytomas, and SDH-deficient gastrointestinal stromal tumors.
• Complex II dysfunction is implicated in mitochondrial myopathies and encephalomyopathies, and in metabolic control of immune cell fate [2,7,8].
• CRISPR knockout, point-mutation, knock-in, and overexpression models are essential to dissect subunit-specific roles in disease and metabolism [3,4].
Description
Respiratory chain complex II (succinate dehydrogenase, SDH) is a mitochondrial inner-membrane enzyme that occupies a unique position in cellular metabolism: it is the only component of the respiratory chain that is also an enzyme of the tricarboxylic acid (TCA) cycle. The Gene Ontology term GO:0045273 captures this dual identity, defining a complex that contains the four polypeptide subunits of succinate dehydrogenase, flavin-adenine dinucleotide (FAD), and iron-sulfur clusters, and that catalyzes the oxidation of succinate by ubiquinone. Because it physically and functionally connects the TCA cycle with the respiratory chain, complex II is a central node for understanding how mitochondrial substrate oxidation is coupled to ATP production and to biosynthetic metabolism. For researchers, GO:0045273 is more than a static annotation. Complex II has emerged as a signaling hub that can trigger apoptosis in response to metabolic stress, and its subunits are recurrently mutated in human tumors [1,5]. The SDHA, SDHB, SDHC, and SDHD genes, together with assembly factors such as SDHAF1 and SDHAF2, determine whether the enzyme is properly assembled and active [1,3]. Mutations that destabilize the complex lead to succinate accumulation, pseudohypoxic signaling, and distinct disease phenotypes ranging from hereditary paraganglioma to gastrointestinal stromal tumors [1,3]. This article synthesizes the authoritative QuickGO definition of GO:0045273 with verified PubMed literature to provide a publication-ready overview of the complex's composition, catalytic mechanism, regulation, disease relevance, and the CRISPR-based experimental models used to study it [1,2,3,4,5,6,7,8]. It is intended for scientists who need a precise, citable reference for the role of respiratory chain complex II in health and disease.
respiratory chain complex II (succinate dehydrogenase) At A Glance
| GO ID | GO:0045273 |
|---|---|
| GO term | respiratory chain complex II (succinate dehydrogenase) |
| Ontology | cellular_component |
| Synonym | electron transport complex II |
| Major function | Catalyzes the oxidation of succinate by ubiquinone, connecting the TCA cycle with the respiratory chain |
| Subunits | Four polypeptide subunits of succinate dehydrogenase (SDHA, SDHB, SDHC, SDHD) |
| Cofactors | Flavin-adenine dinucleotide (FAD) and iron-sulfur clusters |
| Localization | Mitochondrial inner membrane |
| Pathological relevance | SDH-deficient tumors, mitochondrial myopathies, and apoptosis signaling [1,2,5] |
What Is GO:0045273?
In our own words, GO:0045273 (respiratory chain complex II, also called succinate dehydrogenase) is a multi-subunit enzyme complex embedded in the mitochondrial inner membrane. It contains the four polypeptide subunits of succinate dehydrogenase, the cofactor flavin-adenine dinucleotide (FAD), and iron-sulfur clusters. The complex catalyzes the oxidation of succinate to fumarate while reducing ubiquinone to ubiquinol, thereby feeding electrons into the respiratory chain. Because the same enzyme oxidizes succinate in the TCA cycle, GO:0045273 represents the physical and functional connection between the TCA cycle and the respiratory chain.
Why Is respiratory chain complex II (succinate dehydrogenase) Important in Cell Biology?
GO:0045273 is important because complex II is the only respiratory chain complex that is also a TCA cycle enzyme, making it a critical integrator of mitochondrial energy production and biosynthesis. Its activity determines the rate at which succinate is oxidized, and its dysfunction causes succinate accumulation that can drive pseudohypoxic signaling and tumorigenesis [1,3]. In addition, complex II has been identified as a general sensor for apoptosis, linking metabolic status to cell death decisions. These features make complex II a focal point for research in cancer metabolism, mitochondrial disease, immunometabolism, and therapeutic targeting [1,2,5,7,8].
• Complex II is the only enzyme shared between the TCA cycle and the respiratory chain, making it a central metabolic hub.
• SDH subunit mutations cause hereditary paragangliomas, pheochromocytomas, and SDH-deficient gastrointestinal stromal tumors.
• Complex II acts as a general sensor for apoptosis, coupling metabolic stress to cell death.
• SDHC has roles in cellular physiology and disease beyond its canonical bioenergetic function.
• SDHB is coupled to NEK7 to orchestrate respiratory chain electron transport homeostasis and impede liver fibrosis.
• Distinct modes of mitochondrial metabolism, including complex II activity, uncouple T cell differentiation from function.
• Mitochondrial myopathies and encephalomyopathies frequently involve respiratory chain complex II defects [2,8].
• Complex II is a source of reactive oxygen species that can act in signaling and pathology.
• FADH2 ambiguities in the electron transfer system highlight the need for precise complex II measurements.
• CRISPR models of SDH genes enable causal testing of subunit-specific functions in disease [1,3,4].
What Happens During respiratory chain complex II (succinate dehydrogenase)?
Succinate oxidation and ubiquinone reduction
In simple terms: Complex II takes electrons from succinate and hands them to ubiquinone.
The catalytic core of complex II oxidizes succinate to fumarate in the TCA cycle and transfers the extracted electrons to the mobile carrier ubiquinone, reducing it to ubiquinol. This reaction is the defining catalytic activity of GO:0045273 and directly links the TCA cycle to the respiratory chain. The reaction requires FAD as the initial electron acceptor and iron-sulfur clusters to relay electrons to the ubiquinone-binding site.
Electron transfer to the respiratory chain
In simple terms: The electrons from succinate enter the respiratory chain to help make ATP.
Once ubiquinone is reduced to ubiquinol, the electrons enter the respiratory chain at the level of complex III, contributing to the proton gradient used for ATP synthesis. Complex II itself does not pump protons, but its electron transfer activity is essential for maintaining respiratory chain electron transport homeostasis [4,6]. NEK7 has been shown to couple SDHB to this homeostatic control, and disruption of this axis impedes liver fibrosis.
Apoptosis sensing
In simple terms: Complex II can sense danger and trigger cell death.
Beyond its bioenergetic role, respiratory chain complex II acts as a general sensor for apoptosis, translating metabolic or oxidative stress into cell death signals. This function is particularly relevant in cancer and neurodegeneration, where dysregulated apoptosis contributes to pathogenesis. The apoptotic sensor function is distinct from the catalytic activity and involves the complex's ability to generate or respond to reactive oxygen species.
Metabolic integration in immune cells
In simple terms: Complex II helps immune cells decide what to become.
Distinct modes of mitochondrial metabolism, including complex II-dependent respiration, uncouple T cell differentiation from effector function. This means that the metabolic state imposed by complex II activity can influence immune cell fate independently of cytokine-driven differentiation. These findings place GO:0045273 at the intersection of immunometabolism and mitochondrial biology.
Key Genes Involved in GO:0045273 respiratory chain complex II (succinate dehydrogenase)
The following genes encode the subunits and assembly factors of respiratory chain complex II (GO:0045273) and are the most frequently studied in disease and metabolism research.
| Gene | Major Role | Research Relevance |
|---|---|---|
| SDHA | Flavoprotein subunit that binds FAD and oxidizes succinate | Mutations cause SDH-deficient tumors and mitochondrial disease [1,6] |
| SDHB | Iron-sulfur subunit that relays electrons to ubiquinone | Most common SDH subunit mutated in hereditary paraganglioma and GIST [1,4] |
| SDHC | Membrane-anchoring subunit that binds ubiquinone | Roles in cellular physiology and disease beyond bioenergetics |
| SDHD | Membrane-anchoring subunit required for complex stability | Germline mutations cause hereditary paraganglioma |
| SDHAF1 | Assembly factor required for SDHB maturation | Defects impair complex II assembly and cause mitochondrial disease |
| SDHAF2 | Assembly factor required for FAD insertion into SDHA | Mutations cause hereditary paraganglioma |
| NEK7 | Kinase that couples SDHB to electron transport homeostasis | Regulates liver fibrosis via complex II |
| UQCRB | Ubiquinone-binding protein of complex III | Interacts functionally with complex II electron transfer |
| UQCRC1 | Core subunit of complex III | Downstream electron acceptor from complex II |
| SDHAF3 | Assembly factor for iron-sulfur cluster insertion | Supports complex II maturation |
| SDHAF4 | Assembly factor that protects SDHA during assembly | Required for efficient complex II assembly |
| FAD | Cofactor that accepts electrons from succinate | Essential for catalytic activity of SDHA |
| Iron-sulfur clusters | Electron relay centers in SDHB | Essential for electron transfer to ubiquinone |
| Succinate | Substrate oxidized to fumarate | Accumulates when complex II is dysfunctional [1,6] |
| Ubiquinone | Electron acceptor reduced to ubiquinol | Links complex II to the respiratory chain |
| SDHD | Membrane anchor and ubiquinone binding | Target for CRISPR knockout studies in paraganglioma |
| SDHC | Membrane anchor and ubiquinone binding | Target for functional studies in disease |
| SDHB | Iron-sulfur subunit | Target for point-mutation models of tumorigenesis [1,4] |
How Is respiratory chain complex II (succinate dehydrogenase) Regulated?
Respiratory chain complex II is regulated at multiple levels. Its assembly requires dedicated assembly factors such as SDHAF1, SDHAF2, SDHAF3, and SDHAF4, which ensure proper FAD insertion and iron-sulfur cluster maturation. The kinase NEK7 has been shown to couple SDHB to respiratory chain electron transport homeostasis, providing a post-translational regulatory mechanism that impedes liver fibrosis. In immune cells, distinct modes of mitochondrial metabolism regulate T cell differentiation and function, indicating that complex II activity is tuned to cellular state. Additionally, complex II acts as a general sensor for apoptosis, meaning its activity can be modulated by oxidative stress and apoptotic signaling. These regulatory layers ensure that complex II function is matched to metabolic demand and stress conditions [1,4,5,7].
respiratory chain complex II (succinate dehydrogenase) and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| SDHA | SDH-deficient GIST | SDHA knockout GIST cell line |
| SDHB | Hereditary paraganglioma, liver fibrosis | SDHB point-mutation knock-in mouse [1,4] |
| SDHC | Paraganglioma, cellular physiology | SDHC knockout cell model |
| SDHD | Hereditary paraganglioma | SDHD knockout mouse |
| SDHAF2 | Hereditary paraganglioma | SDHAF2 knockout cell line |
SDH-deficient gastrointestinal stromal tumors
SDH-deficient gastrointestinal stromal tumors (GISTs) are a distinct subtype of GIST that lack KIT or PDGFRA mutations and instead harbor mutations in SDH subunits, most commonly SDHA. These tumors are characterized by loss of complex II immunostaining and succinate accumulation, which drives pseudohypoxic signaling. The diagnosis and classification of SDH-deficient GISTs rely on immunohistochemistry for SDHB, and their clinical behavior differs from conventional GISTs.
Hereditary paraganglioma and pheochromocytoma
Germline mutations in SDHB, SDHC, SDHD, SDHAF1, and SDHAF2 cause hereditary paragangliomas and pheochromocytomas, often with multiple tumors and a high risk of malignancy for SDHB mutations. The unifying mechanism is complex II dysfunction leading to succinate accumulation and activation of hypoxia-inducible factors [1,3]. These syndromes illustrate how a single respiratory chain complex can be a tumor suppressor hub.
Mitochondrial myopathies and encephalomyopathies
Defects in respiratory chain complex II are among the causes of mitochondrial myopathies and encephalomyopathies, a heterogeneous group of disorders that often present with muscle weakness, exercise intolerance, and neurological symptoms [2,8]. Complex II deficiency can be isolated or part of a broader respiratory chain defect, and diagnosis requires biochemical and genetic testing [2,8]. These diseases highlight the importance of complex II for normal muscle and brain function [2,8].
Liver fibrosis and metabolic regulation
Recent work has shown that NEK7 couples SDHB to respiratory chain electron transport homeostasis, and that disruption of this axis impedes liver fibrosis. This finding expands the disease relevance of complex II beyond classical mitochondrial disorders and cancer, implicating it in fibrotic remodeling. The study suggests that targeting complex II regulatory proteins could be a therapeutic strategy for liver fibrosis.
From respiratory chain complex II (succinate dehydrogenase)-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of SDHB cause succinate accumulation and pseudohypoxia? | SDHB knockout cell line |
| Does a specific SDHA mutation impair FAD binding? | SDHA point-mutation knock-in |
| Can wild-type SDHC rescue complex II deficiency? | SDHC overexpression |
| How does NEK7 regulate SDHB in liver fibrosis? | NEK7 knockout or SDHB knock-in mouse |
| What is the role of complex II in T cell differentiation? | Inducible SDHA knockout in T cells |
| Does complex II act as an apoptosis sensor? | SDHC knockout with apoptotic challenge |
How to Study the respiratory chain complex II (succinate dehydrogenase) Process
| Method | What It Measures | Typical Application |
|---|---|---|
| High-resolution respirometry | Complex II-dependent oxygen consumption | Mitochondrial function |
| SDHB immunohistochemistry | Loss of complex II subunit | Tumor diagnosis |
| Immunoblotting | Subunit protein levels | Assembly defects |
| Metabolomics | Succinate and TCA intermediates | SDH deficiency [1,6] |
| CRISPR knockout screen | Gene essentiality and synthetic lethality | Cancer and immune cells [1,7] |
| RNA-seq | Transcriptional changes | Pathway analysis |
| Proteomics | Complex II interactome | Assembly and regulation |
Respirometry and enzyme assays
High-resolution respirometry and spectrophotometric assays measure complex II activity by following succinate-dependent ubiquinone reduction or oxygen consumption. These methods are essential to distinguish complex II from other respiratory chain contributions and to quantify FADH2-linked respiration. They are typically applied to isolated mitochondria or permeabilized cells.
Immunohistochemistry and immunoblotting
Immunohistochemistry for SDHB is the standard diagnostic tool for SDH-deficient tumors, as loss of SDHB staining indicates complex II dysfunction. Immunoblotting for SDHA, SDHB, SDHC, and SDHD can confirm subunit loss and assess assembly defects. These methods are widely used in pathology and research laboratories.
Metabolomics and succinate measurement
Mass spectrometry-based metabolomics quantifies succinate and other TCA cycle intermediates, providing a direct readout of complex II dysfunction [1,6]. Succinate accumulation is a hallmark of SDH-deficient tumors and can be used as a biomarker. This approach is often combined with stable isotope tracing to assess flux.
CRISPR screening and functional genomics
CRISPR knockout screens targeting SDH subunits and assembly factors can identify synthetic lethal interactions and pathways that depend on complex II [1,3]. These screens are typically performed in cancer cell lines or primary immune cells to uncover context-specific vulnerabilities [1,7]. Bioinformatics analysis of screen data then prioritizes candidate genes for validation.
How CRISPR Can Be Used to Study GO:0045273 respiratory chain complex II (succinate dehydrogenase)
Knockout
CRISPR knockout of SDHA, SDHB, SDHC, or SDHD abolishes complex II activity and causes succinate accumulation, mimicking SDH-deficient tumors. These models are used to study the consequences of complex II loss on metabolism, signaling, and tumorigenesis [1,3]. Knockout of assembly factors such as SDHAF2 also impairs complex II assembly and is used to dissect assembly pathways.
Point Mutation
Point-mutation knock-in models introduce specific patient-derived mutations in SDH subunits to test their effects on complex II assembly, catalysis, and stability [1,6]. These models are particularly valuable for distinguishing loss-of-function from dominant-negative effects. They can also be used to study FAD-binding defects in SDHA.
Knock-in
Knock-in of tagged SDH subunits (e.g., HA or GFP) allows visualization and immunoprecipitation of complex II in living cells. Tagged knock-in models are used to map interactors such as NEK7 and to track complex II localization. They also enable quantitative proteomics of the complex.
Overexpression
Overexpression of wild-type or mutant SDH subunits can rescue or exacerbate complex II defects, providing a gain-of-function complement to knockout studies. Overexpression models are used to test whether a candidate mutation is sufficient to drive a phenotype. They are also useful for producing recombinant complex II for structural studies.
How EDITGENE Supports respiratory chain complex II (succinate dehydrogenase) Research
Researchers studying respiratory chain complex II (succinate dehydrogenase)-related genes often need to determine whether a candidate gene is causally involved in complex II assembly, catalysis, or disease. EDITGENE provides a comprehensive suite of CRISPR services to generate precisely engineered cell models for such investigations.
Contact EDITGENE today to design your custom CRISPR model for respiratory chain complex II (succinate dehydrogenase) research.
Frequently Asked Questions About respiratory chain complex II (succinate dehydrogenase)
What is GO:0045273?
GO:0045273 is the Gene Ontology term for respiratory chain complex II (succinate dehydrogenase), a mitochondrial inner membrane complex that contains four polypeptide subunits, FAD, and iron-sulfur clusters, and catalyzes succinate oxidation by ubiquinone.
What genes are involved in respiratory chain complex II?
The core genes are SDHA, SDHB, SDHC, and SDHD, with assembly factors including SDHAF1, SDHAF2, SDHAF3, and SDHAF4.
What does succinate dehydrogenase do?
Succinate dehydrogenase oxidizes succinate to fumarate in the TCA cycle and transfers electrons to ubiquinone in the respiratory chain.
How is complex II linked to cancer?
Mutations in SDH subunits cause SDH-deficient gastrointestinal stromal tumors and hereditary paragangliomas/pheochromocytomas through succinate accumulation and pseudohypoxic signaling.
What diseases are associated with complex II deficiency?
Complex II deficiency is associated with mitochondrial myopathies, encephalomyopathies, hereditary paraganglioma, pheochromocytoma, and SDH-deficient GIST [1,2,8].
Is complex II involved in apoptosis?
Yes, respiratory chain complex II acts as a general sensor for apoptosis, linking metabolic stress to cell death.
How can I study complex II in the lab?
Common methods include high-resolution respirometry, SDHB immunohistochemistry, metabolomics, and CRISPR knockout screens [1,6].
What is the role of SDHB in liver fibrosis?
NEK7 couples SDHB to respiratory chain electron transport homeostasis, and disruption of this axis impedes liver fibrosis.
Does complex II affect immune cells?
Distinct modes of mitochondrial metabolism, including complex II activity, uncouple T cell differentiation from function.
What CRISPR models are available for complex II research?
Knockout, point-mutation knock-in, tagged knock-in, and overexpression models for SDH subunits and assembly factors are available from EDITGENE [1,3,4].
Conclusion
Respiratory chain complex II (GO:0045273) is a unique mitochondrial enzyme that bridges the TCA cycle and the respiratory chain, with essential roles in bioenergetics, apoptosis sensing, and immune cell metabolism [5,6,7]. Its dysfunction causes a spectrum of human diseases, including SDH-deficient tumors, hereditary paragangliomas, and mitochondrial myopathies [1,2,8]. Understanding the complex's assembly, regulation, and subunit-specific functions requires precise experimental models, and CRISPR-based approaches are indispensable for causal studies [1,3,4]. By combining authoritative GO annotation with verified literature, this article provides a foundation for researchers to design rigorous experiments on complex II biology.
References
- 1. Ibrahim A et al.. 2020. Succinate Dehydrogenase-Deficient Gastrointestinal Stromal Tumors.. Arch Pathol Lab Med 144(5):655-660 PMID: 31169996
- 2. DiMauro S et al.. 1987. Mitochondrial myopathies.. J Inherit Metab Dis 10 Suppl 1:113-28 PMID: 2824920
- 3. Wang Q et al.. 2023. Succinate dehydrogenase complex subunit C: Role in cellular physiology and disease.. Exp Biol Med (Maywood) 248(3):263-270 PMID: 36691338
- 4. Sun Z et al.. 2025. NEK7 couples SDHB to orchestrate respiratory chain electron transport homeostasis that impedes liver fibrosis.. Nat Commun 16(1):10751 PMID: 41315267
- 5. Grimm S. 2013. Respiratory chain complex II as general sensor for apoptosis.. Biochim Biophys Acta 1827(5):565-72 PMID: 23000077
- 6. Gnaiger E. 2024. Complex II ambiguities-FADH(2) in the electron transfer system.. J Biol Chem 300(1):105470 PMID: 38118236
- 7. Bailis W et al.. 2019. Distinct modes of mitochondrial metabolism uncouple T cell differentiation and function.. Nature 571(7765):403-407 PMID: 31217581
- 8. DiMauro S et al.. 1993. Mitochondrial encephalomyopathies.. Arch Neurol 50(11):1197-208 PMID: 8215979
