GO:0006121 mitochondrial electron transport, succinate to ubiquinone: Complex II Mechanism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006121 describes the transfer of electrons from succinate to ubiquinone during oxidative phosphorylation, mediated by the multisubunit enzyme complex II (succinate dehydrogenase).
• Complex II is unique among respiratory chain complexes because it is the only enzyme shared between the tricarboxylic acid (TCA) cycle and the mitochondrial electron transport chain.
• The reaction catalyzed is the oxidation of succinate to fumarate, with electrons transferred via FAD and iron-sulfur clusters to ubiquinone, linking carbon metabolism to ATP production.
• Complex II is a major source of reactive oxygen species (ROS) at the mitochondrial level, particularly under conditions of reverse electron transport.
• Dysfunction or altered expression of complex II subunits is implicated in cancer, neurodegeneration, and metabolic disorders, making it a therapeutic target.
• Advanced CRISPR models (knockout, point mutation, knock-in, overexpression) enable precise dissection of complex II function in health and disease.
Description
GO:0006121, mitochondrial electron transport, succinate to ubiquinone, is a biological process that defines the transfer of electrons from succinate to ubiquinone during oxidative phosphorylation, mediated by the multisubunit enzyme complex II. This process is a critical node where the TCA cycle intersects with the electron transport chain, allowing cells to couple carbon oxidation to energy production. Complex II, also known as succinate dehydrogenase, catalyzes the oxidation of succinate to fumarate while reducing ubiquinone to ubiquinol, thereby feeding electrons into the ubiquinone pool. Researchers study this term to understand mitochondrial bioenergetics, ROS generation, and metabolic reprogramming in diseases such as cancer and neurodegeneration. The process is also a target for therapeutic interventions, as highlighted by recent studies on iron-addicted colorectal cancers that exploit the heme-complex II axis. Moreover, alternative electron acceptors like fumarate and rhodoquinone have been shown to participate in mammalian electron transport, expanding the known roles of complex II under specific conditions.
mitochondrial electron transport, succinate to ubiquinone At A Glance
| GO ID | GO:0006121 |
|---|---|
| GO term | mitochondrial electron transport, succinate to ubiquinone |
| Ontology | biological_process |
| Synonym | complex II (succinate to ubiquinone); mitochondrial electron transport, succinate to coenzyme Q; oxidative phosphorylation, succinate to ubiquinone |
| Major function | Electron transfer from succinate to ubiquinone via complex II, linking TCA cycle to oxidative phosphorylation |
| Cellular location | Mitochondrial inner membrane |
| Key enzyme | Complex II (succinate dehydrogenase; SDH) |
| Cofactors | FAD, iron-sulfur clusters, heme b |
| Pathological relevance | Cancer, neurodegeneration, metabolic disorders |
What Is GO:0006121?
GO:0006121 is defined as the transfer of electrons from succinate to ubiquinone that occurs during oxidative phosphorylation, mediated by the multisubunit enzyme known as complex II. In simpler terms, it is the step in cellular respiration where succinate, a TCA cycle intermediate, is oxidized and the resulting electrons are passed to ubiquinone, a lipid-soluble electron carrier in the mitochondrial inner membrane.
Why Is mitochondrial electron transport, succinate to ubiquinone Important in Cell Biology?
GO:0006121 is fundamental to cellular energy metabolism because it directly connects the TCA cycle to the electron transport chain, enabling efficient ATP production. Dysregulation of this process contributes to a wide range of diseases, including cancer, where complex II mutations or altered expression can affect tumor growth and survival. Additionally, complex II is a significant source of reactive oxygen species, which can damage cellular components and contribute to aging and neurodegenerative diseases. Understanding this process is therefore essential for developing targeted therapies and for interpreting metabolic phenotypes in research and clinical settings.
• Links TCA cycle to oxidative phosphorylation, central to ATP production.
• Complex II is the only enzyme shared between TCA cycle and electron transport chain.
• Mutations in SDH subunits cause hereditary paragangliomas and pheochromocytomas.
• Complex II contributes to ROS generation, impacting cellular signaling and damage.
• Altered complex II activity is observed in colorectal cancer and other malignancies.
• Fumarate can act as a terminal electron acceptor under certain conditions, expanding metabolic flexibility.
• Rhodoquinone can carry electrons in mammalian electron transport chain, highlighting alternative pathways.
• Complex II is a target for metabolic inhibitors and therapeutic interventions.
• Studying this process aids in understanding mitochondrial diseases and metabolic reprogramming.
• CRISPR-based models enable precise manipulation of complex II genes for functional studies.
What Happens During mitochondrial electron transport, succinate to ubiquinone?
Succinate Oxidation at the FAD Cofactor
In simple terms: Succinate is converted to fumarate, and the electrons are captured by FAD.
The process begins when succinate binds to the catalytic site of complex II, where it is oxidized to fumarate. This reaction is coupled to the reduction of the covalently bound FAD cofactor to FADH2. The electrons extracted from succinate are then transferred through a series of iron-sulfur clusters within the enzyme.
Electron Transfer Through Iron-Sulfur Clusters
In simple terms: Electrons travel through a chain of iron-sulfur clusters to reach ubiquinone.
From FADH2, electrons are passed sequentially through three iron-sulfur clusters (2Fe-2S, 4Fe-4S, 3Fe-4S) located in the hydrophilic subunit of complex II. This transfer is essential for delivering electrons to the ubiquinone-binding site. The iron-sulfur clusters facilitate efficient electron tunneling and are critical for the enzyme's function.
Ubiquinone Reduction to Ubiquinol
In simple terms: Ubiquinone accepts the electrons and becomes ubiquinol, a mobile electron carrier.
At the membrane-embedded domain of complex II, ubiquinone is reduced to ubiquinol by accepting two electrons and two protons. This step links the oxidation of succinate to the ubiquinone pool, which then feeds electrons into complex III. The reduction of ubiquinone is a key regulatory point and can be influenced by the redox state of the cell.
Integration with Oxidative Phosphorylation
In simple terms: The electrons from succinate ultimately help produce ATP through the electron transport chain.
The ubiquinol produced by complex II enters the Q-cycle in complex III, eventually reducing cytochrome c and contributing to the proton gradient across the inner mitochondrial membrane. This gradient drives ATP synthesis via ATP synthase. Thus, GO:0006121 is directly coupled to cellular energy production.
Reverse Electron Transport and ROS Generation
In simple terms: Under certain conditions, electrons can flow backwards, producing reactive oxygen species.
Complex II can participate in reverse electron transport (RET), where electrons from ubiquinol are driven back to reduce NAD+ at complex I, leading to ROS production at complex I. This mechanism is particularly relevant in ischemia-reperfusion injury and cancer. The balance between forward and reverse electron flow is critical for cellular redox homeostasis.
Key Genes Involved in GO:0006121 mitochondrial electron transport, succinate to ubiquinone
The following genes encode subunits and assembly factors of complex II, which directly mediate or regulate the succinate to ubiquinone electron transfer.
| Gene | Major Role | Research Relevance |
|---|---|---|
| SDHA | Catalytic subunit with FAD binding site; oxidizes succinate | Mutations cause Leigh syndrome and paragangliomas; target for cancer studies |
| SDHB | Iron-sulfur cluster subunit; transfers electrons to ubiquinone | Mutations linked to pheochromocytoma and paraganglioma; biomarker in cancers |
| SDHC | Membrane anchor subunit; binds ubiquinone | Mutations associated with paragangliomas and gastrointestinal stromal tumors |
| SDHD | Small membrane subunit; required for complex II stability | Implicated in hereditary paraganglioma; model for assembly studies |
| SDHAF1 | Assembly factor for SDHB iron-sulfur cluster | Defects cause infantile leukoencephalopathy; research on complex II assembly |
| SDHAF2 | Flavination factor for SDHA | Mutations cause paraganglioma; target for functional studies |
| SDHAF3 | Involved in iron-sulfur cluster insertion | Potential role in complex II maturation; understudied |
| SDHAF4 | Stabilizes SDHA during assembly | Knockout models show reduced complex II activity |
| UQCRFS1 | Rieske iron-sulfur protein of complex III; interacts with ubiquinol | Not directly in complex II but downstream; relevant for electron transfer |
| NDUFS1 | Complex I subunit; site of RET-mediated ROS | Studied in context of reverse electron transport from complex II |
| FH | Fumarate hydratase; converts fumarate to malate | Mutations cause hereditary leiomyomatosis; fumarate can act as electron acceptor |
| COQ2 | Ubiquinone biosynthesis | Defects affect ubiquinone pool; relevant for complex II function |
| COQ6 | Ubiquinone biosynthesis | Mutations cause nephropathy; impacts electron transport |
| COQ9 | Ubiquinone biosynthesis | Knockout models show mitochondrial dysfunction |
| HIF1A | Hypoxia-inducible factor; downstream of SDH mutations | Mediates pseudohypoxia in SDH-mutant tumors |
| EPAS1 | Hypoxia-inducible factor 2 alpha; associated with paraganglioma | Co-operates with SDH mutations |
| KIT | Receptor tyrosine kinase; often mutated in GIST with SDH loss | Therapeutic target in SDH-deficient GIST |
| SDH5 | Alias for SDHAF2; flavination factor | Same as SDHAF2 |
How Is mitochondrial electron transport, succinate to ubiquinone Regulated?
The activity of complex II and the succinate to ubiquinone electron transfer are regulated at multiple levels. Transcriptional regulation of SDH subunits responds to metabolic cues and hypoxia. Post-translational modifications, such as phosphorylation and acetylation, can modulate complex II activity. The availability of ubiquinone, determined by COQ gene expression, also influences the rate of electron transfer. Additionally, reverse electron transport is regulated by the proton motive force and the redox state of the ubiquinone pool. Under conditions of iron deficiency, heme availability can impact complex II assembly and function, as seen in iron-addicted cancers.
mitochondrial electron transport, succinate to ubiquinone and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| SDHB | Paraganglioma, pheochromocytoma | Knockout in chromaffin cells; xenograft models |
| SDHA | Leigh syndrome, paraganglioma | Patient-derived fibroblasts; CRISPR point mutation |
| SDHC | Gastrointestinal stromal tumor | Knockout in GIST cell lines; organoids |
| SDHD | Hereditary paraganglioma | Mouse models with conditional knockout |
| FH | Hereditary leiomyomatosis and renal cell cancer | Knockout in renal cells; fumarate accumulation studies |
Complex II Mutations in Hereditary Paraganglioma and Pheochromocytoma
Germline mutations in SDHB, SDHC, SDHD, and SDHAF2 predispose individuals to paragangliomas and pheochromocytomas, often with a pseudohypoxic signature due to succinate accumulation and inhibition of prolyl hydroxylases. These tumors exhibit altered metabolism and are a paradigm for complex II-linked tumorigenesis.
Complex II in Colorectal Cancer and Iron Addiction
Recent studies show that iron-addicted colorectal cancers exploit a heme-complex II axis to resist oxidative cell death. Targeting this axis may offer therapeutic strategies. Complex II dysfunction can also lead to increased ROS, which promotes genomic instability and tumor progression.
Neurodegeneration and Mitochondrial Dysfunction
Impaired complex II activity has been observed in neurodegenerative diseases such as Parkinson's and Huntington's, where mitochondrial dysfunction and oxidative stress contribute to neuronal loss. Mutations in SDHA cause Leigh syndrome, a severe neurometabolic disorder.
Metabolic Disorders and Alternative Electron Acceptors
Fumarate can act as a terminal electron acceptor in the mammalian electron transport chain under hypoxia or complex III inhibition, revealing metabolic flexibility that may be relevant in ischemia and cancer. Rhodoquinone, typically found in invertebrates, can also carry electrons in mammalian mitochondria, suggesting alternative pathways that could be exploited therapeutically.
From mitochondrial electron transport, succinate to ubiquinone-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of SDHB impair complex II assembly and increase ROS? | SDHB knockout cell lines (CRISPR) |
| What is the effect of a specific SDHA point mutation on enzyme kinetics? | Point mutation knock-in via CRISPR |
| Can overexpression of SDHA rescue complex II deficiency? | Overexpression cell models |
| How does tagged SDHC affect ubiquinone binding? | Tagged knock-in for imaging and pull-down |
| What is the role of SDHAF2 in flavination? | Knockout and rescue with wild-type or mutant SDHAF2 |
| Does fumarate act as an electron acceptor in complex II-deficient cells? | Metabolic flux analysis in knockout models |
How to Study the mitochondrial electron transport, succinate to ubiquinone Process
| Method | What It Measures | Typical Application |
|---|---|---|
| High-resolution respirometry | Oxygen consumption rates | Assessing complex II activity in cells |
| CRISPR knockout screens | Gene essentiality and synthetic lethality | Identifying modifiers of complex II function |
| Proteomics | Protein abundance and interactions | Complex II assembly and stoichiometry |
| MitoSOX imaging | Mitochondrial ROS levels | Detecting reverse electron transport |
| Metabolomics | Succinate, fumarate, and other metabolites | Linking complex II to metabolic pathways |
| Western blot | Protein expression and modification | Validating knockout or overexpression |
| Blue native PAGE | Intact respiratory chain complexes | Assessing complex II assembly |
| Seahorse assay | Extracellular acidification and oxygen consumption | Real-time metabolic phenotyping |
Respirometry and Metabolic Flux Analysis
High-resolution respirometry (e.g., Oroboros) measures oxygen consumption to assess complex II activity in intact cells or isolated mitochondria. This method is essential for quantifying succinate-driven respiration and detecting defects in electron transfer.
CRISPR Screening and Functional Genomics
Genome-wide CRISPR knockout screens can identify genes that modulate sensitivity to complex II inhibitors or that are synthetic lethal with SDH mutations. Such screens are powerful for discovering new regulators of GO:0006121.
Proteomics and Complexome Profiling
Mass spectrometry-based proteomics and complexome analysis can reveal the assembly state of complex II and its interactors. These methods help identify assembly factors and post-translational modifications.
ROS Detection and Redox Imaging
Fluorescent probes (e.g., MitoSOX) and genetically encoded redox sensors allow real-time monitoring of ROS production from complex II, particularly under reverse electron transport conditions.
How CRISPR Can Be Used to Study GO:0006121 mitochondrial electron transport, succinate to ubiquinone
Knockout
CRISPR knockout of SDH subunits (e.g., SDHB, SDHA) creates models to study loss of complex II function, succinate accumulation, and downstream effects on HIF signaling and ROS. These models are valuable for cancer and metabolic research.
Point Mutation
Introducing specific point mutations (e.g., SDHA R451C) via CRISPR allows precise interrogation of catalytic residues and their impact on enzyme kinetics and assembly. Such models mimic patient mutations and help understand genotype-phenotype relationships.
Knock-in
Knock-in of tagged versions of SDH subunits (e.g., HA-tagged SDHC) enables imaging, immunoprecipitation, and interaction studies. This approach is useful for tracking complex II localization and dynamics.
Overexpression
CRISPR activation (CRISPRa) or cDNA overexpression can increase levels of complex II subunits to study rescue of deficiency or gain-of-function effects. Overexpression models help determine rate-limiting steps in electron transfer.
How EDITGENE Supports mitochondrial electron transport, succinate to ubiquinone Research
Researchers studying mitochondrial electron transport, succinate to ubiquinone-related genes often need to determine whether a candidate gene is causally involved in complex II function, metabolic regulation, or disease. EDITGENE provides a comprehensive suite of CRISPR services to generate precisely engineered cell models, enabling rigorous functional validation and accelerating discovery in mitochondrial biology.
Contact EDITGENE today to design your custom CRISPR model for mitochondrial electron transport, succinate to ubiquinone research.
Frequently Asked Questions About mitochondrial electron transport, succinate to ubiquinone
What is GO:0006121?
GO:0006121 is the Gene Ontology term for mitochondrial electron transport, succinate to ubiquinone, the process where electrons from succinate are transferred to ubiquinone via complex II during oxidative phosphorylation.
What genes are involved in mitochondrial electron transport, succinate to ubiquinone?
Key genes include SDHA, SDHB, SDHC, SDHD, and assembly factors SDHAF1-4, which encode subunits of complex II.
What is complex II?
Complex II, also known as succinate dehydrogenase, is the multisubunit enzyme that catalyzes the oxidation of succinate to fumarate and reduces ubiquinone, linking the TCA cycle to the electron transport chain.
How is succinate to ubiquinone electron transfer regulated?
It is regulated by transcriptional control of SDH genes, post-translational modifications, ubiquinone availability, and the redox state of the cell.
What diseases are associated with complex II dysfunction?
Mutations in SDH genes cause hereditary paragangliomas, pheochromocytomas, Leigh syndrome, and are implicated in cancers such as colorectal and gastrointestinal stromal tumors.
What is the role of ROS in this process?
Complex II can generate reactive oxygen species, especially during reverse electron transport, contributing to cellular signaling and damage in various diseases.
Can fumarate act as an electron acceptor in the electron transport chain?
Yes, recent studies show that fumarate can serve as a terminal electron acceptor in mammalian electron transport, particularly under conditions of complex III inhibition or hypoxia.
What is rhodoquinone and its role in mammalian electron transport?
Rhodoquinone is a lipid-soluble electron carrier that can carry electrons in the mammalian electron transport chain, providing an alternative to ubiquinone.
How can CRISPR be used to study complex II?
CRISPR knockout, point mutation, knock-in, and overexpression models allow precise manipulation of SDH genes to study their function in metabolism, ROS production, and disease.
What methods are used to measure complex II activity?
High-resolution respirometry, Seahorse assays, blue native PAGE, and ROS imaging are commonly used to assess complex II activity and electron transfer.
Conclusion
GO:0006121, mitochondrial electron transport, succinate to ubiquinone, is a cornerstone of mitochondrial bioenergetics and metabolic regulation. Its central enzyme, complex II, bridges the TCA cycle and oxidative phosphorylation, with far-reaching implications for cancer, neurodegeneration, and metabolic diseases. Advances in CRISPR technology now enable precise modeling of complex II dysfunction, offering new avenues for therapeutic discovery. Continued research into this process will illuminate fundamental mechanisms and translate into clinical benefits.
References
- 1. Zhao RZ et al.. 2019. Mitochondrial electron transport chain, ROS generation and uncoupling (Review).. Int J Mol Med 44(1):3-15 PMID: 31115493
- 3. Spinelli JB et al.. 2021. Fumarate is a terminal electron acceptor in the mammalian electron transport chain.. Science 374(6572):1227-1237 PMID: 34855504
- 4. Valeros J et al.. 2025. Rhodoquinone carries electrons in the mammalian electron transport chain.. Cell 188(4):1084-1099.e27 PMID: 39909039
- 5. Jain C et al.. 2026. Iron-addicted colorectal cancers exploit heme-complex II axis to resist oxidative cell death.. Cell Metab 38(8):1599-1617.e8 PMID: 42202783
- 6. Gnaiger E. 2024. Complex II ambiguities-FADH(2) in the electron transfer system.. J Biol Chem 300(1):105470 PMID: 38118236
- 8. Gutman M. 1980. Electron flux through the mitochondrial ubiquinone.. Biochim Biophys Acta 594(1):53-84 PMID: 7006698
