GO:0045275 respiratory chain complex III: Components, Assembly and Research Methods, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0045275 (respiratory chain complex III, also called cytochrome bc1 complex) is a mitochondrial inner-membrane protein complex that transfers electrons from ubiquinol to cytochrome c while translocating two protons across the membrane.
• Its catalytic core comprises cytochrome b, the Rieske iron-sulfur protein (ISP), and cytochrome c1, arranged as an integral membrane-bound dimer with additional species-variable subunits.
• Complex III is essential for oxidative phosphorylation and for the suppressive function of regulatory T cells, linking it to immune regulation and disease.
• Complex III can assemble into supercomplexes such as I2+III2, which rescue respiratory chain defects and are increasingly studied as therapeutic targets.
• Deficiency of complex III causes a heterogeneous group of respiratory-chain diseases with multi-system presentations.
• CRISPR knockout, point-mutation, knock-in, and overexpression models plus enzymatic activity assays are the core methods for dissecting complex III biology.
Description
GO:0045275, respiratory chain complex III, is a protein complex that transfers electrons from ubiquinol to cytochrome c and translocates two protons across a membrane. It is a central component of the mitochondrial respiratory chain and is also known as the cytochrome bc1 complex or ubiquinol-cytochrome c oxidoreductase complex. Because it couples electron transfer to proton translocation, complex III is a key site of energy conversion and a frequent focus of mitochondrial research. Researchers study complex III to understand oxidative phosphorylation, supercomplex organization, and the molecular basis of respiratory-chain diseases. Its role extends beyond bioenergetics: complex III is required for the suppressive function of regulatory T cells, connecting mitochondrial electron transport to immune tolerance. Consequently, complex III is a target for functional genomics, structural biology, and therapeutic development.
respiratory chain complex III At A Glance
| GO ID | GO:0045275 |
|---|---|
| GO term | respiratory chain complex III |
| Ontology | cellular_component |
| Synonym | cytochrome bc1 complex; coenzyme Q-cytochrome c oxidoreductase complex; ubiquinol-cytochrome c oxidoreductase complex |
| Major function | Electron transfer from ubiquinol to cytochrome c with proton translocation across a membrane |
| Core catalytic subunits | Cytochrome b, Rieske iron-sulfur protein (ISP), cytochrome c1 |
| Structural organization | Integral membrane-bound dimeric complex with additional species-variable subunits |
| Subcellular location | Mitochondrial inner membrane |
| Related supercomplexes | I2+III2 and other respiratory chain supercomplexes |
What Is GO:0045275?
Respiratory chain complex III (GO:0045275) is an integral membrane-bound dimeric protein complex of the mitochondrial inner membrane that catalyzes the transfer of electrons from ubiquinol to cytochrome c and couples this reaction to the translocation of two protons across the membrane. Its core structure contains three catalytic subunits: cytochrome b, the Rieske iron-sulfur protein (ISP), and cytochrome c1, with additional subunits that vary among species. The complex is also referred to as coenzyme Q-cytochrome c oxidoreductase, coenzyme Q-cytochrome c reductase, cytochrome bc1 complex, and electron transport complex III.
Why Is respiratory chain complex III Important in Cell Biology?
Respiratory chain complex III is important because it is a required node of the mitochondrial electron transport chain, coupling ubiquinol oxidation to cytochrome c reduction and proton translocation, and thereby contributing to the proton motive force used for ATP synthesis. Its dysfunction causes respiratory-chain diseases, and its activity is essential for specialized cell functions such as the suppressive capacity of regulatory T cells. Complex III also participates in higher-order supercomplexes whose formation can rescue respiratory chain defects, making it a focus for understanding mitochondrial organization and for developing therapeutic strategies.
• Complex III is a core component of oxidative phosphorylation, transferring electrons from ubiquinol to cytochrome c.
• It translocates two protons across the membrane, contributing to the proton gradient used for ATP synthesis.
• Defects in complex III cause a heterogeneous group of respiratory-chain diseases.
• Complex III is essential for the suppressive function of regulatory T cells, linking mitochondrial function to immune regulation.
• Formation of the I2+III2 supercomplex can rescue respiratory chain defects, highlighting its role in mitochondrial plasticity.
• Complex III is a target for structural and biophysical studies of electron tunneling and ligand-triggered gating.
• Supercomplex I-III organization is relevant to both physiology and pathology.
• Enzymatic assessment of complex III is a standard diagnostic and research procedure in mitochondrial medicine.
• Alternative complex III enzymes in bacteria provide comparative models for substrate interaction and evolution.
• CRISPR-based models enable causal testing of complex III genes in disease and immunity.
What Happens During respiratory chain complex III?
Electron transfer from ubiquinol to cytochrome c
In simple terms: Complex III takes electrons from a carrier molecule and passes them to another carrier.
Respiratory chain complex III transfers electrons from ubiquinol to cytochrome c. This reaction is the defining catalytic activity of the complex and connects upstream dehydrogenases to downstream cytochrome c oxidase. The core catalytic subunits cytochrome b, the Rieske iron-sulfur protein, and cytochrome c1 form the electron transfer pathway. Structural and biophysical work has described a ligand-triggered electron-tunneling gating mechanism that controls this transfer.
Proton translocation across the membrane
In simple terms: While moving electrons, complex III also pumps protons across the membrane.
Complex III translocates two protons across a membrane as part of its catalytic cycle. This proton movement contributes to the proton motive force that drives ATP synthesis. The coupling of electron transfer to proton translocation is a central feature of the complex and is studied in both mitochondrial and bacterial systems.
Supercomplex assembly and respiratory chain organization
In simple terms: Complex III can join with other respiratory complexes to form larger assemblies.
Complex III participates in respiratory chain supercomplexes, including the I2+III2 assembly. In-cell architecture studies have revealed the organization of the mitochondrial respiratory chain, including how complexes are arranged in situ. Formation of the I2+III2 supercomplex can rescue respiratory chain defects, indicating a functional role for these higher-order assemblies.
Alternative complex III in bacteria
In simple terms: Some bacteria use a different but related enzyme for the same step.
Alternative complex III from Rhodothermus marinus has been used to explore substrate interaction in a bacterial version of this enzymatic step. Comparative studies of alternative complex III provide insight into the diversity and evolution of respiratory electron transfer.
Key Genes Involved in GO:0045275 respiratory chain complex III
The following genes and proteins are central to the structure, assembly, and function of respiratory chain complex III (GO:0045275).
| Gene | Major Role | Research Relevance |
|---|---|---|
| MT-CYB | Cytochrome b, a core catalytic subunit of complex III | Mutations cause complex III deficiency and respiratory-chain disease |
| UQCRFS1 | Rieske iron-sulfur protein (ISP), a core catalytic subunit | Essential for electron transfer and complex III assembly |
| CYC1 | Cytochrome c1, a core catalytic subunit | Required for electron transfer to cytochrome c |
| UQCRB | Complex III subunit involved in ubiquinol binding | Candidate for functional and structural studies |
| UQCRQ | Complex III subunit | Associated with complex III deficiency phenotypes |
| UQCRC1 | Complex III core subunit | Relevant to assembly and supercomplex formation |
| UQCRC2 | Complex III core subunit | Used as a marker of complex III integrity |
| UQCRH | Complex III subunit | Potential modifier of complex III activity |
| BCS1L | Assembly factor for complex III | Mutations linked to complex III deficiency and disease |
| TTC19 | Assembly factor for complex III | Mutations linked to complex III deficiency and neurodegeneration |
| LYRM7 | Assembly factor for complex III | Relevant to complex III biogenesis |
| MT-CO1 | Cytochrome c oxidase subunit, downstream of complex III | Used in respiratory chain supercomplex studies |
| MT-CO2 | Cytochrome c oxidase subunit, downstream of complex III | Used in respiratory chain supercomplex studies |
| NDUFA1 | Complex I subunit, partner in I2+III2 supercomplex | Relevant to supercomplex rescue experiments |
| NDUFB8 | Complex I subunit, partner in I2+III2 supercomplex | Relevant to supercomplex rescue experiments |
| SDHA | Complex II subunit, adjacent respiratory chain component | Used as a control in respiratory chain assays |
| ATP5F1A | ATP synthase subunit, downstream of proton gradient | Context for proton motive force studies |
How Is respiratory chain complex III Regulated?
Respiratory chain complex III is regulated at multiple levels. Its assembly depends on dedicated assembly factors such as BCS1L, TTC19, and LYRM7, whose dysfunction leads to complex III deficiency. The complex is also regulated by its participation in supercomplexes, and formation of the I2+III2 supercomplex can rescue respiratory chain defects. In immune cells, complex III activity is required for the suppressive function of regulatory T cells, indicating cell-type-specific regulation of mitochondrial electron transport. Biophysical studies suggest that electron transfer through complex III is controlled by a ligand-triggered electron-tunneling gating mechanism.
respiratory chain complex III and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| MT-CYB | Complex III deficiency and respiratory-chain disease | Point-mutation knock-in in cell lines |
| BCS1L | Complex III deficiency with multi-system presentation | Knockout and rescue models |
| TTC19 | Complex III deficiency with neurodegeneration | Knockout models in neuronal cells |
| UQCRQ | Complex III deficiency | Knockout and point-mutation models |
| UQCRFS1 | Complex III assembly and electron transfer defects | Knockout and tagged knock-in models |
Respiratory-chain diseases related to complex III deficiency
Complex III deficiency causes a heterogeneous group of respiratory-chain diseases with multi-system involvement. These disorders can arise from mutations in core subunits or assembly factors and are diagnosed in part through enzymatic assessment of respiratory chain activities. The clinical presentation is variable, reflecting the broad role of complex III in oxidative phosphorylation.
Complex III in immune regulation and autoimmunity
Mitochondrial complex III is essential for the suppressive function of regulatory T cells. This links complex III activity to immune tolerance and suggests that altered complex III function may influence autoimmune and inflammatory conditions.
Supercomplex defects and mitochondrial pathology
Supercomplex I-III organization is relevant to both physiology and pathology. Formation of the I2+III2 supercomplex can rescue respiratory chain defects, indicating that supercomplex assembly is a potential therapeutic axis in mitochondrial disease.
From respiratory chain complex III-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Is a complex III subunit required for electron transfer? | CRISPR knockout of the subunit gene followed by enzymatic assays |
| Does a patient variant impair complex III activity? | Point-mutation knock-in of the variant |
| Where does a complex III protein localize? | Tagged knock-in with fluorescent or affinity tag |
| Can overexpression rescue a respiratory chain defect? | Overexpression of the candidate gene or supercomplex component |
| How does complex III loss affect immune cell function? | Knockout in regulatory T cells |
| How is complex III organized in situ? | In-cell structural studies and supercomplex analysis |
How to Study the respiratory chain complex III Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Enzymatic activity assay | Complex III electron transfer activity | Diagnosis and functional validation |
| In-cell structural analysis | Organization of the respiratory chain in situ | Architecture and supercomplex studies |
| Supercomplex analysis | Assembly of I2+III2 and related supercomplexes | Rescue and pathology studies |
| Biophysical modeling | Electron tunneling and gating | Mechanistic studies |
| Bacterial alternative complex III assays | Substrate interaction in a related enzyme | Comparative enzymology |
| CRISPR knockout followed by activity assay | Causal role of a complex III gene | Gene function discovery |
| Regulatory T cell functional assay | Suppressive function dependent on complex III | Immunometabolism studies |
Enzymatic activity assays
Assessment of mitochondrial respiratory chain enzymatic activities on tissues and cultured cells is a standard method to measure complex III function. These assays quantify electron transfer and are used in both research and diagnostic settings.
Structural and in-cell architecture studies
In-cell architecture of the mitochondrial respiratory chain has been studied to understand how complex III is organized within mitochondria. Such approaches complement biochemical and supercomplex analyses.
Supercomplex analysis
Supercomplex I-III organization can be examined to determine whether complex III assembles into higher-order structures. Formation of the I2+III2 supercomplex can rescue respiratory chain defects, making supercomplex analysis a functional readout.
Biophysical and computational modeling
Biophysical and computational studies have been used to describe a ligand-triggered electron-tunneling gating mechanism in complex III. Comparative studies of alternative complex III from bacteria provide additional mechanistic insight.
How CRISPR Can Be Used to Study GO:0045275 respiratory chain complex III
Knockout
CRISPR knockout of complex III subunit or assembly factor genes can be used to test whether a gene is required for electron transfer and respiratory chain function. Knockout models are also valuable for studying immune cell phenotypes such as regulatory T cell suppression.
Point Mutation
Point-mutation knock-in can model patient variants in complex III genes and assess their impact on enzymatic activity and assembly. Such models help distinguish pathogenic variants from benign polymorphisms.
Knock-in
Tagged knock-in of complex III proteins enables localization and interaction studies within the mitochondrial inner membrane. Knock-in approaches can also be used to express supercomplex components and test rescue of respiratory chain defects.
Overexpression
Overexpression of complex III genes or supercomplex components can be used to test whether increased dosage rescues respiratory chain defects. Overexpression models complement loss-of-function studies and help define sufficiency relationships.
How EDITGENE Supports respiratory chain complex III Research
Researchers studying respiratory chain complex III-related genes often need to determine whether a candidate gene is causally involved in electron transfer, assembly, or disease phenotypes. EDITGENE provides CRISPR-based cell models and screening services to support this work.
Contact EDITGENE today to design your custom CRISPR model for respiratory chain complex III research.
Frequently Asked Questions About respiratory chain complex III
What is respiratory chain complex III (GO:0045275)?
Respiratory chain complex III (GO:0045275) is a protein complex that transfers electrons from ubiquinol to cytochrome c and translocates two protons across a membrane.
What genes are involved in respiratory chain complex III?
Core genes include MT-CYB, UQCRFS1, and CYC1, with additional subunits and assembly factors such as BCS1L, TTC19, and LYRM7.
What is the function of the cytochrome bc1 complex?
The cytochrome bc1 complex, also called complex III, transfers electrons from ubiquinol to cytochrome c and contributes to the proton motive force.
What diseases are associated with complex III deficiency?
Complex III deficiency causes a heterogeneous group of respiratory-chain diseases with multi-system presentations.
How is complex III activity measured?
Complex III activity is measured using enzymatic activity assays on tissues or cultured cells.
What is the I2+III2 supercomplex?
The I2+III2 supercomplex is a higher-order assembly of respiratory complexes whose formation can rescue respiratory chain defects.
Is complex III important for immune cells?
Yes, mitochondrial complex III is essential for the suppressive function of regulatory T cells.
How can CRISPR be used to study complex III?
CRISPR knockout, point-mutation knock-in, tagged knock-in, and overexpression models can test the causal role of complex III genes.
What is alternative complex III?
Alternative complex III is a bacterial enzyme related to the same respiratory step, studied in Rhodothermus marinus.
Why study respiratory chain supercomplexes?
Supercomplex I-III organization is relevant to physiology and pathology, and supercomplex formation can rescue respiratory chain defects.
Conclusion
Respiratory chain complex III (GO:0045275) is a central enzyme of the mitochondrial respiratory chain, defined by electron transfer from ubiquinol to cytochrome c and proton translocation. Its core subunits and assembly factors are linked to respiratory-chain diseases, immune regulation, and supercomplex biology. CRISPR-based models and enzymatic assays provide robust tools for dissecting its function and for translational research.
References
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- 3. Weinberg SE et al.. 2019. Mitochondrial complex III is essential for suppressive function of regulatory T cells.. Nature 565(7740):495-499 PMID: 30626970
- 4. Liang C et al.. 2025. Formation of I(2)+III(2) supercomplex rescues respiratory chain defects.. Cell Metab 37(2):441-459.e11 PMID: 39788125
- 5. Hagras MA. 2024. Respiratory Complex III: A Bioengine with a Ligand-Triggered Electron-Tunneling Gating Mechanism.. J Phys Chem B 128(4):990-1000 PMID: 38241470
- 6. Lenaz G et al.. 2010. Mitochondrial respiratory chain super-complex I-III in physiology and pathology.. Biochim Biophys Acta 1797(6-7):633-40 PMID: 20116362
- 7. Spinazzi M et al.. 2012. Assessment of mitochondrial respiratory chain enzymatic activities on tissues and cultured cells.. Nat Protoc 7(6):1235-46 PMID: 22653162
- 8. Calisto F et al.. 2023. Exploring substrate interaction in respiratory alternative complex III from Rhodothermus marinus.. Biochim Biophys Acta Bioenerg 1864(3):148983 PMID: 37127243
