GO:0008121 quinol-cytochrome-c reductase activity: Mechanism, Genes and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0008121 quinol-cytochrome-c reductase activity describes the molecular function that transfers electrons from a quinol to cytochrome c, a central step in respiratory and photosynthetic electron transport chains.
• The reaction catalyzed is: a quinol + 2 Fe(III)-cytochrome c = a quinone + 2 Fe(II)-cytochrome c + 2 H+(out), coupling electron transfer to proton translocation across a membrane.
• This activity is carried out by cytochrome bc1 complexes in mitochondria and many bacteria, and by cytochrome b6f complexes in chloroplasts and cyanobacteria, which share a common activated Q-cycle mechanism.
• Alternative Complex III (ACIII) represents a structurally distinct enzyme that also catalyzes quinol:cytochrome c oxidoreductase activity in some bacteria.
• The proton/electron stoichiometry of ubiquinol:cytochrome c reductase is regulated by the membrane potential, linking catalytic efficiency to cellular energy status.
• In Escherichia coli and other organisms, alternative respiratory pathways involving quinol-cytochrome c oxidoreductase activity allow adaptation to changing electron acceptors and environmental conditions.
Description
Quinol-cytochrome-c reductase activity (GO:0008121) is a molecular function that enables the transfer of electrons from a quinol substrate to cytochrome c, a process fundamental to biological energy conversion. This activity is the defining catalytic property of cytochrome bc1 complexes in mitochondria and many bacteria, as well as cytochrome b6f complexes in chloroplasts and cyanobacteria. The reaction is a key component of respiratory and photosynthetic electron transport chains, contributing to the generation of a proton motive force across energy-transducing membranes. Researchers study this activity to understand how cells produce ATP, how electron flow is regulated, and how defects in these complexes lead to human disease and metabolic disorders. The enzyme is also a target for natural inhibitors and synthetic compounds, making it relevant to drug discovery and biotechnology.
quinol-cytochrome-c reductase activity At A Glance
| GO ID | GO:0008121 |
|---|---|
| GO term | quinol-cytochrome-c reductase activity |
| Ontology | molecular_function |
| Synonym | ubiquinol-cytochrome c oxidoreductase activity; complex III (mitochondrial electron transport) activity; ubiquinol:ferricytochrome-c oxidoreductase activity; mitochondrial electron transport complex III |
| Major function | Catalyzes electron transfer from a quinol to cytochrome c, contributing to proton translocation across a membrane |
| Reaction | a quinol + 2 Fe(III)-cytochrome c = a quinone + 2 Fe(II)-cytochrome c + 2 H+(out) |
| Cellular location | Inner mitochondrial membrane, bacterial plasma membrane, thylakoid membrane (as cytochrome b6f) |
| Representative complexes | Cytochrome bc1 (complex III), cytochrome b6f, Alternative Complex III (ACIII) |
| Inhibitors | Myxothiazol, antimycin A, stigmatellin (classic inhibitors of cytochrome bc1) |
What Is GO:0008121?
Quinol-cytochrome-c reductase activity (GO:0008121) is defined as the catalysis of the reaction: a quinol + 2 Fe(III)-cytochrome c = a quinone + 2 Fe(II)-cytochrome c + 2 H+(out). In other words, it is the molecular function that transfers electrons from a reduced quinol molecule to cytochrome c, releasing protons on the outside of a membrane. This activity is synonymous with ubiquinol-cytochrome c oxidoreductase, complex III (mitochondrial electron transport) activity, and several other names listed in QuickGO. It is a molecular_function term in the Gene Ontology and is carried out by multi-subunit membrane protein complexes such as cytochrome bc1 and cytochrome b6f.
Why Is quinol-cytochrome-c reductase activity Important in Cell Biology?
Quinol-cytochrome-c reductase activity is essential for cellular respiration and photosynthesis, as it connects the oxidation of quinols to the reduction of cytochrome c, thereby contributing to the proton gradient used for ATP synthesis. In mitochondria, this activity is part of oxidative phosphorylation, and its dysfunction has been linked to mitochondrial diseases and altered drug responses. In bacteria, alternative respiratory pathways that include this activity allow adaptation to different electron acceptors and environmental conditions. In photosynthetic organisms, the analogous cytochrome b6f complex performs the same catalytic function in the thylakoid membrane. Understanding this activity is therefore critical for basic bioenergetics, disease research, and the development of inhibitors targeting respiratory complexes.
• Central to oxidative phosphorylation and ATP production in mitochondria.
• Key component of photosynthetic electron transport in chloroplasts and cyanobacteria via cytochrome b6f.
• Target of natural and synthetic inhibitors such as myxothiazol, used to study respiratory chain function.
• Involved in bacterial alternative respiration, allowing growth under varying oxygen and electron acceptor conditions.
• Dysfunction or altered regulation can affect cellular energy balance and contribute to metabolic stress.
• Provides a model system for studying proton-coupled electron transfer and the Q-cycle mechanism.
• Relevant to drug discovery, as cytochrome bc1 is a validated target for antiparasitic and antifungal agents.
• Studied in diverse organisms including Anabaena variabilis, Rhodopseudomonas sphaeroides, Corynebacterium glutamicum, and Rhodothermus marinus.
• Membrane potential regulates the proton/electron stoichiometry, linking activity to cellular energetics.
• Alternative Complex III expands the structural diversity of enzymes performing this activity.
What Happens During quinol-cytochrome-c reductase activity?
Substrate binding and electron bifurcation
In simple terms: The enzyme first grabs a quinol molecule and splits its electrons into two different paths.
The catalytic cycle begins when a quinol (e.g., ubiquinol) binds to the Qo site of the cytochrome bc1 complex. The enzyme then bifurcates the two electrons from the quinol: one electron is transferred through the high-potential chain to cytochrome c1 and then to cytochrome c, while the other electron is sent through the low-potential chain to the Qi site via cytochromes b. This bifurcation is a hallmark of the activated Q-cycle mechanism shared by cytochrome bc1 and cytochrome b6f complexes.
Proton release and membrane potential
In simple terms: Protons are released on one side of the membrane, helping to build an energy gradient.
Upon quinol oxidation at the Qo site, two protons are released to the positive side of the membrane (outside in mitochondria or inside in bacteria). This proton release contributes to the proton motive force. The proton/electron stoichiometry of ubiquinol:cytochrome c reductase is regulated by the membrane potential, meaning the efficiency of proton pumping can vary with the energy state of the membrane.
Electron transfer to cytochrome c
In simple terms: The electrons are passed to cytochrome c, a small mobile carrier.
The high-potential electron from the Qo site is transferred via the Rieske iron-sulfur protein to cytochrome c1 and finally to cytochrome c, a soluble electron carrier. In some bacteria, Alternative Complex III (ACIII) can directly donate electrons to caa3 oxygen reductase, bypassing cytochrome c in certain pathways. This step completes the reduction of cytochrome c, which then carries electrons to the next complex in the respiratory chain.
Q-cycle and proton translocation
In simple terms: The enzyme uses a cycle to pump more protons than a simple transfer would allow.
The Q-cycle is a mechanism in which the two electrons from a quinol are split, with one electron reducing cytochrome c and the other reducing a quinone at the Qi site to form a semiquinone, which is later fully reduced to quinol by a second turnover. This cycle results in the net translocation of four protons per two electrons transferred to cytochrome c, maximizing the proton motive force. The activated Q-cycle is considered a common mechanism for both cytochrome bc1 and cytochrome b6f complexes.
Alternative respiratory pathways
In simple terms: Some organisms use different versions of this enzyme to adapt to their environment.
In Escherichia coli and other bacteria, alternative respiratory pathways allow the use of different electron donors and acceptors, including quinol-cytochrome c oxidoreductase activity, depending on environmental conditions. In Euglena mitochondria, an alternative respiratory pathway can support oxidative phosphorylation, demonstrating metabolic flexibility. These pathways often involve enzymes structurally distinct from the canonical cytochrome bc1, such as Alternative Complex III.
Key Genes Involved in GO:0008121 quinol-cytochrome-c reductase activity
The following genes encode subunits or accessory proteins of complexes that carry out quinol-cytochrome-c reductase activity, based on experimental evidence from bacteria, mitochondria, and chloroplasts.
| Gene | Major Role | Research Relevance |
|---|---|---|
| petA (cytochrome f) | Subunit of cytochrome b6f complex in chloroplasts and cyanobacteria | Model for photosynthetic electron transport and Q-cycle studies |
| petB (cytochrome b6) | Core subunit of cytochrome b6f complex | Target for mutagenesis to study proton translocation |
| petC (Rieske protein) | Iron-sulfur subunit of cytochrome b6f | Key component for electron bifurcation |
| qcrA | Rieske iron-sulfur protein of cytochrome bc1 in bacteria | Studied in Corynebacterium glutamicum QcrCBA operon |
| qcrB | Cytochrome b subunit of cytochrome bc1 | Essential for quinol oxidation and proton pumping |
| qcrC | Diheme c-type cytochrome subunit of cytochrome bc1 | Novel hydrophobic diheme cytochrome in Corynebacterium glutamicum |
| cyc1 (cytochrome c1) | Electron carrier subunit of cytochrome bc1 | Target for studying electron transfer to cytochrome c |
| RIP1 (Rieske protein) | Iron-sulfur protein in mitochondrial complex III | Mutations linked to respiratory deficiency |
| COB (cytochrome b) | Core subunit of mitochondrial complex III | Mutations affect proton/electron stoichiometry |
| ACIII operon genes | Subunits of Alternative Complex III | Direct electron donor to caa3 oxygen reductase in Rhodothermus marinus |
| cydA | Subunit of cytochrome bd oxidase (alternative pathway) | Studied in E. coli alternative respiration |
| cyoA | Subunit of cytochrome bo oxidase | Model for quinol oxidase in E. coli |
| ndh genes | Type II NADH dehydrogenase, feeds electrons to quinone pool | Linked to alternative respiratory pathways |
| AOX (alternative oxidase) | Bypasses complex III and IV in some organisms | Studied in Euglena and other organisms |
| UQCRFS1 | Rieske iron-sulfur protein in human complex III | Mutations associated with mitochondrial disease |
| UQCRC1 | Core protein 1 of human complex III | Potential target for inhibitor studies |
| UQCRC2 | Core protein 2 of human complex III | Involved in complex assembly and stability |
| MT-CYB | Mitochondrially encoded cytochrome b | Mutations cause myxothiazol resistance and disease |
How Is quinol-cytochrome-c reductase activity Regulated?
The activity of quinol-cytochrome-c reductase is regulated by the membrane potential, which influences the proton/electron stoichiometry of the reaction. In Escherichia coli, alternative respiratory pathways are transcriptionally regulated in response to electron acceptors, allowing adaptation to different growth conditions. The Q-cycle mechanism itself is an intrinsic regulatory feature that maximizes proton translocation efficiency. Additionally, inhibitors such as myxothiazol can modulate activity by binding to the Qo site, and resistance mutations in cytochrome b have been identified in human mitochondria.
quinol-cytochrome-c reductase activity and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| MT-CYB | Myxothiazol resistance, mitochondrial myopathy | Point mutation knock-in in human cells |
| UQCRFS1 | Complex III deficiency | Knockout in HEK293 cells |
| UQCRC2 | Mitochondrial disease, metabolic stress | Overexpression in patient fibroblasts |
| qcrC | Bacterial respiration and virulence | Knockout in Corynebacterium glutamicum |
| ACIII operon | Alternative respiration in Rhodothermus marinus | Knockout and complementation |
Mitochondrial dysfunction and myxothiazol resistance
Mutations in mitochondrial complex III subunits, such as cytochrome b, can confer resistance to inhibitors like myxothiazol and may be associated with mitochondrial myopathies and encephalopathies. These mutations can alter the catalytic activity of quinol-cytochrome-c reductase and affect cellular energy production.
Metabolic and neurodegenerative implications
Impaired quinol-cytochrome-c reductase activity can lead to reduced ATP synthesis and increased reactive oxygen species, contributing to metabolic stress and neurodegeneration. While specific disease links are still under investigation, the enzyme is a known target for drugs and toxins that affect mitochondrial function.
Bacterial pathogenesis and drug targeting
In bacteria, alternative respiratory pathways involving quinol-cytochrome-c oxidoreductase activity are important for survival in host environments and can influence susceptibility to antibiotics. Targeting these enzymes is a potential strategy for developing new antimicrobials.
From quinol-cytochrome-c reductase activity-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of quinol-cytochrome-c reductase activity affect cell viability? | Knockout of UQCRFS1 or MT-CYB in human cell lines |
| How do point mutations in cytochrome b alter inhibitor resistance? | Point mutation knock-in of MT-CYB in mitochondria |
| Can a tagged subunit be used to study complex assembly? | Knock-in of HA-tagged UQCRC1 |
| Does overexpression of Alternative Complex III enhance respiration? | Overexpression of ACIII operon in Rhodothermus marinus |
| What is the role of qcrC in bacterial growth? | Knockout of qcrC in Corynebacterium glutamicum |
| How does membrane potential regulate proton pumping? | Point mutations in cytochrome b combined with electrophysiology |
How to Study the quinol-cytochrome-c reductase activity Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Spectrophotometric cytochrome c reduction assay | Rate of cytochrome c reduction by quinol | Measure specific activity of complex III |
| Inhibitor sensitivity assay | Effect of myxothiazol or antimycin A | Distinguish Qo and Qi site contributions |
| Knockout and complementation | Requirement of specific genes for activity | Study qcrC and ACIII operon |
| RNA-seq / transcriptomics | Expression of respiratory pathway genes | Analyze regulation by electron acceptors |
| Proteomics / mass spectrometry | Subunit composition and modifications | Purify and characterize cytochrome b/c complexes |
| EPR spectroscopy | Iron-sulfur cluster environment | Study Rieske protein and Q-cycle intermediates |
| Membrane potential measurements | Proton motive force and stoichiometry | Assess regulation by membrane potential |
| X-ray crystallography | Three-dimensional structure of complex | Understand Q-cycle mechanism |
Enzymatic assays for quinol-cytochrome-c reductase activity
The activity can be measured spectrophotometrically by monitoring the reduction of cytochrome c at 550 nm using a quinol substrate such as ubiquinol or decylubiquinol. Inhibitors like myxothiazol and antimycin A are used to distinguish specific contributions.
Genetic and molecular biology approaches
Knockout, point mutation, and complementation studies in bacteria such as Corynebacterium glutamicum and Rhodothermus marinus have been used to dissect the roles of individual subunits like qcrC and ACIII components. Transcriptional regulation of alternative respiratory pathways in E. coli has been studied using reporter fusions and RNA analysis.
Biochemical purification and proteomics
Cytochrome b/c complexes can be purified from membranes using detergents, and their subunit composition analyzed by mass spectrometry. The QcrCBA operon in Corynebacterium glutamicum was characterized by purification and sequence analysis.
Structural and biophysical methods
The Q-cycle mechanism has been studied using site-directed mutagenesis, electron paramagnetic resonance (EPR), and X-ray crystallography of cytochrome bc1 and b6f complexes. Membrane potential effects on proton/electron stoichiometry were measured using potentiometric dyes and kinetic assays.
How CRISPR Can Be Used to Study GO:0008121 quinol-cytochrome-c reductase activity
Knockout
CRISPR knockout of genes encoding subunits such as UQCRFS1 or MT-CYB can abolish quinol-cytochrome-c reductase activity, allowing researchers to study its role in respiration and cell growth. In bacteria, knockout of qcrC or ACIII genes can reveal their contribution to alternative respiration.
Point Mutation
Point mutations in cytochrome b, such as those conferring myxothiazol resistance, can be introduced using CRISPR base editing or homology-directed repair to study inhibitor binding and electron transfer. Mutations in the Rieske protein can alter the redox potential and Q-cycle efficiency.
Knock-in
Knock-in of epitope tags (e.g., HA or FLAG) into endogenous subunits like UQCRC1 allows for affinity purification and localization studies of the complex. Knock-in of disease-associated mutations can create isogenic models for mitochondrial dysfunction.
Overexpression
Overexpression of the entire ACIII operon or individual subunits can enhance respiratory capacity and allow biochemical characterization of the complex. In E. coli, overexpression of alternative respiratory pathway components can shift electron flow.
How EDITGENE Supports quinol-cytochrome-c reductase activity Research
Researchers studying quinol-cytochrome-c reductase activity-related genes often need to determine whether a candidate gene is causally involved in respiratory function, disease, or drug response. EDITGENE provides a comprehensive suite of CRISPR-based services to create precisely engineered cell models, enabling functional validation of genes encoding subunits and regulators of this essential enzyme.
Contact EDITGENE today to design your custom CRISPR model for quinol-cytochrome-c reductase activity research.
Frequently Asked Questions About quinol-cytochrome-c reductase activity
What is quinol-cytochrome-c reductase activity?
It is a molecular function (GO:0008121) that catalyzes the transfer of electrons from a quinol to cytochrome c, coupled to proton translocation across a membrane.
What genes are involved in quinol-cytochrome-c reductase activity?
Genes include petA, petB, petC in chloroplasts, qcrA, qcrB, qcrC in bacteria, and UQCRFS1, UQCRC1, UQCRC2, MT-CYB in humans.
What is the Q-cycle?
The Q-cycle is a mechanism in cytochrome bc1 and b6f complexes where electrons from a quinol are bifurcated, leading to net proton translocation.
Which diseases are linked to quinol-cytochrome-c reductase dysfunction?
Mutations in complex III subunits can cause mitochondrial myopathies and resistance to inhibitors like myxothiazol.
How can I measure quinol-cytochrome-c reductase activity?
It is commonly measured by spectrophotometric cytochrome c reduction assays using ubiquinol or decylubiquinol as substrate.
What is Alternative Complex III?
Alternative Complex III (ACIII) is a structurally distinct enzyme that can donate electrons directly to caa3 oxygen reductase in bacteria like Rhodothermus marinus.
Is quinol-cytochrome-c reductase activity found in bacteria?
Yes, it is present in many bacteria, including Corynebacterium glutamicum and Rhodothermus marinus, often as part of alternative respiratory pathways.
How is quinol-cytochrome-c reductase activity regulated?
It is regulated by membrane potential, which affects proton/electron stoichiometry, and by transcriptional control of alternative respiratory pathways in bacteria.
What inhibitors target quinol-cytochrome-c reductase?
Myxothiazol and antimycin A are classic inhibitors that bind to the Qo and Qi sites of cytochrome bc1.
Can CRISPR be used to study quinol-cytochrome-c reductase activity?
Yes, CRISPR knockout, point mutation, knock-in, and overexpression models enable functional dissection of genes encoding subunits and regulators.
Conclusion
Quinol-cytochrome-c reductase activity (GO:0008121) is a fundamental molecular function in respiratory and photosynthetic electron transport, catalyzed by cytochrome bc1, cytochrome b6f, and Alternative Complex III. Its mechanism involves the Q-cycle, proton translocation, and regulation by membrane potential. Understanding this activity is crucial for bioenergetics, disease research, and drug development. CRISPR-based models provide powerful tools to dissect the roles of individual genes and subunits in this process.
References
- 1. Hauska G et al.. 1982. Cytochrome b/c complexes with polyprenyl quinol:cytochrome c oxidoreductase activity from Anabaena variabilis and Rhodopseudomonas sphaeroides GA: comparison of preparations from chloroplasts and mitochondria.. Biochem Soc Trans 10(5):340-1 PMID: 6292024
- 2. Sone N et al.. 2001. A novel hydrophobic diheme c-type cytochrome. Purification from Corynebacterium glutamicum and analysis of the QcrCBA operon encoding three subunit proteins of a putative cytochrome reductase complex.. Biochim Biophys Acta 1503(3):279-90 PMID: 11115640
- 3. Bechmann G et al.. 1991. Regulation of the proton/electron stoichiometry of mitochondrial ubiquinol:cytochrome c reductase by the membrane potential.. Eur J Biochem 195(2):431-8 PMID: 1847681
- 4. Refojo PN et al.. 2017. The monoheme cytochrome c subunit of Alternative Complex III is a direct electron donor to caa3 oxygen reductase in Rhodothermus marinus.. Biol Chem 398(9):1037-1044 PMID: 28141544
- 5. Mulkidjanian AY. 2010. Activated Q-cycle as a common mechanism for cytochrome bc1 and cytochrome b6f complexes.. Biochim Biophys Acta 1797(12):1858-68 PMID: 20650262
- 6. Parker WD Jr et al.. 1988. Myxothiazol resistance in human mitochondria.. Biochim Biophys Acta 936(1):133-8 PMID: 2846049
- 7. Moreno-Sánchez R et al.. 2000. Oxidative phosphorylation supported by an alternative respiratory pathway in mitochondria from Euglena.. Biochim Biophys Acta 1457(3):200-10 PMID: 10773165
- 8. Unden G et al.. 1997. Alternative respiratory pathways of Escherichia coli: energetics and transcriptional regulation in response to electron acceptors.. Biochim Biophys Acta 1320(3):217-34 PMID: 9230919
