Cytochrome c oxidase subunit 5B, nuclear-encoded peripheral membrane protein (~12.5 kDa) that is a structural component of Complex IV (cytochrome c oxidase) in mitochondrial inner membrane. Complex IV is terminal enzyme of electron transport chain, catalyzing reduction of Oβ to HβO using electrons from cytochrome c, contributing to proton electrochemical gradient for ATP synthesis. Complex IV comprises 14 subunits (3 mtDNA-encoded catalytic core: COX1/2/3, plus 11 nuclear-encoded including COX5B). COX5B lacks transmembrane helix, instead attaches to matrix-exposed surface of enzyme. Primary role is structural - helps form quaternary structure and is necessary for complex stability and optimal function, though doesn't bind substrate or perform redox chemistry. Localized to matrix side of inner mitochondrial membrane. Critical for Complex IV assembly and maintenance - incorporated early during assembly (with COX2/3 modules), required for progression to fully assembled holoenzyme. Knockdown causes accumulation of assembly intermediates and overall Complex IV deficiency, leading to decreased COX activity, loss of mitochondrial membrane potential, impaired respiration, energy deficits, and excess ROS. Interacts with PKA regulatory subunit (PKA-R) on matrix side - this binding inhibits cytochrome c oxidase activity, suggesting cAMP/PKA pathway can regulate respiration by targeting COX5B. Interacts with MAVS (mitochondrial antiviral signaling protein) on outer membrane, negatively regulating antiviral signaling - during viral infection MAVS triggers ROS production and upregulates COX5B, which in turn dampens MAVS signaling by reducing ROS and coordinating with autophagy to limit MAVS aggregation. Links mitochondrial electron transport status to immune signaling, preventing excessive ROS/apoptosis during antiviral defense. "Moonlighting" regulatory roles beyond structural function - integrates into broader cellular networks, modulating signals and balancing metabolic activity with cell signaling and stress responses. Essential for oxidative phosphorylation - as part of Complex IV contributes to respiratory electron transport and ATP synthesis coupled to proton transport. Synthesized in cytosol with targeting signal, imported to mitochondrial matrix for Complex IV integration. No disease-causing mutations in COX5B definitively identified in humans, but experimental knockdowns produce clear mitochondrial dysfunction.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: COX5B is synthesized in cytosol and imported to mitochondria where it assembles into Complex IV on the inner mitochondrial membrane. This broad mitochondrial localization is well-supported. Reason: Core localization confirmed by phylogenetic inference (IBA), direct experimental evidence (UniProt states mitochondrial targeting), and multiple structural studies showing COX5B as integral component of mitochondrial Complex IV. Supporting Evidence: PMID:30030519 we obtained the entire CIV structure containing 14 subunits, which includes the extra subunit NDUFA4 file:human/COX5B/COX5B-uniprot.txt SUBCELLULAR LOCATION Mitochondrion inner membrane; Peripheral membrane protein; Matrix side. file:human/COX5B/COX5B-deep-research-perplexity.md See deep research file for comprehensive analysis file:human/COX5B/COX5B-deep-research-falcon.md As a **nuclear-encoded COX subunit**, COX5B is synthesized outside mitochondria and incorporated during coordinated COX biogenesis |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | IBA GO_REF:0000033 | ACCEPT | Summary: COX5B is essential structural subunit of Complex IV which catalyzes final step of electron transport chain - transfer of electrons from cytochrome c to oxygen. While COX5B itself doesn't directly catalyze electron transfer (performed by catalytic core subunits COX1/2/3), it is required for complex assembly, stability, and optimal activity. Reason: Core biological process annotation. Phylogenetic inference combined with extensive experimental evidence shows COX5B is absolutely required for this process. Knockdown studies demonstrate loss of COX5B causes accumulation of assembly intermediates, decreased COX activity, and impaired electron transport. Supporting Evidence: PMID:30030519 26 β 29 It accepts electrons from cytochrome c to reduce the oxygen to water and meanwhile pumps two protons from the matrix side to the intermembrane space (IMS) file:human/COX5B/COX5B-uniprot.txt Electrons originating from reduced cytochrome c in the intermembrane space (IMS) are transferred via the dinuclear copper A center (CU(A)) of subunit 2 and heme A of subunit 1 to the active site in subunit 1, a binuclear center (BNC) formed by heme A3 and copper B (CU(B)). The BNC reduces molecular oxygen to 2 water molecules using 4 electrons from cytochrome c in the IMS and 4 protons from the mitochondrial matrix file:human/COX5B/COX5B-deep-research-falcon.md reducing COX5B in a macrophage cell line led to decreased **cytochrome c oxidase activity**, decreased **mitochondrial membrane potential (ΞΟm)**, increased **reactive oxygen species (ROS)**, and accumulation of **COX assembly intermediates** |
| GO:0005740 mitochondrial envelope | IEA GO_REF:0000002 | MODIFY | Summary: COX5B is specifically on inner mitochondrial membrane, not outer membrane. The mitochondrial envelope includes both inner and outer membranes, making this annotation too broad. Reason: While technically correct (inner membrane is part of envelope), this term is too general. More specific term GO:0005743 (mitochondrial inner membrane) better captures COX5B's precise localization and is already annotated with stronger evidence. Proposed replacements: mitochondrial inner membrane Supporting Evidence: file:human/COX5B/COX5B-uniprot.txt SUBCELLULAR LOCATION Mitochondrion inner membrane; Peripheral membrane protein; Matrix side. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: COX5B localizes specifically to mitochondrial inner membrane as peripheral protein on matrix side, where it integrates into Complex IV. Reason: Core and precise localization. COX5B is peripheral membrane protein (lacks transmembrane helix) attached to matrix-facing surface of inner membrane as confirmed by structural studies and UniProt annotation. Supporting Evidence: PMID:30030519 we obtained the entire CIV structure containing 14 subunits, which includes the extra subunit NDUFA4 file:human/COX5B/COX5B-uniprot.txt SUBCELLULAR LOCATION Mitochondrion inner membrane; Peripheral membrane protein; Matrix side. file:human/COX5B/COX5B-deep-research-falcon.md COX5B is described as **matrix-facing** and **lacking a transmembrane domain**, consistent with a peripheral location on the matrix side of CIV rather than forming the membrane-embedded catalytic core |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | IEA GO_REF:0000002 | ACCEPT | Summary: Duplicate of IBA annotation above for same term. Both correct. Reason: Same biological process as IBA annotation, just from different evidence source (InterPro domain mapping). Acceptable redundancy. |
| GO:0045277 respiratory chain complex IV | IEA GO_REF:0000002 | ACCEPT | Summary: COX5B is stoichiometric subunit of respiratory chain Complex IV (cytochrome c oxidase), the 14-subunit enzyme complex in mitochondrial inner membrane. Reason: Core and precise cellular component annotation. COX5B is integral structural component of Complex IV, incorporated during assembly and required for complex stability. Multiple structural studies confirm this. Supporting Evidence: PMID:30030519 we propose that the intact complex-IV is a monomer containing 14 subunits file:human/COX5B/COX5B-uniprot.txt Component of the cytochrome c oxidase (complex IV, CIV), a multisubunit enzyme composed of 14 subunits. file:human/COX5B/COX5B-deep-research-falcon.md COX5B is a **stoichiometric nuclear-encoded accessory subunit** of mammalian Complex IV that surrounds the mtDNA-encoded catalytic core rather than forming the catalytic center itself |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | MODIFY | Summary: While Complex IV contains multiple metal centers (heme A, copper), COX5B itself coordinates zinc via conserved cysteine residues. This annotation is imprecise as it doesn't specify which metal. Reason: COX5B has zinc-finger motif with three conserved cysteines coordinating Zn2+. More specific term GO:0008270 (zinc ion binding) would be more accurate than generic metal ion binding. UniProt explicitly documents Zn binding sites. Proposed replacements: zinc ion binding Supporting Evidence: file:human/COX5B/COX5B-uniprot.txt BINDING 91; /ligand=Zn(2+). BINDING 93; /ligand=Zn(2+). BINDING 113; /ligand=Zn(2+). BINDING 116; /ligand=Zn(2+). |
| GO:1902600 proton transmembrane transport | IEA GO_REF:0000108 | ACCEPT | Summary: Complex IV pumps protons across inner mitochondrial membrane coupled to electron transfer. COX5B doesn't directly pump protons but is essential structural component required for this activity. Reason: Appropriate biological process annotation. While catalytic subunits (COX1) perform actual proton pumping, COX5B is required for complex assembly and optimal function. Annotation inferred from GO:0004129 (cytochrome-c oxidase activity) which inherently involves proton pumping. Supporting Evidence: PMID:30030519 It accepts electrons from cytochrome c to reduce the oxygen to water and meanwhile pumps two protons from the matrix side to the intermembrane space (IMS) file:human/COX5B/COX5B-uniprot.txt The BNC reduces molecular oxygen to 2 water molecules using 4 electrons from cytochrome c in the IMS and 4 protons from the mitochondrial matrix. file:human/COX5B/COX5B-deep-research-falcon.md catalysis is coupled to proton uptake and proton translocation, supporting efficient four-electron reduction and minimizing partial reduction/ROS formation |
| GO:0005515 protein binding | IPI PMID:17500595 Huntingtin interacting proteins are genetic modifiers of neu... | REMOVE | Summary: Generic protein binding term with experimental evidence from high-throughput interaction study. This term is too vague to be informative about COX5B's actual molecular functions. Reason: Per curation guidelines, avoid generic protein binding annotations as they don't tell us about actual function. COX5B has more specific and informative interactions (e.g., structural role in Complex IV assembly, regulatory interactions with PKA-R and MAVS). This annotation from Huntingtin interactome study provides no functional insight. Supporting Evidence: PMID:17500595 In this study we report a large set of novel Htt-fragment-interacting proteins using yeast two-hybrid (Y2H) and affinity pull-down/mass spectrometry (MS) protein interaction screens. |
| GO:0005515 protein binding | IPI PMID:21516116 Next-generation sequencing to generate interactome datasets. | REMOVE | Summary: Next-generation sequencing interactome - likely high-throughput data without specific functional relevance. Reason: Generic protein binding from high-throughput study. Not informative about COX5B's specific molecular functions. Per guidelines, avoid vague protein binding terms. Supporting Evidence: PMID:21516116 Next-generation sequencing to generate interactome datasets. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | REMOVE | Summary: Proteome-scale human interactome network - high-throughput interactome mapping. Reason: Generic protein binding from high-throughput proteomics. Not informative about COX5B function. Per guidelines, seek more informative MF terms. Supporting Evidence: PMID:25416956 A proteome-scale map of the human interactome network. |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | REMOVE | Summary: Study on genetic variants disrupting protein interactions - high-throughput analysis. Reason: Generic protein binding from HTP variant analysis. Doesn't provide functional insight into COX5B's role. Per guidelines, avoid uninformative protein binding annotations. Supporting Evidence: PMID:31515488 Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | REMOVE | Summary: Reference map of human binary protein interactome - large-scale interaction mapping. Reason: Generic protein binding from HTP binary interactome. Not functionally informative. Per guidelines, protein binding doesn't tell us about actual function. Supporting Evidence: PMID:32296183 Apr 8. A reference map of the human binary protein interactome. |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | REMOVE | Summary: Neurodegenerative disease protein interactome - HTP aggregation study. Reason: Generic protein binding from neurodegeneration interactome study. No evidence this is functionally relevant to COX5B's core role in Complex IV. Supporting Evidence: PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins and Uncovers Widespread Protein Aggregation in Affected Brains. |
| GO:0005515 protein binding | IPI PMID:34446781 First identification of ITM2B interactome in the human retin... | REMOVE | Summary: ITM2B interactome in human retina - tissue-specific interactome. Reason: Generic protein binding from retinal interactome. Not informative about COX5B's molecular function in Complex IV or oxidative phosphorylation. Supporting Evidence: PMID:34446781 First identification of ITM2B interactome in the human retina. |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | NAS PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: High-quality structural paper (Zong et al. 2018) on 14-subunit human Complex IV providing direct evidence for COX5B's role in electron transport. Reason: Strong support from definitive structural study. NAS (non-traceable author statement) code appropriate for established knowledge about Complex IV function. Same process as IBA and IEA annotations above. Supporting Evidence: PMID:30030519 It accepts electrons from cytochrome c to reduce the oxygen to water |
| GO:0031966 mitochondrial membrane | IDA PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | MODIFY | Summary: Broad term for mitochondrial membranes (includes both inner and outer). COX5B specifically on inner membrane. Reason: While technically correct, this is less precise than GO:0005743 (mitochondrial inner membrane) which has multiple annotations with stronger evidence. The more specific term better captures COX5B's localization. Proposed replacements: mitochondrial inner membrane Supporting Evidence: PMID:30030519 we propose that the intact complex-IV is a monomer containing 14 subunits |
| GO:0045277 respiratory chain complex IV | IPI PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: Structural study showing COX5B as one of 14 subunits in monomeric Complex IV. Direct experimental evidence. Reason: Excellent evidence from cryo-EM structure. IPI code indicates protein-protein interaction demonstrated, which is appropriate for showing COX5B assembles into Complex IV. Duplicate of IEA annotation but with stronger experimental evidence. Supporting Evidence: PMID:30030519 we propose that the intact complex-IV is a monomer containing 14 subunits file:human/COX5B/COX5B-deep-research-falcon.md COX5B is present in defined CIV assembly intermediates. In particular, it is included in the **S3 assembly intermediate** of CIV, indicating incorporation during stepwise assembly rather than being a late peripheral add-on |
| GO:0045333 cellular respiration | NAS PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | KEEP AS NON CORE | Summary: Very broad biological process term. While COX5B does participate in cellular respiration via Complex IV, more specific terms exist. Reason: Too general - does not capture COX5B's specific role. More precise terms like GO:0006123 (mitochondrial electron transport, cytochrome c to oxygen) and GO:0006119 (oxidative phosphorylation) better describe COX5B's function. Keep as valid but non-core annotation. Supporting Evidence: PMID:30030519 Jul 20. Structure of the intact 14-subunit human cytochrome c oxidase. file:human/COX5B/COX5B-deep-research-falcon.md COX5B is an accessory structural/regulatory component of Complex IV** required for proper **assembly and/or maintenance** of functional cytochrome c oxidase and normal mitochondrial respiratory performance |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: Immunofluorescence data confirming mitochondrial localization. Broad but correct. Reason: Direct experimental evidence (IDA) from immunofluorescence confirms mitochondrial localization. While broad, this is valid core localization. More specific terms for inner membrane also annotated. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: High-throughput mitochondrial proteome study. COX5B identified in quantitative mitochondrial proteome. Reason: High-confidence mitochondrial proteomics data. HTP evidence code indicates high-throughput but this is from reputable mitochondrial proteome study. Confirms core localization. Supporting Evidence: PMID:34800366 Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context |
| GO:0006119 oxidative phosphorylation | IEA GO_REF:0000041 | ACCEPT | Summary: COX5B is essential component of oxidative phosphorylation as structural subunit of Complex IV. Complex IV couples electron transfer to proton pumping, contributing to protonmotive force that drives ATP synthesis. Reason: Core metabolic pathway annotation. UniPathway-based inference is appropriate. COX5B is integral to OXPHOS through its role in Complex IV. This captures the broader metabolic context beyond just electron transport. Supporting Evidence: file:human/COX5B/COX5B-uniprot.txt PATHWAY: Energy metabolism; oxidative phosphorylation. file:human/COX5B/COX5B-deep-research-falcon.md COX5B functions within the **oxidative phosphorylation (OXPHOS)** pathway by contributing to the structure/assembly/regulation of **Complex IV**, which performs terminal electron transfer from cytochrome c to oxygen and contributes to proton motive force generation |
| GO:0005515 protein binding | IPI PMID:31170524 Pyridine nucleotide-disulphide oxidoreductase domain 2 (PYRO... | REMOVE | Summary: PYROXD2 mitochondrial function study - may identify COX5B interaction but generic protein binding uninformative. Reason: Generic protein binding annotation. Not informative about COX5B's molecular function. Per guidelines, avoid vague protein binding terms that don't elucidate actual functional role. Supporting Evidence: PMID:31170524 Jun 3. Pyridine nucleotide-disulphide oxidoreductase domain 2 (PYROXD2): Role in mitochondrial function. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-163214 | ACCEPT | Summary: Reactome pathway for electron transfer from reduced cytochrome c to molecular oxygen - core Complex IV function. Reason: Traceable author statement from Reactome curated pathway. COX5B is part of Complex IV which localizes to and functions at inner mitochondrial membrane. Appropriate localization annotation. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9709406 | ACCEPT | Summary: Reactome reaction - CO binds to Cytochrome c oxidase. Documents Complex IV as inner membrane complex. Reason: Reactome curated reaction showing Complex IV at inner membrane. Valid TAS evidence for COX5B localization as subunit of Complex IV. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9837978 | ACCEPT | Summary: LONP1 binds mitochondrial inner membrane proteins including Complex IV subunits. Reason: Reactome pathway on mitochondrial protein quality control. LONP1 protease interacts with inner membrane proteins including COX5B. Confirms localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9838004 | ACCEPT | Summary: LONP1 degrades mitochondrial inner membrane proteins - part of protein quality control. Reason: Reactome pathway documenting degradation of inner membrane proteins. COX5B as inner membrane-associated protein subject to quality control. Valid localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865663 | ACCEPT | Summary: Complex IV assembly reaction - incorporation of nuclear-encoded subunits including COX5B into inner membrane complex. Reason: Reactome reaction for Complex IV assembly showing incorporation of COX5B and other subunits. Documents COX5B localization to inner membrane as part of assembly process. |
| GO:0004129 cytochrome-c oxidase activity | TAS PMID:1646156 Structure of the human cytochrome c oxidase subunit Vb gene ... | ACCEPT | Summary: COX5B is structural subunit of Complex IV which catalyzes electron transfer from cytochrome c to O2. While COX5B doesn't directly perform catalysis (done by COX1/2/3 core subunits), it is essential for complex assembly and optimal activity. Reason: Appropriate molecular function annotation. Though COX5B is not catalytic subunit, GO allows annotating structural subunits to the activity of the complex they enable. COX5B is absolutely required for cytochrome-c oxidase activity - knockdown studies show loss of COX activity when COX5B depleted. Early paper (Lomax et al. 1991) establishing COX5B as Complex IV subunit. Supporting Evidence: PMID:1646156 Structure of the human cytochrome c oxidase subunit Vb gene and chromosomal mapping of the coding gene and of seven pseudogenes file:human/COX5B/COX5B-uniprot.txt Cytochrome c oxidase is the component of the respiratory chain that catalyzes the reduction of oxygen to water file:human/COX5B/COX5B-deep-research-falcon.md COX5B is **not the catalytic redox center**; like other nuclear-encoded accessory CIV subunits, it is understood to **modulate assembly, stability, and regulation** of the holoenzyme |
| GO:0007585 respiratory gaseous exchange by respiratory system | TAS PMID:1646156 Structure of the human cytochrome c oxidase subunit Vb gene ... | REMOVE | Summary: This term refers to organismal physiological gas exchange in lungs/respiratory organs, not mitochondrial cellular respiration. Clear misannotation. Reason: Incorrect biological process. GO:0007585 is for organismal gas exchange in lungs/gills/respiratory system - COX5B functions in cellular/mitochondrial respiration (GO:0045333, GO:0006119, GO:0006123), not organismal gas exchange. This appears to be error from early annotation conflating cellular and organismal respiration. Same PMID correctly supports other annotations. Supporting Evidence: GO:0007585 GO term definition respiratory gaseous exchange by respiratory system is The process of gaseous exchange between an organism and its environment [organismal-level process, not appropriate for mitochondrial protein]. PMID:1646156 Structure of the human cytochrome c oxidase subunit Vb gene and chromosomal mapping of the coding gene and of seven pseudogenes. |
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Download this section (compressed HTML)Q: Is the matrix-facing zinc site of COX5B (Cys91/93/113/116) required for stable incorporation into Complex IV, for catalytic activity of the holoenzyme, or for an additional regulatory function such as a putative redox-sensing role?
Suggested experts: Kadenbach B, HΓΌttemann M
Q: At which CIV assembly intermediate (e.g. S3 module) is COX5B obligately incorporated, and does its absence cause specific stalling phenotypes resolvable by complex-IV biogenesis assays?
Suggested experts: Fernandez-Vizarra E, Mick DU
Q: Are the reported COX5B interactions with PYROXD2 and with MAVS (mitochondrial antiviral signaling) functionally significant in vivo, and do they represent regulated moonlighting roles or background mitochondrial co-purification?
Suggested experts: Yu F, Hou F
Experiment: Generate CRISPR knock-in human cells expressing cysteine-to-serine COX5B variants that abolish zinc coordination; assay CIV assembly intermediates (BN-PAGE), supercomplex formation (CN-PAGE), COX5B incorporation (immunoblot of native gels), and CIV activity (spectrophotometric assays).
Hypothesis: The matrix-side Zn2+ site of COX5B (Cys91/93/113/116) is required for stable CIV incorporation and is dispensable for early assembly intermediates but essential for holoenzyme stability.
Type: structure-function knock-in with native complex profiling
Experiment: Compare BN/CN-PAGE supercomplex patterns and CI/CIII/CIV in-gel activity assays in COX5B-null vs COX5B-rescue cells; correlate with CL/MLCL phospholipid composition and SCAFI/COX7A2L levels.
Hypothesis: COX5B is required for stabilization of the I+III2+IV1 respiratory supercomplex in addition to monomeric CIV.
Type: blue-native PAGE of respiratory supercomplexes
Experiment: Use co-immunoprecipitation, proximity ligation assay, and split-luciferase reporters to validate COX5B-MAVS association on outer membrane microdomains. Quantify type-I IFN reporter activity, mtROS, and MAVS aggregation in COX5B-knockdown, knockout, and rescue cells challenged with RNA virus mimetics.
Hypothesis: The COX5B-MAVS interaction modulates antiviral signaling by tuning ROS production downstream of viral RNA sensing.
Type: signal-transduction assays of mitochondrial-immune crosstalk
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