Cytochrome c oxidase subunit 6B1 (COX6B1) is a nuclear-encoded, peripheral intermembrane-side subunit of mitochondrial Complex IV. It is part of the mature 14-subunit cytochrome c oxidase complex and contributes structurally to normal Complex IV assembly/stability and activity, but it is not one of the mtDNA-encoded catalytic redox subunits. Existing annotations should therefore keep Complex IV membership and electron-transport participation while avoiding assignment of independent cytochrome-c oxidase catalytic activity to COX6B1 alone. Pathogenic variants cause mitochondrial Complex IV deficiency, nuclear type 7.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0045277 respiratory chain complex IV | IBA GO_REF:0000033 | ACCEPT | Summary: COX6B1 is a bona fide subunit of respiratory chain Complex IV, supported by the intact human Complex IV structure and ComplexPortal annotation. The 3.3 Å cryo-EM structure (PDB 5Z62) of the 14-subunit human Complex IV places COX6B1 in the assembled holoenzyme. Reason: Core complex-membership annotation. Supporting Evidence: PMID:30030519 we obtained the entire CIV structure containing 14 subunits, which includes the extra subunit NDUFA4 |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: COX6B1 is a mitochondrial protein as part of mitochondrial cytochrome c oxidase. Reason: Correct broad localization. |
| GO:0005743 mitochondrial inner membrane | IBA GO_REF:0000033 | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. |
| GO:0005739 mitochondrion | IEA GO_REF:0000002 | ACCEPT | Summary: COX6B1 is a mitochondrial protein as part of mitochondrial cytochrome c oxidase. Reason: Correct broad localization. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. |
| GO:0045277 respiratory chain complex IV | IEA GO_REF:0000120 | ACCEPT | Summary: COX6B1 is a bona fide subunit of respiratory chain Complex IV, supported by the intact human Complex IV structure and ComplexPortal annotation. The 3.3 Å cryo-EM structure (PDB 5Z62) of the 14-subunit human Complex IV places COX6B1 in the assembled holoenzyme. Reason: Core complex-membership annotation. Supporting Evidence: PMID:30030519 we obtained the entire CIV structure containing 14 subunits, which includes the extra subunit NDUFA4 |
| GO:1902600 proton transmembrane transport | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Complex IV pumps protons, but COX6B1 is not itself the proton-translocation path or catalytic core. The automated inference projects a whole-complex process onto a peripheral subunit. Reason: Over-annotated at the individual gene-product level. The appropriate core statement is Complex IV membership and contribution to the complex-level activity. Falcon deep research confirms COX6B1 is a small accessory subunit, not a catalytic/proton-translocation component, and that its loss does not change proton stoichiometry (only enzyme activity and cooperativity). Supporting Evidence: file:human/COX6B1/COX6B1-deep-research-falcon.md Experimental removal/loss of COX6B1 (e.g., during mild solubilization) leads to **monomerization** of COX and is associated with a **~two-fold increase in enzyme activity** without changing proton stoichiometry, interpreted as loss of inter-monomer cooperativity and altered cytochrome c binding kinetics. |
| GO:0006119 oxidative phosphorylation | IEA GO_REF:0000041 | KEEP AS NON CORE | Summary: COX6B1 participates in oxidative phosphorylation through Complex IV, but the term is broader than the gene product's specific role. The 3.3 Å cryo-EM structure (PDB 5Z62) of the 14-subunit human Complex IV places COX6B1 within the assembled terminal oxidase of the electron transport chain. Reason: Keep as a broad pathway-level annotation, not the core function statement. Supporting Evidence: PMID:30030519 CIV is the terminal oxidase of the electron transport chain in mitochondria. file:human/COX6B1/COX6B1-deep-research-falcon.md COX6B1 functions within the **oxidative phosphorylation** pathway as a complex IV accessory subunit that contributes to **quaternary structure** (dimerization) and is linked to **assembly/biogenesis** of the complex. |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | NAS PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: COX6B1 participates in cytochrome-c-to-oxygen electron transport as part of the intact Complex IV holoenzyme. Reason: Correct complex-level process annotation for a structural Complex IV subunit. Supporting Evidence: PMID:30030519 CIV is the terminal oxidase of the electron transport chain in mitochondria. file:human/COX6B1/COX6B1-deep-research-falcon.md Cytochrome c oxidase (complex IV; COX/CCO) is the terminal enzyme of the mitochondrial electron transport chain. Its catalytic core transfers electrons from cytochrome c to molecular oxygen and contributes to the proton gradient used for ATP synthesis. |
| GO:0031966 mitochondrial membrane | IDA PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: Mitochondrial membrane is a correct broader localization for a peripheral inner-membrane Complex IV subunit. Reason: Accept as correct but less specific than mitochondrial inner membrane. Supporting Evidence: PMID:30030519 Current opinions point out that CIV exists in two states under physiological conditions, either being assembled into supercomplexes or freely scattered on mitochondrial inner membrane. |
| GO:0045277 respiratory chain complex IV | IPI PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: COX6B1 is a bona fide subunit of respiratory chain Complex IV, supported by the intact human Complex IV structure and ComplexPortal annotation. Reason: Core complex-membership annotation. Supporting Evidence: PMID:30030519 we obtained the entire CIV structure containing 14 subunits, which includes the extra subunit NDUFA4 file:human/COX6B1/COX6B1-deep-research-falcon.md COX6B1 is **not catalytic**; it is a **small, nuclear-encoded accessory subunit** that modulates structure/assembly and function of the complex. |
| GO:0045333 cellular respiration | NAS PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | KEEP AS NON CORE | Summary: Cellular respiration is correct at the pathway level but too broad for the specific COX6B1 role. Reason: Keep as non-core. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: COX6B1 is a mitochondrial protein as part of mitochondrial cytochrome c oxidase. Reason: Correct broad localization. |
| GO:0005743 mitochondrial inner membrane | EXP PMID:30030519 Structure of the intact 14-subunit human cytochrome c oxidas... | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. Supporting Evidence: PMID:30030519 Current opinions point out that CIV exists in two states under physiological conditions, either being assembled into supercomplexes or freely scattered on mitochondrial inner membrane. file:human/COX6B1/COX6B1-deep-research-falcon.md COX6B1 is positioned on the **intermembrane-space (IMS)-facing side** of complex IV. Structural placement from reviews and figure evidence shows COX6B1 exposed on the IMS side and situated at/near the **dimer interface**. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: COX6B1 is a mitochondrial protein as part of mitochondrial cytochrome c oxidase. Reason: Correct broad localization. Supporting Evidence: file:human/COX6B1/COX6B1-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0021762 substantia nigra development | HEP PMID:22926577 Quantitative proteomic analysis of human substantia nigra in... | KEEP AS NON CORE | Summary: The substantia nigra development annotation comes from phenotype/proteomic evidence and does not describe the core molecular role of COX6B1. Reason: Keep as non-core/phenotype-associated rather than a primary gene function. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-163214 | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9709406 | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865663 | ACCEPT | Summary: COX6B1 associates with Complex IV at the mitochondrial inner membrane on the intermembrane-space side. Reason: Correct localization for a peripheral membrane subunit of Complex IV. |
| GO:0004129 cytochrome-c oxidase activity | NAS PMID:2172092 Isolation of cDNAs encoding subunit VIb of cytochrome c oxid... | MARK AS OVER ANNOTATED | Summary: Cytochrome-c oxidase activity is the activity of the assembled Complex IV enzyme. COX6B1 contributes to this activity as a subunit but does not independently catalyze electron transfer or oxygen reduction. Reason: Classic example of whole-complex activity being over-attributed to a non-catalytic subunit. Represent this as contributes_to in the synthesized core function, not as independently enabled activity. Falcon deep research corroborates that COX6B1 is "not catalytic" and is instead a nuclear-encoded accessory subunit that modulates complex structure/assembly and function. Supporting Evidence: file:human/COX6B1/COX6B1-deep-research-falcon.md COX6B1 is **not catalytic**; it is a **small, nuclear-encoded accessory subunit** that modulates structure/assembly and function of the complex. |
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Download this section (compressed HTML)Q: Beyond the dimer-bridging structural role captured by current GO terms, does COX6B1 enable a specific, early redox-sensitive step in MT-CO2 maturation / copper-delivery (via COA6 / SCO1 / SCO2)? Confirming this would justify a more specific "mitochondrial respiratory chain complex IV assembly" annotation and ideally an MT-CO2-metalation-related BP child term.
Q: Are the R19/R20 N-terminal arginine variants (R19H/R19C or R20H/R20C) causing pathology because they destabilize the COX6B1 fold, prevent incorporation into assembly intermediate S3, or specifically disrupt CIV dimerization at the intermembrane-space face? Distinguishing among these would refine the functional consequence (assembly vs. dimerization vs. catalytic cooperativity).
Q: Why does loss of COX6B1 lead to a ~2-fold rise in monomeric CIV enzyme activity in vitro yet manifest as isolated Complex IV deficiency in patients? Is the in vivo bottleneck CIV assembly failure (low steady-state holoenzyme), loss of supercomplex stabilization, or loss of negative-cooperative regulation needed to match O2 reduction to electron supply?
Experiment: Cryo-EM of human Complex IV reconstituted with R19H/R20H/R20C COX6B1 variants alongside an in vitro CIV assembly assay using CRISPR COX6B1-KO HEK293 mitochondria complemented with WT vs. variant COX6B1. Quantify CIV holoenzyme levels by BN-PAGE, assembly-intermediate occupancy (S1/S2/S3), MT-CO2 metalation status (Cu content + SCO1/SCO2 co-IP), and respirometry.
Hypothesis: R19/R20 variants act primarily by blocking incorporation of COX6B1 into the S3 assembly intermediate and impair MT-CO2 copper delivery, rather than by destabilizing the assembled dimer interface.
Type: structural biology / mitochondrial biochemistry
Experiment: Native single-particle cytochrome c kinetic analyses of isolated dimeric versus monomeric COX from WT and COX6B1-KO human cells, pairing this with high-resolution respirometry under physiological cytochrome c concentrations and at varying ATP/ADP ratios.
Hypothesis: The 2-fold activity gain of monomeric CIV in vitro reflects loss of negative cooperativity that is essential in vivo for matching CIV turnover to upstream electron supply; under physiological cytochrome c and energy-charge constraints, monomeric CIV under-performs.
Type: enzyme kinetics / respirometry
Experiment: Patient-derived iPSC differentiation to cardiomyocytes and cortical/spinal neurons from R19H/R20C COX6B1 patients vs. isogenic CRISPR-corrected controls. Assess CIV assembly (BN-PAGE), MT-CO2 maturation, supercomplex content, respirometry, ROS, and Ca2+ handling; pair with proteomic interactome mapping of COA6/SCO1/SCO2 around COX6B1.
Hypothesis: Patient cardiomyocytes show selective Complex IV assembly failure and MT-CO2 hypo-metalation that is rescued by wild-type COX6B1, with a steeper bioenergetic penalty than fibroblasts owing to higher CIV turnover demand.
Type: stem-cell biology / clinical model
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