Cytochrome c oxidase subunit 4 isoform 2 (COX4-2) is a nuclear-encoded, tissue-biased alternative COX4 subunit of mitochondrial Complex IV. COX4I2 is homologous to the ubiquitous COX4I1 subunit and is incorporated into cytochrome c oxidase as a supernumerary, non-catalytic subunit rather than as one of the mtDNA-encoded redox-core subunits. Its annotations should therefore distinguish Complex IV membership and participation in mitochondrial electron transport from independent cytochrome-c oxidase catalytic activity. COX4I2 is an inner mitochondrial membrane protein with intermembrane-space topology and is highly expressed in lung; pathogenic variants cause exocrine pancreatic insufficiency, dyserythropoietic anemia, and calvarial hyperostosis (EPIDACH).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0045277 respiratory chain complex IV | IBA GO_REF:0000033 | ACCEPT | Summary: COX4I2 is the alternative COX4 isoform incorporated into mitochondrial respiratory chain Complex IV. The annotation captures its core role as a supernumerary subunit of the mature cytochrome c oxidase complex. Reason: Core complex-membership annotation. COX4I2 should be represented as part_of respiratory chain Complex IV, while catalytic activity remains attributable to the intact complex and the mtDNA-encoded catalytic core. Supporting Evidence: file:human/COX4I2/COX4I2-deep-research-falcon.md COX4I2 encodes an alternative isoform of the COX4 subunit of cytochrome c oxidase (complex IV). COX4 is one of the larger nuclear-encoded complex IV subunits with a matrix-facing extrinsic domain that contributes to complex IV regulation, and structural interactions that influence cytochrome c docking/architecture near COX2. |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | IBA GO_REF:0000033 | ACCEPT | Summary: COX4I2 participates in mitochondrial electron transport from cytochrome c to oxygen through its membership in Complex IV. It is not the catalytic redox core, but the intact complex requires its nuclear-encoded subunit set for normal activity and regulation. Reason: Correct complex-level biological-process annotation for a bona fide Complex IV subunit. This should not be interpreted as COX4I2 independently enabling cytochrome-c oxidase activity. Supporting Evidence: file:human/COX4I2/COX4I2-deep-research-falcon.md COX4I2 does not create a new catalytic reaction; complex IV's canonical chemistry remains the terminal step of the respiratory chain (electron transfer to O2 with reduction to water and proton pumping). Instead, COX4I2 is best understood as a regulatory/kinetic tuning subunit that changes how complex IV responds to oxygen tension and cellular metabolic state (e.g., ATP/ADP control), thereby influencing downstream signaling (NADH/ROS) in specialized O2-sensing contexts. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. Supporting Evidence: file:human/COX4I2/COX4I2-deep-research-falcon.md COX4I2 is synthesized in the cytosol as a mitochondrial precursor and becomes incorporated into complex IV in the inner mitochondrial membrane, with functional surfaces facing the matrix and the intermembrane space as part of the assembled oxidase. |
| GO:0005758 mitochondrial intermembrane space | IEA GO_REF:0000117 | ACCEPT | Summary: The intermembrane-space annotation reflects the topology of COX4-family subunits and their exposure toward the cytochrome c side of Complex IV. COX4I2 should not be treated as a soluble IMS protein, but the topology statement is acceptable. Reason: Accept as a topology-aware component annotation. The primary location remains mitochondrial inner membrane. Supporting Evidence: file:human/COX4I2/COX4I2-deep-research-falcon.md COX4I2 is synthesized in the cytosol as a mitochondrial precursor and becomes incorporated into complex IV in the inner mitochondrial membrane, with functional surfaces facing the matrix and the intermembrane space as part of the assembled oxidase. |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | IEA GO_REF:0000002 | ACCEPT | Summary: COX4I2 participates in mitochondrial electron transport from cytochrome c to oxygen through its membership in Complex IV. It is not the catalytic redox core, but the intact complex requires its nuclear-encoded subunit set for normal activity and regulation. Reason: Correct complex-level biological-process annotation for a bona fide Complex IV subunit. This should not be interpreted as COX4I2 independently enabling cytochrome-c oxidase activity. |
| GO:0045277 respiratory chain complex IV | IEA GO_REF:0000002 | ACCEPT | Summary: COX4I2 is the alternative COX4 isoform incorporated into mitochondrial respiratory chain Complex IV. The annotation captures its core role as a supernumerary subunit of the mature cytochrome c oxidase complex. Reason: Core complex-membership annotation. COX4I2 should be represented as part_of respiratory chain Complex IV, while catalytic activity remains attributable to the intact complex and the mtDNA-encoded catalytic core. |
| GO:0031966 mitochondrial membrane | IEA GO_REF:0000107 | ACCEPT | Summary: Mitochondrial membrane is a broader parent localization consistent with the more specific mitochondrial inner membrane annotation. Reason: Correct but broad cellular-component annotation. |
| GO:0006119 oxidative phosphorylation | IEA GO_REF:0000041 | KEEP AS NON CORE | Summary: Complex IV is part of oxidative phosphorylation, and COX4I2 participates in this pathway as a Complex IV subunit. The term is broad relative to COX4I2's specific role. Reason: Keep as a valid pathway-level annotation, but the core reviewed function should emphasize Complex IV membership and cytochrome-c-to-oxygen electron transport. |
| GO:0005743 mitochondrial inner membrane | ISS GO_REF:0000024 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0045277 respiratory chain complex IV | IC PMID:11311561 Mammalian subunit IV isoforms of cytochrome c oxidase. | ACCEPT | Summary: COX4I2 is the alternative COX4 isoform incorporated into mitochondrial respiratory chain Complex IV. The annotation captures its core role as a supernumerary subunit of the mature cytochrome c oxidase complex. Reason: Core complex-membership annotation. COX4I2 should be represented as part_of respiratory chain Complex IV, while catalytic activity remains attributable to the intact complex and the mtDNA-encoded catalytic core. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: COX4I2 is a mitochondrial Complex IV subunit, so the broad mitochondrion localization is correct. Reason: Correct broad localization, although mitochondrial inner membrane is the preferred specific term. Supporting Evidence: file:human/COX4I2/COX4I2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-163214 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9709406 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865412 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865449 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865579 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9865663 | ACCEPT | Summary: COX4I2 is annotated by UniProt and Reactome as a single-pass mitochondrial inner membrane component of Complex IV. Reason: Correct core localization. Complex IV is embedded in the mitochondrial inner membrane and COX4I2 is a membrane subunit of the complex. |
| GO:0005758 mitochondrial intermembrane space | TAS Reactome:R-HSA-9865412 | ACCEPT | Summary: The intermembrane-space annotation reflects the topology of COX4-family subunits and their exposure toward the cytochrome c side of Complex IV. COX4I2 should not be treated as a soluble IMS protein, but the topology statement is acceptable. Reason: Accept as a topology-aware component annotation. The primary location remains mitochondrial inner membrane. |
| GO:0006123 mitochondrial electron transport, cytochrome c to oxygen | IDA PMID:11311561 Mammalian subunit IV isoforms of cytochrome c oxidase. | ACCEPT | Summary: COX4I2 participates in mitochondrial electron transport from cytochrome c to oxygen through its membership in Complex IV. It is not the catalytic redox core, but the intact complex requires its nuclear-encoded subunit set for normal activity and regulation. Reason: Correct complex-level biological-process annotation for a bona fide Complex IV subunit. This should not be interpreted as COX4I2 independently enabling cytochrome-c oxidase activity. Supporting Evidence: file:human/COX4I2/COX4I2-deep-research-falcon.md A controlled isoform-exchange system in human cells (HEK293 COX4i1/2 knockout background with single-isoform knock-in) showed that replacing COX4I1 with COX4I2 produced a ~2-fold increase in p50 (oxygen partial pressure at half-maximal respiration), indicating a decrease in complex IV oxygen affinity while leaving overall complex IV activity and cytochrome c affinity broadly similar. |
| GO:0006091 generation of precursor metabolites and energy | NAS PMID:11311561 Mammalian subunit IV isoforms of cytochrome c oxidase. | KEEP AS NON CORE | Summary: Generation of precursor metabolites and energy is a very broad parent-level process for mitochondrial respiration. Reason: Valid but too general to represent the core evolved function of COX4I2. |
| GO:0045333 cellular respiration | NAS PMID:11911854 Differentiation-dependent repression of c-myc, B22, COX II a... | KEEP AS NON CORE | Summary: Cellular respiration is correct at the pathway level because COX4I2 is a Complex IV subunit, but it is less informative than the specific mitochondrial electron-transport annotation. Reason: Keep as non-core; more specific terms capture the relevant Complex IV role. |
Loading supporting content…
Download this section (compressed HTML)Q: Is the COX4I2-specific N-terminal cysteine cluster modelled to be in proximity required for hypoxic O2-affinity tuning, and does it form an isoform-specific intramolecular or inter-subunit disulfide in vivo?
Suggested experts: Hüttemann M, Grossman LI
Q: How does COX4I2 incorporation reshape the cytochrome c docking environment of Complex IV to lower O2 affinity (~2-fold p50 shift) without altering bulk CIV activity or cytochrome c affinity?
Suggested experts: Hüttemann M, Kadenbach B
Q: Does the COX4I2-dependent hypoxic ventilatory response require canonical chemoreceptor (carotid body) signalling alone, or is COX4I2-tuned Complex IV in vascular smooth muscle and lung an obligate parallel sensor?
Suggested experts: Moreno-Domínguez A, López-Barneo J
Q: Are COX4I1 → COX4I2 isoform switches reversible during chronic hypoxia, and are dysregulated switches contributing to pulmonary hypertension and other O2-sensing pathologies?
Suggested experts: Sommer N, Weissmann N
Experiment: Generate cysteine-to-serine COX4I2 variants in the HEK293 COX4I1/2 double-KO knock-in background; measure p50 (high-resolution respirometry across O2 tensions), CIV activity, and cytochrome c affinity. Detect disulfide formation by non-reducing SDS-PAGE / mass spectrometry under graded O2.
Hypothesis: The isoform-specific cysteine cluster of COX4I2 forms an O2-/redox-responsive disulfide that gates Complex IV oxygen affinity.
Type: structure-function knock-in respirometry
Experiment: Use smooth-muscle-specific inducible Cox4i2 deletion in mice; combine isolated perfused lung HPV measurements, patch-clamp recording of L-type Ca2+ and Kv channels, and FRET-based mitochondrial NADH/H2O2 probes in PASMCs under graded O2.
Hypothesis: Acute hypoxic vasoconstriction in pulmonary artery smooth muscle requires COX4I2-dependent ETC redox signalling upstream of K+/Ca2+ channel modulation.
Type: conditional knockout with electrophysiology and live-cell redox imaging
Experiment: Apply BN-/CN-PAGE and complexome profiling to COX4I1-only and COX4I2-only HEK293 cells under normoxia and chronic hypoxia. Correlate supercomplex composition with ROS, NAD+/NADH, and respiratory parameters.
Hypothesis: COX4I1 and COX4I2 differentially partition into Complex IV-containing respiratory supercomplexes, altering supercomplex composition under hypoxia.
Type: complexome profiling under controlled oxygenation
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)