COQ4 is a mitochondrial protein that functions in the biosynthesis of coenzyme Q (ubiquinone, CoQ10 in humans), a lipid-soluble redox cofactor of the respiratory chain. It is a subunit of the multi-protein CoQ biosynthetic complex (the COQ "synthome" or metabolon), assembled with COQ3, COQ5, COQ6, COQ7 and COQ9 on the matrix face of the inner mitochondrial membrane. COQ4 has a dual character: it is required for the integrity and assembly of the complex (a long-recognized structural/organizing role), and it is itself a zinc-dependent enzyme that catalyzes the C1 decarboxylation step of the CoQ head-group modification pathway (4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase; EC 4.1.1.130), converting a polyprenyl-benzoate precursor to a polyprenyl-phenol with release of CO2. A bound Zn2+ ion, coordinated by a conserved His/Asp/Glu motif, is essential for this activity. Loss of COQ4 function reduces CoQ10 synthesis and causes autosomal recessive primary coenzyme Q10 deficiency-7 (encephalopathy, cardiomyopathy, Leigh syndrome) and a spastic-ataxia/hereditary spastic paraplegia spectrum disorder.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) localization of COQ4 to mitochondrion. Correct but general; the more specific inner-membrane/matrix-face localization is better captured by other annotations. Reason: COQ4 is a mitochondrial protein, consistent with all direct evidence, but this generic compartment term is subsumed by the specific mitochondrial inner membrane annotations. |
| GO:0006744 ubiquinone biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference that COQ4 is involved in ubiquinone biosynthesis. This is the central, well-supported biological process for COQ4 and is independently confirmed by experimental (IDA/IMP) annotations. |
| GO:0120539 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) inference of the C1 decarboxylase molecular function. This is now confirmed by two independent 2024 experimental (IDA) studies and is the core molecular function of COQ4. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic localization to the mitochondrial inner membrane, consistent with the experimental IDA annotation (PMID:27499296) and with UniProt (peripheral membrane protein on the matrix side of the inner membrane). Supporting Evidence: file:human/COQ4/COQ4-uniprot.txt Mitochondrion inner membrane |
| GO:0006744 ubiquinone biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic annotation to ubiquinone biosynthesis (InterPro/UniRule/UniPathway). Redundant with the strong experimental support for this core process. |
| GO:0008270 zinc ion binding | IEA GO_REF:0000104 | ACCEPT | Summary: Zinc ion binding is genuinely supported: UniProt lists Zn(2+) as the catalytic cofactor with defined binding residues (163, 164, 167, 179), and 2024 work showed COQ4 decarboxylase activity is Zn-dependent (abolished by EDTA and by metal-site mutations). This underpins the catalytic function. Supporting Evidence: file:human/COQ4/COQ4-uniprot.txt Name=Zn(2+) |
| GO:0016831 carboxy-lyase activity | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: Carboxy-lyase activity is a correct but general parent of the specific COQ4 activity, 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity (GO:0120539). Keep as a non-core ancestor; the specific decarboxylase term is the core molecular function. Reason: Ancestor of the specific decarboxylase term already annotated with IDA evidence. Proposed replacements: 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity |
| GO:0031314 extrinsic component of mitochondrial inner membrane | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: COQ4 is a peripheral (extrinsic) membrane protein on the matrix side of the inner mitochondrial membrane, consistent with UniProt. This term correctly captures the extrinsic, matrix-facing topology. Supporting Evidence: file:human/COQ4/COQ4-uniprot.txt Matrix side. |
| GO:0120539 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity | IEA GO_REF:0000003 | ACCEPT | Summary: EC-based (GO_REF:0000003) electronic mapping to the C1 decarboxylase activity (EC 4.1.1.130). Correct and redundant with the experimental IDA annotations. |
| GO:0005515 protein binding | IPI PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | MARK AS OVER ANNOTATED | Summary: IntAct binary interactions of COQ4 with other COQ synthome subunits (COQ3/COQ5/COQ6/COQ7) from the mitochondrial interaction mapping study. These are real physical partners within the CoQ biosynthetic complex, but the bare "protein binding" term is uninformative; the biology is better captured by complex membership (GO:0110142). Reason: Uninformative generic MF term; the underlying interactions are complex-subunit contacts represented by ubiquinone biosynthesis complex membership. Supporting Evidence: PMID:27499296 identify a dynamic human coenzyme Q biosynthetic complex that includes multiple MXPs |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Binary interactome (Y2H) interaction (with MAGEA2B). Bare "protein binding" is uninformative and this single high-throughput interaction does not inform COQ4's core CoQ-biosynthesis function. Reason: Uninformative generic MF term from a large-scale interactome screen. |
| GO:0006744 ubiquinone biosynthetic process | TAS Reactome:R-HSA-2142789 | ACCEPT | Summary: Reactome traceable-author statement placing COQ4 in ubiquinol biosynthesis. Consistent with the core biological process. |
| GO:0120539 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity | TAS Reactome:R-HSA-2162195 | ACCEPT | Summary: Reactome reaction "COQ4 decarboxylates MHDB" assigns the C1 decarboxylase activity to COQ4. Consistent with the experimentally established core molecular function. |
| GO:0005743 mitochondrial inner membrane | IDA PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | ACCEPT | Summary: Direct-assay (ComplexPortal-curated IDA) localization of COQ4 to the mitochondrial inner membrane, from the study that mapped the human CoQ biosynthetic complex. Strong support for the inner-membrane location. Supporting Evidence: file:human/COQ4/COQ4-uniprot.txt Mitochondrion inner membrane |
| GO:0006744 ubiquinone biosynthetic process | NAS PMID:28927698 Biochemistry of Mitochondrial Coenzyme Q Biosynthesis. | ACCEPT | Summary: Non-traceable author statement (from a CoQ biosynthesis review) that COQ4 is part of the terminal CoQ biosynthetic complex. Consistent with the core process and with the stronger IDA/IMP annotations. Supporting Evidence: PMID:28927698 The enzymes in the terminal phase of CoQ biosynthesis form a biosynthetic complex termed complex Q |
| GO:0110142 ubiquinone biosynthesis complex | IPI PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | ACCEPT | Summary: COQ4 is a subunit of the CoQ biosynthetic complex (COQ synthome / metabolon) composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9. This is a core, accurate cellular-component annotation of complex membership. Supporting Evidence: file:human/COQ4/COQ4-uniprot.txt Component of a multi-subunit COQ enzyme complex, composed of PMID:27499296 identify a dynamic human coenzyme Q biosynthetic complex that includes multiple MXPs |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Immunofluorescence-based (HPA) mitochondrial localization. Correct but general; subsumed by the specific inner-membrane annotations. |
| GO:0120539 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity | IDA PMID:38295803 COQ4 is required for the oxidative decarboxylation of the C1... | ACCEPT | Summary: Direct experimental demonstration that COQ4 catalyzes the oxidative decarboxylation of the C1 carbon of CoQ precursors: COQ4 complemented an E. coli strain deficient for C1 decarboxylation/hydroxylation and showed oxidative decarboxylation activity in a non-CoQ-producer bacterium. This is one of two primary studies establishing COQ4's core catalytic function. Supporting Evidence: PMID:38295803 COQ4 displays oxidative decarboxylation activity in the non-CoQ producer |
| GO:0120539 4-hydroxy-3-methoxy-5-polyprenylbenzoate decarboxylase activity | IDA PMID:38425362 In vitro construction of the COQ metabolon unveils the molec... | ACCEPT | Summary: In vitro reconstruction of the animal COQ metabolon showed that purified COQ4, the only synthome subunit with a canonical Zn-binding motif, exercises Zn-dependent decarboxylase activity on the C1 precursor. Directly establishes the core catalytic molecular function. Supporting Evidence: PMID:38425362 we found that COQ4 exercised decarboxylase activity and production of 4a |
| GO:0006744 ubiquinone biosynthetic process | IDA PMID:38295803 COQ4 is required for the oxidative decarboxylation of the C1... | ACCEPT | Summary: Experimental evidence that COQ4 contributes to CoQ biosynthesis via oxidative decarboxylation of CoQ precursors (in addition to its structural role). Strong support for the core biological process. Supporting Evidence: PMID:38295803 COQ4 contributes to CoQ biosynthesis, not only via its previously proposed structural |
| GO:0006744 ubiquinone biosynthetic process | IDA PMID:38425362 In vitro construction of the COQ metabolon unveils the molec... | ACCEPT | Summary: In vitro metabolon reconstruction placed COQ4's decarboxylase step within the CoQ biosynthetic pathway. Strong support for the core biological process. Supporting Evidence: PMID:38425362 These findings attest to COQ4 |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput high-confidence mitochondrial proteome assignment. Correct but general; subsumed by the specific inner-membrane localization. |
| GO:0006744 ubiquinone biosynthetic process | IMP PMID:38014483 Biallelic COQ4 Variants in Hereditary Spastic Paraplegia: Cl... | ACCEPT | Summary: Mutant-phenotype evidence: loss-of-function COQ4 variants in patient fibroblasts and knockout complementation lines caused decreased ubiquinone biosynthesis, confirming COQ4's requirement for CoQ synthesis. Supporting Evidence: PMID:38014483 lower ubiquinone biosynthesis |
| GO:0006744 ubiquinone biosynthetic process | IMP PMID:30659264 Clinical phenotype, in silico and biomedical analyses, and i... | ACCEPT | Summary: Mutant-phenotype evidence: the homozygous G124S COQ4 variant caused CoQ10 deficiency with lower CoQ10 levels in patient fibroblasts, supporting COQ4's role in ubiquinone biosynthesis. Supporting Evidence: PMID:30659264 The levels of CoQ10 and mitochondrial respiratory chain complex (C) II + III activity were |
| GO:0005515 protein binding | IPI PMID:25152161 Molecular characterization of the human COQ5 C-methyltransfe... | MARK AS OVER ANNOTATED | Summary: Physical interaction with COQ5 (Q5HYK3) within the CoQ-synthome. A real complex partner, but the bare "protein binding" term is uninformative; complex membership (GO:0110142) captures the biology. Reason: Uninformative generic MF term; the interaction is a CoQ-synthome subunit contact. Supporting Evidence: PMID:25152161 assemble with the multi-subunit complex termed the CoQ-synthome |
| GO:0032991 protein-containing complex | IDA PMID:25152161 Molecular characterization of the human COQ5 C-methyltransfe... | MARK AS OVER ANNOTATED | Summary: Generic protein-containing complex membership. Correct but over-general; the specific complex is the ubiquinone biosynthesis complex (GO:0110142) already annotated. Reason: Over-general ancestor of the specific ubiquinone biosynthesis complex term. Proposed replacements: ubiquinone biosynthesis complex |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162186 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane. Consistent with the experimentally supported location. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162187 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane (redundant Reactome reaction). Consistent with the supported location. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162188 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane (redundant Reactome reaction). Consistent with the supported location. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162193 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane (redundant Reactome reaction). Consistent with the supported location. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162194 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane (redundant Reactome reaction). Consistent with the supported location. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2162195 | ACCEPT | Summary: Reactome traceable-author localization to the mitochondrial inner membrane, associated with the COQ4 decarboxylation reaction. Consistent with the supported location. |
| GO:0005739 mitochondrion | IDA GO_REF:0000054 | KEEP AS NON CORE | Summary: LIFEdb GFP-fusion localization to mitochondrion. Correct but general; subsumed by the specific inner-membrane annotations. |
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Download this section (compressed HTML)Q: Does human COQ4 catalyze only C1 decarboxylation, or also a coupled C1 hydroxylation? PMID:38295803 proposes a single oxidative decarboxylation (decarboxylation + hydroxylation), whereas PMID:38425362 assigns C1 hydroxylation to COQ6; resolving this would refine the molecular-function annotation.
Experiment: Determine a structure of the human COQ4 (or COQ synthome subcomplex) with bound Zn2+ and a polyprenyl-benzoate substrate analog to define the decarboxylase active site and the basis of substrate presentation within the metabolon.
Experiment: Reconstitute COQ4 metal-site point mutants (e.g. the His/Asp motif) in COQ4-null human cells and quantify CoQ10 and pathway intermediates to separate the catalytic contribution from the structural/assembly contribution of COQ4.
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