COX10 is a multi-pass mitochondrial membrane enzyme required for heme A biosynthesis and Complex IV biogenesis. It catalyzes the first committed heme A pathway reaction, converting protoheme IX/heme b and farnesyl diphosphate to heme O. COX10 is therefore a heme O synthase/protoheme IX farnesyltransferase, not a stable structural subunit of cytochrome c oxidase. Loss of COX10 impairs heme A production, blocks early Complex IV assembly, and causes mitochondrial Complex IV deficiency, nuclear type 3.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: COX10 is a mitochondrial heme A biosynthesis enzyme. Reason: Correct broad localization. |
| GO:0006784 heme A biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: COX10 catalyzes the heme O-forming step that is required for heme A biosynthesis. Reason: Core biological-process annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md Heme A is produced by a two-step pathway in mitochondria: 1) **COX10 (heme o synthase): heme b → heme o** (prenylation), and 2) **COX15 (heme a synthase): heme o → heme a** |
| GO:0008495 protoheme IX farnesyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: COX10 converts protoheme IX/heme b and farnesyl diphosphate to heme O. Reason: Core molecular-function annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md **COX10 catalyzes conversion of heme b (protoheme IX) to heme o** by transferring a **farnesyl moiety from farnesyl diphosphate** to the **vinyl group at C2 (pyrrole ring A) of heme b**, producing heme o (a prenylated heme intermediate). |
| GO:0004659 prenyltransferase activity | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Prenyltransferase activity is a correct broad parent for COX10's protoheme IX farnesyltransferase activity. Reason: Keep as non-core because GO:0008495 is the specific core molecular function. |
| GO:0006783 heme biosynthetic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Heme biosynthetic process is a correct broad parent of heme A biosynthesis. Reason: Keep as non-core because GO:0006784 is the more precise process. |
| GO:0006784 heme A biosynthetic process | IEA GO_REF:0000117 | ACCEPT | Summary: COX10 catalyzes the heme O-forming step that is required for heme A biosynthesis. Reason: Core biological-process annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md COX10 catalyzes the **first committed step of heme a biosynthesis**, transferring a **farnesyl group from farnesyl diphosphate** to the **vinyl group at C2 / pyrrole ring A of heme b (protoheme IX)** to form **heme o** |
| GO:0008495 protoheme IX farnesyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: COX10 converts protoheme IX/heme b and farnesyl diphosphate to heme O. Reason: Core molecular-function annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md **COX10 catalyzes conversion of heme b (protoheme IX) to heme o** by transferring a **farnesyl moiety from farnesyl diphosphate** to the **vinyl group at C2 (pyrrole ring A) of heme b**, producing heme o (a prenylated heme intermediate). |
| GO:0016020 membrane | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: COX10 is a membrane protein, but this cellular-component term is too general. Reason: Keep as non-core; mitochondrial inner/mitochondrial membrane terms are more informative. |
| GO:0016765 transferase activity, transferring alkyl or aryl (other than methyl) groups | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Transferase activity transferring alkyl or aryl groups is a broad parent description of the farnesyltransferase reaction. Reason: Keep as non-core because the specific GO:0008495 activity is present. |
| GO:0017004 cytochrome complex assembly | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: COX10 deficiency disrupts cytochrome c oxidase assembly by limiting heme A availability. This is a consequence of its biosynthetic enzyme role rather than direct structural assembly-factor activity. Reason: Keep as non-core; heme A biosynthesis is the direct function. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md Loss-of-function COX10 alleles disrupt heme A supply to cytochrome c oxidase, leading to **complex IV deficiency** |
| GO:0031966 mitochondrial membrane | IEA GO_REF:0000120 | ACCEPT | Summary: COX10 is a mitochondrial membrane protein. Reason: Correct localization. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md COX10 is an **integral mitochondrial inner membrane protein**. |
| GO:0005739 mitochondrion | IEA GO_REF:0000107 | ACCEPT | Summary: COX10 is a mitochondrial heme A biosynthesis enzyme. Reason: Correct broad localization. |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: COX10 is a multi-pass mitochondrial inner-membrane enzyme; the is_active_in matrix qualifier is potentially misleading if interpreted as a soluble matrix localization or as the main cellular-component context for the activity. Reason: Over-specific as an activity-location assertion for a membrane enzyme; mitochondrial inner membrane is the clearer location for COX10's protoheme IX farnesyltransferase activity. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: COX10 is a mitochondrial heme A biosynthesis enzyme. Reason: Correct broad localization. |
| GO:0006783 heme biosynthetic process | TAS Reactome:R-HSA-189451 | KEEP AS NON CORE | Summary: Heme biosynthetic process is a correct broad parent of heme A biosynthesis. Reason: Keep as non-core because GO:0006784 is the more precise process. |
| GO:0008495 protoheme IX farnesyltransferase activity | TAS Reactome:R-HSA-2995330 | ACCEPT | Summary: COX10 converts protoheme IX/heme b and farnesyl diphosphate to heme O. Reason: Core molecular-function annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md **COX10 catalyzes conversion of heme b (protoheme IX) to heme o** by transferring a **farnesyl moiety from farnesyl diphosphate** to the **vinyl group at C2 (pyrrole ring A) of heme b**, producing heme o (a prenylated heme intermediate). |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: COX10 is a mitochondrial heme A biosynthesis enzyme. Reason: Correct broad localization. Supporting Evidence: file:human/COX10/COX10-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0004311 farnesyl-diphosphate farnesyltransferase activity | IGI PMID:8078902 Isolation of a human cDNA for heme A:farnesyltransferase by ... | REMOVE | Summary: GO:0004311 is now labeled farnesyl-diphosphate farnesyltransferase activity, but it is still not COX10's reaction. COX10 uses farnesyl diphosphate to farnesylate heme b/protoheme IX, forming heme O. Reason: Incorrect molecular-function assignment; keep protoheme IX farnesyltransferase activity (GO:0008495), which is already present, as the COX10-specific activity. |
| GO:0070069 cytochrome complex | IMP PMID:12928484 Mutations in COX10 result in a defect in mitochondrial heme ... | MARK AS OVER ANNOTATED | Summary: COX10 is required for Complex IV biogenesis, but it is a heme A biosynthesis enzyme and not a stable component of a cytochrome complex. Reason: Over-annotates a biosynthetic/assembly factor as if it were part of the respiratory cytochrome complex. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md COX10 is an **integral mitochondrial inner membrane protein**. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-2995330 | ACCEPT | Summary: Reactome places the COX10 heme O-forming reaction at the mitochondrial inner membrane. Reason: Correct specific localization for the heme A biosynthesis enzyme. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md COX10 is an **integral mitochondrial inner membrane protein**. |
| GO:0005739 mitochondrion | IC PMID:14607829 Cytochrome c oxidase subassemblies in fibroblast cultures fr... | ACCEPT | Summary: COX10 is a mitochondrial heme A biosynthesis enzyme. Reason: Correct broad localization. |
| GO:0006784 heme A biosynthetic process | IMP PMID:12928484 Mutations in COX10 result in a defect in mitochondrial heme ... | ACCEPT | Summary: COX10 catalyzes the heme O-forming step that is required for heme A biosynthesis. Reason: Core biological-process annotation. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md COX10 catalyzes the **first committed step of heme a biosynthesis**, transferring a **farnesyl group from farnesyl diphosphate** to the **vinyl group at C2 / pyrrole ring A of heme b (protoheme IX)** to form **heme o** |
| GO:0008535 respiratory chain complex IV assembly | IMP PMID:14607829 Cytochrome c oxidase subassemblies in fibroblast cultures fr... | ACCEPT | Summary: COX10 mutations impair Complex IV assembly because heme A is required for assembly of the cytochrome c oxidase catalytic core. Reason: Correct downstream biological-process annotation supported by patient-cell assembly studies, while the direct molecular function remains heme O synthesis. Supporting Evidence: file:human/COX10/COX10-deep-research-falcon.md Heme A is uniquely used by **cytochrome c oxidase (Complex IV)** and is required not only for catalysis but also for proper maturation/stability of the catalytic core subunit **COX1**. |
| GO:0004311 farnesyl-diphosphate farnesyltransferase activity | TAS PMID:8078902 Isolation of a human cDNA for heme A:farnesyltransferase by ... | REMOVE | Summary: GO:0004311 is now labeled farnesyl-diphosphate farnesyltransferase activity, but it is still not COX10's reaction. COX10 uses farnesyl diphosphate to farnesylate heme b/protoheme IX, forming heme O. Reason: Incorrect molecular-function assignment; keep protoheme IX farnesyltransferase activity (GO:0008495), which is already present, as the COX10-specific activity. |
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Download this section (compressed HTML)Q: Are COX10 and COX15 organized as a single physical heme A biosynthesis super-complex that channels heme O without release into the bulk inner membrane lipid bilayer?
Suggested experts: Khalimonchuk O, Shoubridge EA
Q: How does coupling of COX10 oligomerization to newly synthesized COX1 ensure that heme O is produced on-demand, and what disassembles COX10 oligomers when CIV assembly stalls?
Suggested experts: Shoubridge EA, Tzagoloff A
Q: For ClinVar COX10 variants of uncertain significance, what fraction are simple loss-of-function vs separation-of-function (e.g. preserved farnesyltransferase activity but disrupted COX1 / COX15 interactions)?
Suggested experts: Voges N, Antonicka H
Q: Is excess heme O an alternative cofactor under heme A synthase deficiency, and does heme O accumulation in COX15-deficient cells contribute to specific pathology beyond CIV loss?
Suggested experts: Khalimonchuk O
Experiment: Apply native MS / crosslinking-MS to immunopurified COX10 and COX15 complexes from human cells; complement with proximity labeling (BioID/TurboID) of active vs catalytically inactive COX10 and COX15 to map the heme-channeling interface; quantify free heme O / heme A pools by LC-MS in the same cells.
Hypothesis: COX10 and COX15 form an obligate heme A synthesis super-complex with COX1 that channels heme O between active sites.
Type: native MS and proximity labeling of heme A biosynthetic complexes
Experiment: Use puromycin-pulse, doxycycline-controlled MT-CO1 translation, and COA3/COX14 depletion in human cells; track COX10 oligomeric state (BN-PAGE, sucrose gradients) and turnover (cycloheximide chase / SILAC).
Hypothesis: COX10 oligomerization is gated by newly synthesized MT-CO1; uncoupling MT-CO1 translation from heme O production triggers COX10 disassembly and degradation.
Type: translational coupling and oligomer dynamics assay
Experiment: Reconstitute recombinant COX10 variants in liposomes / nanodiscs and measure protoheme IX farnesyltransferase kinetics; compare against complementation rescue of COX10-null human cells (CIV activity, BN-PAGE, mitochondrial respirometry) for the same panel of variants.
Hypothesis: A subset of ClinVar VUS COX10 alleles are separation-of-function variants that preserve farnesyltransferase activity in vitro but fail to support CIV assembly in vivo.
Type: paired in-vitro enzymology and human-cell complementation of ClinVar VUS
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