| Topic | Current understanding | Key sources |
|---|---|---|
| Identity / names / family | Human **COX10** (UniProt **Q12887**) corresponds to **protoheme IX farnesyltransferase, mitochondrial**, also called **heme O synthase**; it belongs to the **UbiA intramembrane aromatic prenyltransferase family** that catalyzes membrane-embedded prenyl transfer reactions. Orthology between human COX10 and yeast Cox10 was established by functional complementation. (pqac-00000000, pqac-00000004) | (pqac-00000000, pqac-00000004) |
| Enzymatic reaction | 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**; **pyrophosphate release is mechanistically implied** by donor ionization and Mg2+-assisted departure. This converts the C2 vinyl into a hydroxyethylfarnesyl substituent. (pqac-00000002, pqac-00000006, pqac-00000007, pqac-00000009) | (pqac-00000002, pqac-00000006, pqac-00000007, pqac-00000009) |
| Subcellular localization / topology | COX10 is an **integral mitochondrial inner membrane** protein; reviews describe it as a **large polytopic membrane enzyme** with predicted **~8–9 transmembrane helices** and a **matrix-facing catalytic site**. It can assemble into **homo-oligomeric complexes of ~300 kDa**. (pqac-00000001, pqac-00000005, pqac-00000012) | (pqac-00000001, pqac-00000005, pqac-00000012) |
| Pathway role | COX10 acts upstream of **COX15** in the two-step pathway **heme b → heme o → heme a**. Heme a is then incorporated into **COX1 / cytochrome c oxidase (complex IV)**, where it is essential for core subunit folding, maturation, and catalytic function. COX10 abundance appears limiting relative to COX15, suggesting COX10 may be rate-limiting for heme a production in some settings. (pqac-00000000, pqac-00000003, pqac-00000005, pqac-00000008) | (pqac-00000000, pqac-00000003, pqac-00000005, pqac-00000008) |
| Key interactions / assembly modules | COX10 function is linked to **COX15**, **COA2**, and **COX1 assembly intermediates**. Human studies place COX10 in complexes with **COX15** and copper/metallochaperone factors including **COX11, SCO1, SCO2, COA3, COX16, PET191, and COX19**; COX10 was also detected in the **SURF1 interactome**, supporting integration of heme a biosynthesis with complex IV assembly. (pqac-00000010, pqac-00000011, pqac-00000012, pqac-00000013) | (pqac-00000010, pqac-00000011, pqac-00000012, pqac-00000013) |
| 2024 functional variant data | A 2024 yeast complementation study reported that **ClinVar listed 102 COX10 variants** as of **17 Jun 2024**, with **nearly three-quarters** classified as **uncertain significance**. The authors tested **25 human variants**; **11/25** retained **~50% or more** of reference COX activity and supported respiratory growth. Examples classified as functional in that assay included **S103A, P104L, A328T, R431W, S103A, D152Y, A174T, F209L, C343R, V356M**; examples with poor/nonfunctional behavior included **T87I, I127T, D132Y, D336V, R339W**. Variants with ~50% activity supported glycerol growth, whereas variants with **<25%** activity generally failed to grow on nonfermentable medium. (pqac-00000014, pqac-00000015) | (pqac-00000014, pqac-00000015) |


*Table: This table condenses the main functional annotation points for human COX10, including identity, catalytic reaction, mitochondrial localization, pathway role in heme a and complex IV biogenesis, interaction partners, and the most relevant 2024 human variant assay statistics.*