| Category | Key points | Best supporting citations |
|---|---|---|
| Identity/Complex membership | Human **CYC1** (UniProt **P08574**) encodes **cytochrome c1**, a **nuclear-encoded core catalytic subunit of mitochondrial respiratory chain complex III (cytochrome bc1 / ubiquinol:cytochrome c oxidoreductase)**. Complex III is an **obligate homodimer** in the inner mitochondrial membrane; cytochrome c1 is one of the three evolutionarily conserved catalytic subunits together with cytochrome b and the Rieske Fe-S protein. | (pqac-00000007, pqac-00000008, pqac-00000004, pqac-00000006) |
| Catalytic role | CYC1 itself is **not a standalone enzyme**; within complex III it carries out the **cytochrome c1 electron-transfer step** of the **Q-cycle**. Electrons derived from **ubiquinol (QH2)** oxidation at the **Qo site** pass via the **Rieske 2Fe-2S protein** to **heme c1 in cytochrome c1**, then to **soluble cytochrome c**, while the second electron travels through cytochrome b hemes toward the **Qi site**. This helps couple electron transfer to proton release into the intermembrane space. | (pqac-00000000, pqac-00000002, pqac-00000006) |
| Cofactor | Cytochrome c1 contains a **single c-type heme, heme c1**, covalently attached and exposed for rapid electron transfer to cytochrome c. The 2024 complex III deficiency update describes CYC1 as bearing a **heme-binding C-terminal domain** facing the intermembrane space; earlier primary work identifies the mature protein as the **heme-containing subunit** of complex III. | (pqac-00000007, pqac-00000008, pqac-00000001) |
| Localization/topology | CYC1 is a **mitochondrial inner membrane** protein. Its **heme-containing domain projects into the intermembrane space**, where it meets cytochrome c; the protein is **anchored by a single C-proximal transmembrane segment**. Thus, its functional redox surface is IMS-exposed while its anchor resides in the inner membrane. | (pqac-00000008, pqac-00000007, pqac-00000005) |
| Processing/maturation | Cytochrome c1 is synthesized as a **precursor**, imported post-translationally into mitochondria, and matures after **proteolytic processing**. Evidence from CYC1 disease literature states the precursor undergoes **two cleavage episodes** before the mature heme-bound form is produced. **IMMP2L** is reported to cleave the CYC1 transit/signal peptide; in Immp2l-deficient mice, **uncleaved Cyc1** accumulates and displays altered predicted interactions within complex III. Yeast biogenesis work further supports that cytochrome c1 cleavage occurs after hemylation in a dedicated maturation context. | (pqac-00000009, pqac-00000015, pqac-00000017) |
| Key interactions | Functionally critical partners are **UQCRFS1/Rieske Fe-S protein** (upstream electron donor), **cytochrome c** (downstream mobile acceptor), and **cytochrome b/CYB** within the same complex. Structural/biophysical analysis indicates **fast heme-to-heme transfer** from heme c1 to cytochrome c via a short-distance docking interface shaped by electrostatic and hydrophobic contacts. IMMP2L-loss modeling predicts altered **Cyc1-Cyb** contacts when the transit peptide is retained. | (pqac-00000002, pqac-00000013, pqac-00000012) |
| Disease associations | Pathogenic **biallelic CYC1 variants** cause **isolated complex III deficiency**. Reported variants include **p.Trp96Cys, p.Leu215Phe, and p.Arg317Trp**. Phenotypes include **recurrent ketoacidosis, lactic acidosis, hyperammonemia, insulin-responsive hyperglycemia**, and in later reports **mitochondrial leukoencephalopathy/optic neuropathy-like presentations**. The 2024 update notes **4 reported cases/families**. | (pqac-00000007, pqac-00000008, pqac-00000009, pqac-00000011) |
| Recent developments 2023-2024 | Recent expert reviews emphasize complex III and supercomplex organization as central to respiratory efficiency and signaling. The **2024 JIMD update** consolidates the current human genetic evidence for **CYC1-related complex III deficiency**. A **2024 IMMP2L study** adds a newer maturation/regulatory angle by linking failed precursor cleavage to altered CYC1-CYB interactions and respiration defects, expanding understanding of how CYC1 processing may affect complex III performance beyond rare coding variants. | (pqac-00000007, pqac-00000012, pqac-00000015, pqac-00000006) |
| Quantitative data points | Reported electron-transfer geometry for cytochrome c1 ↔ cytochrome c: **~17.4 Å Fe-to-Fe** and **~9.4 Å edge-to-edge**, with predicted transfer rates up to **~8.3 × 10^6 s^-1**. In CYC1-deficient patients, complex III activity was reported at **4%, 24%, and 25% of controls** in liver, muscle, and fibroblasts, respectively. In IMMP2L-deficient models affecting CYC1 processing, MEFs showed **~27% lower total respiration** and **~12% lower mitochondrial respiration**; kidney succinate-driven respiration was **~31% lower**, and modeling predicted **56 new contacts** between uncleaved Cyc1 transit peptide and Cyb. | (pqac-00000002, pqac-00000008, pqac-00000014, pqac-00000013) |


*Table: This table summarizes the core functional annotation for human CYC1 (UniProt P08574), including its role in mitochondrial complex III, topology, maturation, disease relevance, and recent research highlights. It is designed as a compact evidence-backed reference for narrative reporting.*