The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Plan status: All preparatory objectives completed. We verified identity, gathered recent and authoritative sources, extracted mechanistic, localization, pathway, and disease evidence, summarized 2023–2024 developments, and prepared an evidence table artifact for embedding.
Gene/protein identity verification
- The SDHD gene (chromosome 11q23.1) encodes the small cytochrome b–like membrane subunit of mitochondrial Complex II (succinate dehydrogenase; SQR), partnering with SDHC to anchor the catalytic SDHA/SDHB dimer, harbor heme b, and form the ubiquinone-binding/channel region. These properties match the UniProt O14521 description and CybS family/domain assignments (CybS; SQR/QFR_C/D; PF05328) (fullerton2020thegeneticbasis pages 1-5, fullerton2020thegeneticbasis pages 5-9, du2023structureofthe pages 2-5).
| Key point | Brief detail | Source (year + URL) |
|---|---|---|
| Identity verification | Human SDHD (gene SDHD, UniProt O14521) encodes the small cytochrome b–like membrane subunit of mitochondrial Complex II (CybS family; SQR/QFR_C/D; PF05328). | Fullerton et al., Mol Genet Metab 2020; https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 1-5) |
| Biochemical role in Complex II | Participates in electron transfer chain by forming the membrane anchor with SDHC; contributes to the ubiquinone-binding/quinone-channel and supports electron transfer from SDHA/SDHB to ubiquinone; heme b is coordinated at the SDHC–SDHD interface. | Du et al., PNAS 2023; https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5), Fullerton et al., 2020; https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 5-9) |
| Localization & topology | Integral inner mitochondrial membrane (IMM) protein with transmembrane helices forming the membrane-anchoring dimer with SDHC; positions facing intermembrane-space side form the quinone entry channel. | Du et al., PNAS 2023; https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5) |
| Pathways & consequences of loss | Central to TCA/ETC coupling (succinate → fumarate; electrons → ubiquinone). SDH loss → succinate accumulation, inhibition of 2-oxoglutarate–dependent dioxygenases, HIF stabilization and epigenetic changes (pseudohypoxia). | Khazal et al., Cancer & Metabolism 2024; https://doi.org/10.1186/s40170-024-00369-9 (khazal2024similardeficienciesdifferent pages 1-2), Fullerton et al., 2020; https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 5-9) |
| Assembly / biogenesis | SDHAF assembly factors required: SDHAF2 (promotes SDHA flavinylation) and SDHAF1/3/4 support Fe–S insertion and subunit maturation/assembly; proper SDHA/SDHB maturation precedes SDHC/SDHD membrane integration. | Fullerton et al., Mol Genet Metab 2020; https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 5-9), Du et al., PNAS 2023; https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5) |
| Structural insights (2023) | Human Complex II cryo‑EM (PNAS 2023) defines quinone-binding entrance formed by SDHC TM1, SDHD TM2 and SDHB C‑terminus; identifies Tyr/other SDHD residues near quinone site and role of heme b as an electron sink. | Du et al., PNAS 2023; https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5) |
| Disease associations | Heterozygous SDHD germline variants → hereditary paraganglioma/pheochromocytoma (PPGL) syndromes (noting parent‑of‑origin effects reported in literature); SDH‑deficient GIST and rare RCC associations; biallelic SDHD variants cause autosomal recessive isolated Complex II deficiency with encephalomyopathy/cardiomyopathy. | Esteban‑Amo et al., Biomedicines 2024; https://doi.org/10.3390/biomedicines12092050 (estebanamo2024succinatedehydrogenaseand pages 1-2), Lin et al., Eur J Hum Genet 2021; https://doi.org/10.1038/s41431-021-00887-w (lin2021consolidatingbiallelicsdhd pages 1-2) |
| Recent 2023–2024 updates | High-resolution human CII structure (Du et al., 2023) clarified quinone channel and SDHD roles; 2024 functional studies/models highlight tissue-specific outcomes of SDH loss and metabolic/immune consequences. | Du et al., PNAS 2023; https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5), Khazal et al., Cancer & Metabolism 2024; https://doi.org/10.1186/s40170-024-00369-9 (khazal2024similardeficienciesdifferent pages 1-2), Esteban‑Amo 2024 review; https://doi.org/10.3390/biomedicines12092050 (estebanamo2024succinatedehydrogenaseand pages 1-2) |
| Key stats & genome context | Chromosomal locus: 11q23.1; protein ~19 kDa (small membrane subunit); isolated Complex II deficiency is rare among OXPHOS defects; SDHD implicated both in tumor predisposition (heterozygous) and recessive mitochondrial disease (biallelic). | Chatoff et al. (table summary) 2025; https://doi.org/10.1002/jcp.70066 (chatoff2025metaboliceffectsof pages 32-32), Fullerton 2020; https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 1-5) |
Table: Compact table summarizing verified facts about human SDHD (UniProt O14521): identity, biochemical role, localization, pathways, assembly, structural insights, disease links, recent updates, and key statistics with source citations. This provides a quick, citable reference for SDHD functional annotation.
Key concepts and definitions (current understanding)
- Enzymatic function: Complex II catalyzes succinate oxidation to fumarate (TCA cycle) by SDHA (FAD cofactor), passes electrons via SDHB Fe–S clusters to ubiquinone at the SDHC/SDHD membrane domain, reducing it to ubiquinol; Complex II does not pump protons. SDHD contributes to the quinone-binding entrance/channel and, together with SDHC, coordinates heme b embedded between the two subunits (du2023structureofthe pages 2-5, fullerton2020thegeneticbasis pages 5-9).
- Subcellular localization and topology: SDHD is an integral protein of the inner mitochondrial membrane (IMM), forming the membrane anchor with SDHC; the quinone-binding entrance is formed by SDHC TM1, SDHD TM2, and the SDHB C-terminus, on the intermembrane-space side (du2023structureofthe pages 2-5).
- Pathways: SDHD links the TCA cycle and the electron transport chain by enabling electron flow from succinate to the CoQ pool. Loss of SDH activity causes succinate accumulation, which inhibits 2‑oxoglutarate–dependent dioxygenases, stabilizes HIFs (pseudohypoxia), and alters epigenetic regulation (khazal2024similardeficienciesdifferent pages 1-2, fullerton2020thegeneticbasis pages 5-9).
- Assembly factors: Biogenesis depends on SDHAFs—SDHAF2 drives covalent FAD attachment to SDHA; SDHAF1/3 assist SDHB Fe–S maturation/protection; SDHAF4 supports assembly of the SDHA/SDHB dimer—before integration with SDHC/SDHD (fullerton2020thegeneticbasis pages 5-9).
Recent developments and latest research (priority 2023–2024)
- Human Complex II structure (2023): High-resolution cryo-EM defined the human quinone-binding entrance (SDHC TM1, SDHD TM2, SDHB C-terminus), local SDHD residues contributing to quinone coordination, and the position/role of heme b as an electron sink equilibrating with quinone/semiquinone. The study also contextualized disease-linked variants (PNAS, Apr 2023; https://doi.org/10.1073/pnas.2216713120) (du2023structureofthe pages 2-5).
- Immunometabolic regulation (2023): Inflammatory activation in macrophages triggers Complex II disassembly and selective SDHB degradation via cardiolipin-dependent mechanisms, redefining how membrane lipids regulate Complex II stability and signaling (Science Advances, Feb 2023; https://doi.org/10.1126/sciadv.ade8701) (estebanamo2024succinatedehydrogenaseand pages 1-2).
- Assembly intermediates (2024): Disordered-to-ordered transitions in assembly factors (notably SDHAF2/SDHAF4) orchestrate the maturation sequence of Complex II, clarifying checkpoints prior to integration with SDHC/SDHD (Nature Communications, Jan 2024; https://doi.org/10.1038/s41467-023-44563-7) (miklovicova2025mitochondrialrespiratorycomplex pages 17-22).
- SDH loss models (2024): Comparative analyses of SDH subunit loss in adrenal-derived vs fibroblast cell lines show tissue-specific adaptations and vulnerabilities, informing why SDH loss drives paraganglioma/pheochromocytoma (Cancer & Metabolism, Dec 2024; https://doi.org/10.1186/s40170-024-00369-9) (khazal2024similardeficienciesdifferent pages 1-2).
Biochemical function and substrate specificity
- Reaction: Succinate + ubiquinone → fumarate + ubiquinol. SDHA provides the succinate site and FAD; SDHB relays electrons via [2Fe‑2S], [4Fe‑4S], [3Fe‑4S]; SDHC/SDHD bind/reduce ubiquinone at the IMM interface. Heme b between SDHC/SDHD is not a direct electron relay to ubiquinone but can act as an electron sink/equilibrator within physiological edge-to-edge distances (<~14 Å) (PNAS 2023; https://doi.org/10.1073/pnas.2216713120; Fullerton 2020; https://doi.org/10.1016/j.ymgme.2020.09.009) (du2023structureofthe pages 2-5, fullerton2020thegeneticbasis pages 5-9).
Cellular localization and membrane topology
- SDHD is an IMM multi-pass subunit; together with SDHC it creates the quinone-binding entrance near the intermembrane space and anchors the hydrophilic SDHA/SDHB head. The cryo‑EM structure maps SDHD TM2 as part of the quinone channel entrance; SDHD residues (e.g., Tyr near the entrance) participate in quinone coordination (PNAS 2023; https://doi.org/10.1073/pnas.2216713120) (du2023structureofthe pages 2-5).
Pathways, regulation, assembly, and higher-order organization
- TCA/ETC integration: Complex II uniquely couples succinate oxidation to CoQ reduction without proton pumping; SDHD’s membrane role is essential for quinone access and reduction (du2023structureofthe pages 2-5).
- Assembly: Maturation proceeds via SDHA flavinylation (SDHAF2) and SDHB Fe–S assembly/protection (SDHAF1/3), then SDHAF4‑mediated dimer assembly, and final docking with SDHC/SDHD (fullerton2020thegeneticbasis pages 5-9). Structural work in 2024 identified metastable intermediates governed by disordered-to-ordered transitions in assembly factors (https://doi.org/10.1038/s41467-023-44563-7) (miklovicova2025mitochondrialrespiratorycomplex pages 17-22).
- Regulation by lipids/inflammation: Cardiolipin orchestrates Complex II disassembly and SDHB turnover in LPS-stimulated macrophages, linking membrane composition to Complex II stability and pro-inflammatory reprogramming (https://doi.org/10.1126/sciadv.ade8701) (estebanamo2024succinatedehydrogenaseand pages 1-2).
- Supercomplexes: Mammalian Complex II is generally not a canonical component of respiratory supercomplexes, although non-mammalian megacomplexes exist; human structural data emphasize SDHD’s local architecture at the CoQ interface rather than supercomplex scaffolding (PNAS 2023) (du2023structureofthe pages 2-5).
Human disease and clinical relevance
- Tumor predisposition: Heterozygous germline SDHD variants predispose to hereditary paraganglioma/pheochromocytoma (PPGL), often with parent‑of‑origin effects described in clinical genetics literature; loss of the remaining allele in tumors produces SDH deficiency with succinate accumulation and pseudohypoxia. SDH-deficient GIST and renal carcinomas have also been reported across SDHx genes (Biomedicines 2024 review; https://doi.org/10.3390/biomedicines12092050) (estebanamo2024succinatedehydrogenaseand pages 1-2, khazal2024similardeficienciesdifferent pages 1-2).
- Pediatric guidance (2024): An international pediatric PPGL consensus provides updated recommendations for diagnosis/management and surveillance in SDHx carriers, relevant to SDHD families (Sep 2024; https://doi.org/10.17863/cam.111911) (miklovicova2025mitochondrialrespiratorycomplex pages 17-22).
- Recessive mitochondrial disease: Biallelic SDHD pathogenic variants cause isolated Complex II deficiency presenting with encephalomyopathy and cardiomyopathy; the p.Glu69Lys variant has been reported in multiple families consolidating SDHD as a disease gene (Eur J Hum Genet 2021; https://doi.org/10.1038/s41431-021-00887-w; J Med Genet 2014; https://doi.org/10.1136/jmedgenet-2013-101932) (lin2021consolidatingbiallelicsdhd pages 1-2, miklovicova2025mitochondrialrespiratorycomplex pages 17-22).
- Epidemiology/penetrance context: Isolated Complex II deficiency is rare among OXPHOS disorders; SDHD lies at 11q23.1 and encodes a ~19 kDa membrane protein. Comparative analyses emphasize lower frequency of SDHD driver variants relative to SDHB/SDHA in some tumor cohorts, but SDHD remains established for PPGL susceptibility (Journal of Cellular Physiology 2025; https://doi.org/10.1002/jcp.70066) (chatoff2025metaboliceffectsof pages 32-32).
Current applications and real-world implementations
- Diagnostics: Structural insights localize SDHD residues at the quinone entrance/heme interface to interpret missense variants; SDH-deficient tumor workups use immunohistochemistry (loss of SDHB staining) and targeted sequencing of SDHx genes to guide surveillance (PNAS 2023; Biomedicines 2024 review) (du2023structureofthe pages 2-5, estebanamo2024succinatedehydrogenaseand pages 1-2).
- Clinical management: Pediatric PPGL consensus (2024) informs genetic testing, biochemical phenotyping, and imaging for SDHx carriers; management is influenced by genotype and tumor location/secretory type (https://doi.org/10.17863/cam.111911) (miklovicova2025mitochondrialrespiratorycomplex pages 17-22).
- Immunometabolism: Targeting pathways controlling Complex II disassembly (e.g., cardiolipin synthesis/turnover) modulates inflammatory outputs in macrophages, suggesting translational avenues in inflammation and cancer contexts where SDH is dysregulated (Sci Adv 2023; https://doi.org/10.1126/sciadv.ade8701) (estebanamo2024succinatedehydrogenaseand pages 1-2).
Expert opinions and analysis (authoritative sources)
- Comprehensive genetics review (2020) synthesizes Complex II structure, assembly (SDHAF1–4), and clinical genetics distinguishing tumor predisposition (heterozygous SDHx) from recessive mitochondrial disease; it remains a standard reference for mechanistic context (Mol Genet Metab 2020; https://doi.org/10.1016/j.ymgme.2020.09.009) (fullerton2020thegeneticbasis pages 1-5, fullerton2020thegeneticbasis pages 5-9).
- Structural experts (2023) provide a definitive human Complex II architecture situating SDHD within the quinone/HEME environment, enabling rational mapping of pathogenic variants (PNAS 2023; https://doi.org/10.1073/pnas.2216713120) (du2023structureofthe pages 2-5).
Relevant statistics and data
- Genome and size: SDHD at 11q23.1; approximate mass ~19 kDa (Journal of Cellular Physiology 2025; https://doi.org/10.1002/jcp.70066) (chatoff2025metaboliceffectsof pages 32-32).
- Rarity: Isolated Complex II deficiency accounts for a small fraction of OXPHOS disorders; SDHD biallelic variants are documented but less frequent than SDHA (Mol Genet Metab 2020; https://doi.org/10.1016/j.ymgme.2020.09.009; Eur J Hum Genet 2021; https://doi.org/10.1038/s41431-021-00887-w) (fullerton2020thegeneticbasis pages 1-5, lin2021consolidatingbiallelicsdhd pages 1-2).
Notes on symbol ambiguity
- SDHD here is human Complex II membrane subunit D (CybS) per UniProt O14521. Retrieved evidence is consistent with this identity; no conflicting gene/protein with the same symbol was used (fullerton2020thegeneticbasis pages 1-5, du2023structureofthe pages 2-5).
References (URLs and dates)
- Du et al. Structure of the human respiratory complex II. PNAS. Apr 24, 2023. URL: https://doi.org/10.1073/pnas.2216713120 (du2023structureofthe pages 2-5)
- Fullerton et al. The genetic basis of isolated mitochondrial complex II deficiency. Mol Genet Metab. Sep 2020. URL: https://doi.org/10.1016/j.ymgme.2020.09.009 (fullerton2020thegeneticbasis pages 1-5, fullerton2020thegeneticbasis pages 5-9)
- Reynolds et al. Cardiolipin coordinates inflammatory metabolic reprogramming through regulation of Complex II disassembly and degradation. Science Advances. Feb 3, 2023. URL: https://doi.org/10.1126/sciadv.ade8701 (estebanamo2024succinatedehydrogenaseand pages 1-2)
- Sharma et al. Disordered-to-ordered transitions in assembly factors allow the complex II catalytic subunit to switch binding partners. Nature Communications. Jan 15, 2024. URL: https://doi.org/10.1038/s41467-023-44563-7 (miklovicova2025mitochondrialrespiratorycomplex pages 17-22)
- Khazal et al. Similar deficiencies, different outcomes: SDH loss in adrenal medulla vs. fibroblast models of paraganglioma. Cancer & Metabolism. Dec 5, 2024. URL: https://doi.org/10.1186/s40170-024-00369-9 (khazal2024similardeficienciesdifferent pages 1-2)
- Lin et al. Consolidating biallelic SDHD variants as a cause of mitochondrial complex II deficiency. Eur J Hum Genet. May 20, 2021. URL: https://doi.org/10.1038/s41431-021-00887-w (lin2021consolidatingbiallelicsdhd pages 1-2)
- Jackson et al. Mutations in SDHD lead to autosomal recessive encephalomyopathy and isolated Complex II deficiency. J Med Genet. Dec 2014. URL: https://doi.org/10.1136/jmedgenet-2013-101932 (miklovicova2025mitochondrialrespiratorycomplex pages 17-22)
- Esteban‑Amo et al. Succinate Dehydrogenase and Human Disease: Novel Insights into a Well‑Known Enzyme. Biomedicines. Sep 17, 2024. URL: https://doi.org/10.3390/biomedicines12092050 (estebanamo2024succinatedehydrogenaseand pages 1-2)
- Casey et al. International consensus statement on the diagnosis and management of phaeochromocytoma and paraganglioma in children and adolescents. Sep 2024. URL: https://doi.org/10.17863/cam.111911 (miklovicova2025mitochondrialrespiratorycomplex pages 17-22)
- Chatoff et al. Metabolic Effects of Succinate Dehydrogenase Loss in Cancer. J Cell Physiol. Jul 2025 (accepted/early). URL: https://doi.org/10.1002/jcp.70066 (chatoff2025metaboliceffectsof pages 32-32)
References
(fullerton2020thegeneticbasis pages 1-5): Millie Fullerton, Robert McFarland, Robert W. Taylor, and Charlotte L. Alston. The genetic basis of isolated mitochondrial complex ii deficiency. Molecular Genetics and Metabolism, 131:53-65, Sep 2020. URL: https://doi.org/10.1016/j.ymgme.2020.09.009, doi:10.1016/j.ymgme.2020.09.009. This article has 74 citations and is from a peer-reviewed journal.
(fullerton2020thegeneticbasis pages 5-9): Millie Fullerton, Robert McFarland, Robert W. Taylor, and Charlotte L. Alston. The genetic basis of isolated mitochondrial complex ii deficiency. Molecular Genetics and Metabolism, 131:53-65, Sep 2020. URL: https://doi.org/10.1016/j.ymgme.2020.09.009, doi:10.1016/j.ymgme.2020.09.009. This article has 74 citations and is from a peer-reviewed journal.
(du2023structureofthe pages 2-5): Zhanqiang Du, Xiaoting Zhou, Yuezheng Lai, Jinxu Xu, Yuying Zhang, Shan Zhou, Ziyan Feng, Long Yu, Yanting Tang, Weiwei Wang, Lu Yu, Changlin Tian, Ting Ran, Hongming Chen, Luke W. Guddat, Fengjiang Liu, Yan Gao, Zihe Rao, and Hongri Gong. Structure of the human respiratory complex ii. Proceedings of the National Academy of Sciences of the United States of America, Apr 2023. URL: https://doi.org/10.1073/pnas.2216713120, doi:10.1073/pnas.2216713120. This article has 58 citations and is from a highest quality peer-reviewed journal.
(khazal2024similardeficienciesdifferent pages 1-2): Fatimah J. Al Khazal, Sanjana Mahadev Bhat, Yuxiang Zhu, Cristina M. de Araujo Correia, Sherry X. Zhou, Brandon A. Wilbanks, Clifford D. Folmes, Gary C. Sieck, Judith Favier, and L. James Maher. Similar deficiencies, different outcomes: succinate dehydrogenase loss in adrenal medulla vs. fibroblast cell culture models of paraganglioma. Cancer & Metabolism, Dec 2024. URL: https://doi.org/10.1186/s40170-024-00369-9, doi:10.1186/s40170-024-00369-9. This article has 6 citations and is from a peer-reviewed journal.
(estebanamo2024succinatedehydrogenaseand pages 1-2): María J. Esteban-Amo, Patricia Jiménez-Cuadrado, Pablo Serrano-Lorenzo, Miguel Á. de la Fuente, and María Simarro. Succinate dehydrogenase and human disease: novel insights into a well-known enzyme. Biomedicines, 12:2050, Sep 2024. URL: https://doi.org/10.3390/biomedicines12092050, doi:10.3390/biomedicines12092050. This article has 25 citations and is from a poor quality or predatory journal.
(lin2021consolidatingbiallelicsdhd pages 1-2): Siying Lin, James Fasham, Fida’ Al-Hijawi, Nouar Qutob, Adam Gunning, Joseph S. Leslie, Lucy McGavin, Nishanka Ubeyratna, Wisam Baker, Ramez Zeid, Peter D. Turnpenny, Andrew H. Crosby, Emma L. Baple, and Reham Khalaf-Nazzal. Consolidating biallelic sdhd variants as a cause of mitochondrial complex ii deficiency. European Journal of Human Genetics, 29:1570-1576, May 2021. URL: https://doi.org/10.1038/s41431-021-00887-w, doi:10.1038/s41431-021-00887-w. This article has 14 citations and is from a domain leading peer-reviewed journal.
(chatoff2025metaboliceffectsof pages 32-32): Adam Chatoff, Daniel S. Kantner, Nathaniel W. Snyder, and Lori Rink. Metabolic effects of succinate dehydrogenase loss in cancer. Journal of cellular physiology, 240 7:e70066, Jul 2025. URL: https://doi.org/10.1002/jcp.70066, doi:10.1002/jcp.70066. This article has 0 citations and is from a peer-reviewed journal.
(miklovicova2025mitochondrialrespiratorycomplex pages 17-22): S Miklovičová. Mitochondrial respiratory complex ii and its function in cancer. Unknown journal, 2025.