Respiratory complex II

evidence_spans

[{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC10161127", "locator": "XML .//body//p", "paragraph_index": 2, "char_start": 0, "char_end": 1105, "evidence_access": "full-text"}]

experimental_model

Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays.

exposure

No nutrient intervention; structural or biochemical characterization.

limitations

UQ1/DCIP are assay reagents; the isolated turnover value is not whole-cell ATP production.

nutrient_topic

Riboflavin research collection; topical membership is not evidence of a direct dietary effect. · Riboflavin (vitamin B2)

organism

Homo sapiens

plain_language

The intact complex moves electrons from succinate into a quinone carrier.

primary_references

[du-2023-human-complex-ii] Structure of the human respiratory complex II (2023). https://pubmed.ncbi.nlm.nih.gov/37098072/ DOI: 10.1073/pnas.2216713120

tissue_or_cell_type

HEK293F-derived purified complex II

Riboflavin: mechanisms, deficiency and nutrient interactions (2026-09-17) · lines 663–674

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays. · source_derived_draft · unverified_draft

### b2-met-human-complex-ii-succinate-quinone Purified human complex II coupled succinate oxidation to ubiquinone reduction in a UQ1/DCIP assay, with reported kcat 0.67 +/- 0.02 per second. Condition category: normal nutrient_topic: Riboflavin research collection; topical membership is not evidence of a direct dietary effect. plain_language: The intact complex moves electrons from succinate into a quinone carrier. organism: Homo sapiens tissue_or_cell_type: HEK293F-derived purified complex II experimental_model: Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays. limitations: UQ1/DCIP are assay reagents; the isolated turnover value is not whole-cell ATP production. exposure: No nutrient intervention; structural or biochemical characterization. evidence_spans: [{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC10161127", "locator": "XML .//body//p", "paragraph_index": 2, "char_start": 0, "char_end": 1105, "evidence_access": "full-text"}] [du-2023-human-complex-ii] Structure of the human respiratory complex II (2023). https://pubmed.ncbi.nlm.nih.gov/37098072/ DOI: 10.1073/pnas.2216713120

cross_nutrient

true

evidence_location

Figure 6D

experimental_model

Human NSCLC cells cultured in RPMI-1640 +10% FBS; L-ascorbic acid stocks neutralized to pH 7.0; isolated lysate respiratory-enzyme activity assays

exposure

15 pmol/cell (approximately 8 mM) ascorbate for 1 h before activity assays.

limitations

Short pharmacological culture exposure; dose per cell, medium and density alter toxicity. Enzyme activity loss is consistent with Fe-S damage but does not directly trace individual iron atoms leaving a cluster. This pattern supports but does not by itself prove direct cluster oxidation; no organism-level mitochondrial benefit or harm claimed.

nutrient_topic

Vitamin C research collection; topical membership is not evidence of a direct dietary effect. · Vitamin C

organism

Homo sapiens

plain_language

The exposure also impaired respiratory complex II, which participates in respiration and the citric acid cycle.

primary_references

[c-reg-schoenfeld] O2⋅- and H2O2-Mediated Disruption of Fe Metabolism Causes the Differential Susceptibility of NSCLC and GBM Cancer Cells to Pharmacological Ascorbate. (2017). https://pubmed.ncbi.nlm.nih.gov/28366679/ DOI: 10.1016/j.ccell.2017.02.018

tissue_or_cell_type

NSCLC cell cultures

Vitamin C: mechanisms, deficiency and nutrient interactions (2026-09-17) · lines 1383–1395

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Human NSCLC cells cultured in RPMI-1640 +10% FBS; L-ascorbic acid stocks neutralized to pH 7.0; isolated lysate respiratory-enzyme activity assays · source_derived_draft · unverified_draft

### c-reg-complex-ii-inactivation The same pharmacological ascorbate exposure reduced NSCLC respiratory-complex II activity, extending the observed impairment to a second Fe-S-containing respiratory complex; complex IV activity was retained. Condition category: normal nutrient_topic: Vitamin C research collection; topical membership is not evidence of a direct dietary effect. plain_language: The exposure also impaired respiratory complex II, which participates in respiration and the citric acid cycle. organism: Homo sapiens tissue_or_cell_type: NSCLC cell cultures experimental_model: Human NSCLC cells cultured in RPMI-1640 +10% FBS; L-ascorbic acid stocks neutralized to pH 7.0; isolated lysate respiratory-enzyme activity assays limitations: Short pharmacological culture exposure; dose per cell, medium and density alter toxicity. Enzyme activity loss is consistent with Fe-S damage but does not directly trace individual iron atoms leaving a cluster. This pattern supports but does not by itself prove direct cluster oxidation; no organism-level mitochondrial benefit or harm claimed. exposure: 15 pmol/cell (approximately 8 mM) ascorbate for 1 h before activity assays. cross_nutrient: true evidence_location: Figure 6D [c-reg-schoenfeld] O2⋅- and H2O2-Mediated Disruption of Fe Metabolism Causes the Differential Susceptibility of NSCLC and GBM Cancer Cells to Pharmacological Ascorbate. (2017). https://pubmed.ncbi.nlm.nih.gov/28366679/ DOI: 10.1016/j.ccell.2017.02.018

cell_type

melanoma

experimental_model

Selenoo deletion

limitations

Does not isolate SDHA AMPylation as the sole cause.

organism

mouse

Selenium: literature corrections and mechanism additions · lines 474–484

Metabolic Ledger literature curation, 17 September 2026; primary papers linked individually · supports · Selenoo deletion · secondary_verified · secondary_verified

## selenoo-loss-increases-complex-ii-activity Removing SELENOO increased respiratory complex-II activity in this model. SELENOO deletion increased complex-II activity in the tested melanoma model. Organism: mouse Cell type: melanoma Experimental model: Selenoo deletion Limitations: Does not isolate SDHA AMPylation as the sole cause. Primary reference: [Selenoprotein O Promotes Melanoma Metastasis and Regulates Mitochondrial Complex II Activity](https://pubmed.ncbi.nlm.nih.gov/39700395/)

evidence_spans

[{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC10161127", "locator": "XML .//body//p", "paragraph_index": 3, "char_start": 0, "char_end": 2164, "evidence_access": "full-text"}]

experimental_model

Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays.

exposure

No nutrient intervention; structural or biochemical characterization.

limitations

Structure establishes location, not nutritional sensitivity.

nutrient_topic

Riboflavin research collection; topical membership is not evidence of a direct dietary effect. · Riboflavin (vitamin B2)

organism

Homo sapiens

plain_language

The B2-derived cofactor sits in the catalytic SDHA part of complex II.

primary_references

[du-2023-human-complex-ii] Structure of the human respiratory complex II (2023). https://pubmed.ncbi.nlm.nih.gov/37098072/ DOI: 10.1073/pnas.2216713120

tissue_or_cell_type

HEK293F-derived purified complex II

Riboflavin: mechanisms, deficiency and nutrient interactions (2026-09-17) · lines 636–647

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays. · source_derived_draft · unverified_draft

### b2-met-human-sdha-fad Cryo-EM of active human complex II resolved FAD within SDHA and located the succinate-fumarate active site between its FAD-binding and capping domains. Condition category: normal nutrient_topic: Riboflavin research collection; topical membership is not evidence of a direct dietary effect. plain_language: The B2-derived cofactor sits in the catalytic SDHA part of complex II. organism: Homo sapiens tissue_or_cell_type: HEK293F-derived purified complex II experimental_model: Human complex II purified from HEK293F cells, cryo-EM, EPR and succinate-quinone activity assays. limitations: Structure establishes location, not nutritional sensitivity. exposure: No nutrient intervention; structural or biochemical characterization. evidence_spans: [{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC10161127", "locator": "XML .//body//p", "paragraph_index": 3, "char_start": 0, "char_end": 2164, "evidence_access": "full-text"}] [du-2023-human-complex-ii] Structure of the human respiratory complex II (2023). https://pubmed.ncbi.nlm.nih.gov/37098072/ DOI: 10.1073/pnas.2216713120

availability_state

machinery_impairment Imported condition classification; unverified.

evidence_spans

[{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC5076811", "locator": "HTML article p", "paragraph_index": 20, "char_start": 0, "char_end": 1274, "evidence_access": "full-text"}]

experimental_model

Human MDA-MB-231 breast cancer cells with CRISPR SDHAF2 disruption and independent siRNA knockdown.

exposure

CRISPR knockout, two independent siRNAs; FAD fluorescence/antibody and enzyme activity.

limitations

Cell-line-specific redundancy is not proof that SDHAF2 is dispensable in every tissue; contextual boundary to yeast and tumor findings.

nutrient_topic

Riboflavin research collection; topical membership is not evidence of a direct dietary effect. · Riboflavin (vitamin B2)

organism

Homo sapiens

plain_language

This breast cancer cell model assembled active, flavinated complex II without SDHAF2.

primary_references

[bezawork-geleta-2016-sdhaf2] The Assembly Factor SDHAF2 Is Dispensable for Flavination of the Catalytic Subunit of Mitochondrial Complex II in Breast Cancer Cells (2016). https://pubmed.ncbi.nlm.nih.gov/27587393/ DOI: 10.1074/jbc.c116.755017

tissue_or_cell_type

MDA-MB-231 breast cancer cells

trigger_kind

machinery_impairment Imported condition classification; unverified.

Riboflavin: mechanisms, deficiency and nutrient interactions (2026-09-17) · lines 715–726

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Human MDA-MB-231 breast cancer cells with CRISPR SDHAF2 disruption and independent siRNA knockdown. · source_derived_draft · unverified_draft

### b2-met-sdhaf2-knockout-context-boundary SDHAF2 knockout and independent siRNA knockdown in MDA-MB-231 cells retained SDHA flavination; knockout cells also retained measurable SDH and succinate-quinone reductase activity. Condition category: machinery_impairment nutrient_topic: Riboflavin research collection; topical membership is not evidence of a direct dietary effect. plain_language: This breast cancer cell model assembled active, flavinated complex II without SDHAF2. organism: Homo sapiens tissue_or_cell_type: MDA-MB-231 breast cancer cells experimental_model: Human MDA-MB-231 breast cancer cells with CRISPR SDHAF2 disruption and independent siRNA knockdown. limitations: Cell-line-specific redundancy is not proof that SDHAF2 is dispensable in every tissue; contextual boundary to yeast and tumor findings. exposure: CRISPR knockout, two independent siRNAs; FAD fluorescence/antibody and enzyme activity. evidence_spans: [{"source_bundle": "artifacts/riboflavin_metabolism_sources.json", "source_key": "PMC5076811", "locator": "HTML article p", "paragraph_index": 20, "char_start": 0, "char_end": 1274, "evidence_access": "full-text"}] [bezawork-geleta-2016-sdhaf2] The Assembly Factor SDHAF2 Is Dispensable for Flavination of the Catalytic Subunit of Mitochondrial Complex II in Breast Cancer Cells (2016). https://pubmed.ncbi.nlm.nih.gov/27587393/ DOI: 10.1074/jbc.c116.755017

evidence_span

{"source_cache": "artifacts/metformin-research/15047621.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "c4c648ab82a982482ec339309f3cc1c84ffe457073f010b546f7a47ffa2fbdf6", "start_char": 0, "end_char": 1613, "text_sha256": "c4c648ab82a982482ec339309f3cc1c84ffe457073f010b546f7a47ffa2fbdf6"}

experimental_model

Tissue homogenates, isolated mitochondria and intact rat soleus muscle

exposure

Metformin 30 mmol/l in homogenates and 270 micromol/l for 24 h in intact muscle, compared with thiazolidinediones

limitations

Millimolar concentrations in homogenates are far above therapeutic exposure. The comparison drugs are included because the paper measured them together, not as a metformin claim.

nutrient_topic

Metformin research collection; topical membership is not evidence of a direct clinical effect, and pharmacological exposure is not dietary intake. · Metformin

organism

Rat tissue and isolated mitochondria

plain_language

The block is specific to the first complex; feeding the chain past it restores respiration.

primary_references

[metformin-p15047621] Thiazolidinediones, like metformin, inhibit respiratory complex I: a common mechanism contributing to their antidiabetic actions? (2004). https://pubmed.ncbi.nlm.nih.gov/15047621/ DOI: 10.2337/diabetes.53.4.1052

tissue_or_cell_type

Skeletal muscle

Metformin: transport, molecular targets, gut mechanisms and nutrient interactions (2026-09-19) · lines 437–448

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Tissue homogenates, isolated mitochondria and intact rat soleus muscle · source_derived_draft · unverified_draft

### metformin-complex-i-muscle Metformin reduced complex I activity in skeletal-muscle homogenates and reduced state 3 respiration of isolated mitochondria consuming glutamate and malate, while respiration on succinate through complex II was unaffected. Condition category: normal nutrient_topic: Metformin research collection; topical membership is not evidence of a direct clinical effect, and pharmacological exposure is not dietary intake. plain_language: The block is specific to the first complex; feeding the chain past it restores respiration. organism: Rat tissue and isolated mitochondria tissue_or_cell_type: Skeletal muscle experimental_model: Tissue homogenates, isolated mitochondria and intact rat soleus muscle limitations: Millimolar concentrations in homogenates are far above therapeutic exposure. The comparison drugs are included because the paper measured them together, not as a metformin claim. exposure: Metformin 30 mmol/l in homogenates and 270 micromol/l for 24 h in intact muscle, compared with thiazolidinediones evidence_span: {"source_cache": "artifacts/metformin-research/15047621.abstract.txt", "locator": "Indexed abstract; zero-based, end-exclusive Unicode character offsets", "file_sha256": "c4c648ab82a982482ec339309f3cc1c84ffe457073f010b546f7a47ffa2fbdf6", "start_char": 0, "end_char": 1613, "text_sha256": "c4c648ab82a982482ec339309f3cc1c84ffe457073f010b546f7a47ffa2fbdf6"} [metformin-p15047621] Thiazolidinediones, like metformin, inhibit respiratory complex I: a common mechanism contributing to their antidiabetic actions? (2004). https://pubmed.ncbi.nlm.nih.gov/15047621/ DOI: 10.2337/diabetes.53.4.1052

availability_state

machinery_impairment Imported condition classification; unverified.

cross_nutrient

true

evidence_location

Full text lines 49–55; Fig. 3; Supplementary Figs. S6–S10

experimental_model

Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria

exposure

Cardiac-specific genetic deletion; 6- and 16-week assembly assays, 6- and 10-week respiratory assays.

limitations

The FeS subunits of complexes II/III were preferentially affected, whereas broader complex-I subunit loss supports an additional assembly role. This experiment does not establish a dietary pantothenate threshold or benefit from B5 supplementation.

nutrient_topic

Pantothenic acid (vitamin B5) research collection; topical membership is not evidence of a direct dietary effect. · Pantothenate (vitamin B5)

organism

Mus musculus

plain_language

Loss of the carrier impaired assembly of several respiratory complexes in mouse heart.

primary_references

[b5-met-ndufab2019] NDUFAB1 confers cardio-protection by enhancing mitochondrial bioenergetics through coordination of respiratory complex and supercomplex assembly. (2019). https://pubmed.ncbi.nlm.nih.gov/31366990/ DOI: 10.1038/s41422-019-0208-x

tissue_or_cell_type

Heart mitochondria

trigger_kind

machinery_impairment Imported condition classification; unverified.

Pantothenic acid (vitamin B5): coenzyme A, deficiency and nutrient interactions (2026-09-17) · lines 949–961

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria · source_derived_draft · unverified_draft

### b5-met-heart-acp-complex-assembly Cardiac Ndufab1 deletion reduced assembled respiratory complexes I–III and supercomplexes in mouse heart mitochondria. Condition category: machinery_impairment nutrient_topic: Pantothenic acid (vitamin B5) research collection; topical membership is not evidence of a direct dietary effect. plain_language: Loss of the carrier impaired assembly of several respiratory complexes in mouse heart. organism: Mus musculus tissue_or_cell_type: Heart mitochondria experimental_model: Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria limitations: The FeS subunits of complexes II/III were preferentially affected, whereas broader complex-I subunit loss supports an additional assembly role. This experiment does not establish a dietary pantothenate threshold or benefit from B5 supplementation. exposure: Cardiac-specific genetic deletion; 6- and 16-week assembly assays, 6- and 10-week respiratory assays. cross_nutrient: true evidence_location: Full text lines 49–55; Fig. 3; Supplementary Figs. S6–S10 [b5-met-ndufab2019] NDUFAB1 confers cardio-protection by enhancing mitochondrial bioenergetics through coordination of respiratory complex and supercomplex assembly. (2019). https://pubmed.ncbi.nlm.nih.gov/31366990/ DOI: 10.1038/s41422-019-0208-x

availability_state

machinery_impairment Imported condition classification; unverified.

cross_nutrient

false

evidence_location

Full text lines 47–49; Fig. 3A

experimental_model

Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria

exposure

Cardiac-specific genetic deletion; 6- and 16-week assembly assays, 6- and 10-week respiratory assays.

limitations

Isolated-mitochondria oxygen consumption does not measure nutrient absorption or human exercise capacity. This experiment does not establish a dietary pantothenate threshold or benefit from B5 supplementation.

nutrient_topic

Pantothenic acid (vitamin B5) research collection; topical membership is not evidence of a direct dietary effect. · Pantothenate (vitamin B5)

organism

Mus musculus

plain_language

The respiratory defect affected several upstream electron-transfer routes but spared the tested complex IV route.

primary_references

[b5-met-ndufab2019] NDUFAB1 confers cardio-protection by enhancing mitochondrial bioenergetics through coordination of respiratory complex and supercomplex assembly. (2019). https://pubmed.ncbi.nlm.nih.gov/31366990/ DOI: 10.1038/s41422-019-0208-x

tissue_or_cell_type

Heart mitochondria

trigger_kind

machinery_impairment Imported condition classification; unverified.

Pantothenic acid (vitamin B5): coenzyme A, deficiency and nutrient interactions (2026-09-17) · lines 963–975

AI-assisted literature curation; primary study URLs and scope retained in the document and extraction. Not publisher full text. · supports · Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria · source_derived_draft · unverified_draft

### b5-met-heart-acp-respiration Cardiac Ndufab1 deletion lowered oxygen consumption supported by complex I-, II-, or III-linked substrates, while complex IV-linked respiration remained unchanged. Condition category: machinery_impairment nutrient_topic: Pantothenic acid (vitamin B5) research collection; topical membership is not evidence of a direct dietary effect. plain_language: The respiratory defect affected several upstream electron-transfer routes but spared the tested complex IV route. organism: Mus musculus tissue_or_cell_type: Heart mitochondria experimental_model: Cardiomyocyte-specific Ndufab1 knockout mice and isolated heart mitochondria limitations: Isolated-mitochondria oxygen consumption does not measure nutrient absorption or human exercise capacity. This experiment does not establish a dietary pantothenate threshold or benefit from B5 supplementation. exposure: Cardiac-specific genetic deletion; 6- and 16-week assembly assays, 6- and 10-week respiratory assays. cross_nutrient: false evidence_location: Full text lines 47–49; Fig. 3A [b5-met-ndufab2019] NDUFAB1 confers cardio-protection by enhancing mitochondrial bioenergetics through coordination of respiratory complex and supercomplex assembly. (2019). https://pubmed.ncbi.nlm.nih.gov/31366990/ DOI: 10.1038/s41422-019-0208-x