alpha-ketoglutarate dehydrogenase
Summary
Alpha-ketoglutarate dehydrogenase is a TCA cycle enzyme complex that converts alpha-ketoglutarate to succinyl-CoA, generating NADH and CO2. It is structurally and mechanistically analogous to pyruvate dehydrogenase and requires the same 5 cofactors (thiamine pyrophosphate, lipoic acid, CoA, FAD, NAD+). It is inhibited by arsenic (binds lipoic acid) and is sensitive to thiamine deficiency.
Detail
**Reaction:** Alpha-ketoglutarate + NAD+ + CoA → Succinyl-CoA + NADH + CO2 (irreversible oxidative decarboxylation; TCA cycle step 4 of 8, mitochondrial matrix).
**Enzyme complex (3 subunits, E1-E3):** - E1: alpha-ketoglutarate dehydrogenase (uses TPP, vitamin B1) - E2: dihydrolipoyl transsuccinylase (lipoic acid, CoA/pantothenate, B5) - E3: dihydrolipoyl dehydrogenase (FAD from B2, NAD+ from B3) Mnemonic for cofactors: "Tender Loving Care For Nancy" (TPP, Lipoic acid, CoA, FAD, NAD+).
**Similarity to PDH and branched-chain alpha-ketoacid dehydrogenase (BCKDH):** All three are alpha-ketoacid dehydrogenase complexes with the same cofactors and the same E3 subunit. PDH: pyruvate → acetyl-CoA. BCKDH: branched-chain ketoacids → acyl-CoA derivatives (deficient in maple syrup urine disease).
**Regulation:** - Inhibited by: succinyl-CoA, NADH (product inhibition). Also inhibited by high ATP in some tissues. - Activated by: Ca2+ (important in muscle contraction and in matching ATP production to demand). - Unlike PDH, it is NOT regulated by phosphorylation/dephosphorylation.
**Energy yield:** The NADH generated yields about 2.5 ATP via the electron transport chain. The next TCA step (succinyl-CoA synthetase) makes GTP/ATP by substrate-level phosphorylation.
**Clinical significance:** - **Thiamine (B1) deficiency** (beriberi, Wernicke-Korsakoff in alcohol use disorder): impaired PDH and alpha-KGDH, causing decreased ATP, especially in the brain and heart. Lactic acidosis occurs from pyruvate accumulation. Giving glucose before thiamine can precipitate or worsen Wernicke encephalopathy. - **Arsenic poisoning:** Arsenite binds lipoic acid sulfhydryl groups, inhibiting PDH and alpha-KGDH. Findings: vomiting, rice-water diarrhea, garlic breath, QT prolongation, and peripheral neuropathy. Treat with dimercaprol (BAL), succimer, or penicillamine. - **Mitochondrial dysfunction/E3 deficiency:** Dihydrolipoamide dehydrogenase deficiency affects all three alpha-ketoacid dehydrogenases (PDH, alpha-KGDH, BCKDH), causing lactic acidosis and neurologic findings. - **Neurodegeneration:** Decreased alpha-KGDH activity is described in Alzheimer disease and other neurodegenerative disorders (research association).
**Related associations:** - Alpha-ketoglutarate is also a key link to amino acid metabolism: it accepts amino groups in transamination (to form glutamate) and is formed from glutamate by glutamate dehydrogenase. It is therefore important in nitrogen transport and the urea cycle (via glutamate). - Fumarate/succinate/2-hydroxyglutarate accumulation: IDH1/2 mutations produce 2-hydroxyglutarate from alpha-KG (gliomas, AML), an oncometabolite.
**High-yield tie-in:** Alpha-KGDH is one of the main NADH-producing steps in the TCA cycle (3 NADH, 1 FADH2, 1 GTP per cycle turn per acetyl-CoA). Cofactor deficiencies and arsenic are the classic test points.
Sources
- First Aid for the USMLE Step 1 (Biochemistry: Metabolism, TCA cycle, Pyruvate dehydrogenase complex)
- Lippincott Illustrated Reviews: Biochemistry (Citric Acid Cycle)
- Lehninger Principles of Biochemistry (Citric Acid Cycle)
- Harper's Illustrated Biochemistry
- Pathoma/Sketchy Biochemistry (TCA cycle and vitamin cofactors)
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.