The Citric Acid Cycle, also known as the Tricarboxylic Acid (TCA) Cycle or Krebs Cycle, is a central metabolic pathway located in the mitochondrial matrix](cell-types/mitochondria). It serves as the hub of cellular metabolism, oxidizing acetyl-CoA derived from carbohydrates, fats, and to generate high-energy electron carriers (NADH and FADH₂) that fuel ATP production through the Electron Transport Chain. In neurons—neurons—highly energy-demanding cells with limited regenerative capacity—proper TCA cycle function is critical for maintaining synaptic activity, membrane potentials, and cellular homeostasis. Dysregulation of this cycle is increasingly recognized as a key contributor to neurodegeneration in Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS), and Huntington's disease (HD) 1. @gibson2020
The TCA cycle consists of eight enzyme-catalyzed reactions that completely oxidize acetyl-CoA to two molecules of CO₂ while capturing energy in the form of NADH, FADH₂, and GTP. This cycle not only generates ATP but also provides metabolic intermediates for biosynthesis, including amino acids, porphyrins, and lipids. In the brain, the TCA cycle is particularly important because: @attwell2001
Reaction: Acetyl-CoA + Oxaloacetate → Citrate + CoA @wiegand1986
Citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate. This is the rate-limiting step of the TCA cycle and is allosterically inhibited by ATP, NADH, and succinyl-CoA, while activated by ADP and Ca²⁺ 3. In AD, reduced CS activity has been reported in the hippocampus and temporal cortex, contributing to impaired energy metabolism 4. @bubber2004
Reaction: Citrate ↔ Isocitrate (via cis-aconitate) @burrage2012
Aconitase catalyzes the isomerization of citrate to isocitrate through a dehydration/hydration mechanism. The enzyme contains an iron-sulfur 4Fe-4S cluster that is highly sensitive to oxidative stress. In neurodegeneration, mitochondrial aconitase (ACO2) is a known target of reactive oxygen species (ROS), and mutations in ACO2 cause hereditary spastic paraplegia and cerebellar ataxia 5. @waitkus2013
Reaction: Isocitrate + NAD⁺ → α-Ketoglutarate + NADH + CO₂ @gibson2002
Isocitrate dehydrogenase (IDH3) is the NAD⁺-dependent enzyme in the TCA cycle, generating NADH. IDH3 is allosterically activated by ADP and inhibited by ATP and NADH. Notably, IDH1 (cytosolic) and IDH2 (mitochondrial) are frequently mutated in cancers, but in neurodegeneration, loss-of-function mutations in IDH1 are associated with reduced α-ketoglutarate levels and impaired DNA methylation 6. @mizuno1990
Reaction: α-Ketoglutarate + NAD⁺ + CoA → Succinyl-CoA + NADH + CO₂ @johnson1998
The α-ketoglutarate dehydrogenase complex is a key regulatory point in the TCA cycle and is particularly vulnerable in neurodegeneration. α-KGDHC requires thiamine pyrophosphate (TPP) as a cofactor, explaining why thiamine deficiency (seen in Wernicke-Korsakoff syndrome) impairs the TCA cycle 7. In AD and PD brains, α-KGDHC activity is significantly reduced in affected regions, contributing to metabolic failure 8. @bayley2010
Reaction: Succinyl-CoA + GDP (or ADP) → Succinate + GTP (or ATP) @yang2011
Succinyl-CoA synthetase (also called succinate thiokinase) generates GTP (or ATP) through substrate-level phosphorylation. This is the only step in the TCA cycle that directly produces high-energy phosphate. Two isoforms exist: SCS-GDP (brain-specific) and SCS-ATP (ubiquitous) 9. @minden1974
Reaction: Succinate + FAD → Fumarate + FADH₂ @gibson2018
Succinate dehydrogenase (SDH) is unique among TCA cycle enzymes because it is also a component of the Electron Transport Chain (Complex II). SDH deficiency is a hallmark of PD, where Complex I inhibition indirectly affects SDH function. Germline SDH mutations cause hereditary paraganglioma and pheochromocytoma 10.
Reaction: Fumarate + H₂O → Malate
Fumarase (fumarate hydratase) catalyzes the hydration of fumarate to malate. Loss-of-function mutations in FH cause hereditary fumarase deficiency, leading to severe neurological deficits, including microcephaly and seizures. In cancer, FH deficiency leads to fumarate accumulation, which inhibits α-ketoglutarate-dependent dioxygenases, causing epigenetic dysregulation 11.
Reaction: Malate + NAD⁺ → Oxaloacetate + NADH
Malate dehydrogenase completes the cycle by regenerating oxaloacetate while generating NADH. The reaction is highly unfavorable (ΔG°' = +29.7 kJ/mol) and is driven forward by the subsequent citrate synthase reaction. MDH2 is also part of the malate-aspartate shuttle, which transfers cytosolic NADH electrons into mitochondria 12.