Untitled Quiz

Tricarboxylic Acid Cycle (TCA) Overview

• Also known as the Citric Acid Cycle or Krebs Cycle
• A metabolic pathway crucial for energy production in most animals
• Oxidative catabolism of carbohydrates (CHO), amino acids (AAs), and fatty acids (FAs) converges here
• Converts carbon skeletons to carbon dioxide (CO₂)

Function of TCA

• Provides energy for ATP production in most animals
• Generates intermediates for various synthetic reactions
• Creates glucose from the carbon skeletons of some amino acids
• Synthesizes amino acids and heme

Other Important Features of TCA

• Anaplerotic reactions replenish intermediates in the pathway
  • These reactions fill up the pathway by replenishing intermediates from the breakdown of amino acids
• Located in the mitochondria, close to the site of the electron transport chain (ETC)
• An oxygen-requiring pathway

Reactions of the TCA Cycle - Oxidative Decarboxylation of Pyruvate

• Acetyl CoA, a 2-carbon substrate, is generated from pyruvate, a 3-carbon compound
• Pyruvate must be transported from the cytosol to the mitochondrion
• The conversion is catalyzed by the pyruvate dehydrogenase complex, a multienzyme complex
  • This step is critically important and significant
    • Deficiencies in this complex can lead to serious central nervous system (CNS) problems
    • Reduced ability of brain cells to produce ATP if the complex is inactive
    • Wernicke-Korsakoff syndrome is one condition associated with thiamine deficiency, particularly in alcoholics

Pyruvate Dehydrogenase Complex Coenzymes

• Requires five coenzymes:
  • Thiamine pyrophosphate (TPP)
  • Lipoic acid
  • Coenzyme A
  • Flavin adenine dinucleotide (FAD)
  • Nicotinamide adenine dinucleotide (NAD)

Regulation of Pyruvate Dehydrogenase (PDH)

• PDH is covalently modified via phosphorylation/dephosphorylation
• Allosterically modified by ATP, acetyl CoA, and NADH
  • These are "high-energy signals"
• High-energy signals switch the enzyme off

PDH Deficiency

• A rare condition—most common cause of congenital lactic acidosis
• Symptoms can include:
  • Neurodegeneration, muscle spasticity, and early death in neonatal onset cases.
• No proven treatment exists
  • Dietary restriction of CHO and supplementation with thiamine may be helpful
  • One possible treatment is an experimental therapy using vitamins and supplements
• Arsenic poisoning inhibits enzymes needing lipoic acid as a coenzyme, interfering with PDH function

EMG and MRI findings in an example patient (Alex)

• EMG showed peripheral nerve damage in arms/legs
• MRI was clear
• Alex was in the ICU, showing symptoms of lactic acidosis—lactic acid in his blood reached dangerous levels
• PDH deficiency diagnosed
• Glucose drip replaced with a protein and lactate regimen.
• This was effective at stabilizing lactate levels
• Peripheral nerve damage occurred in infancy
• The treatment included a lifelong ketogenic diet

TCA Deficiency and Survival

• Defects in TCA cycle enzymes are rare
• This highlights the importance of the pathway for survival
• Fumarase Deficiency has resulted in:
  • Encephalopathy, severe intellectual disabilities, unusual facial features, brain malformation, and epileptic seizures.

Synthesis of Citrate from Acetyl CoA and Oxaloacetate

• Catalyzed by Citrate Synthase
• Inhibited by citrate and substrate availability
• This is one example of metabolism of metabolites converting to other metabolites

Isomerization of Citrate By Aconitase

• Catalyzed by aconitase
• Inhibited by fluoracetate, a plant toxin used as a pesticide
• This enzyme is an important part of the cycle

Oxidative Decarboxylation of Isocitrate By Isocitrate Dehydrogenase

• Catalyzed by isocitrate dehydrogenase
• Produces the first NADH of the cycle
• Releases a carbon dioxide (CO₂) molecule
• Rate-limiting step

Oxidative Decarboxylation of α-Ketoglutarate By α-Ketoglutarate Dehydrogenase

• Releases the second CO₂
• Produces the second NADH of the cycle
• Uses TPP, Lipoic acid, FAD, NAD+, and CoA as coenzymes
• Inhibited by NADH and succinyl CoA and activated by Ca²+
• A-ketoglutarate is also produced from breakdown of amino acid glutamate

Cleavage of Succinyl Coenzyme A By Succinate Thiokinase

• Results in GTP production (substrate-level phosphorylation)
• Succinyl CoA can build up from propionyl CoA (FA metabolism)

Oxidation of Succinate By Succinate Dehydrogenase

• Produces FADH₂
• The only TCA cycle enzyme embedded in the inner mitochondrial membrane
• Functions as Complex II of the ETC

Hydration of Fumarate By Fumarase

• Produces L-malate
• Part of the Urea cycle, purine synthesis, and amino acid catabolism

Oxidation of Malate By Malate Dehydrogenase

• Produces the third NADH of the cycle.
• Produces oxaloacetate (OAA) via transamination of the amino acid aspartate

Energy Produced by TCA per cycle

• One turn of the cycle generates:
• 3 NADH
• 1 FADH2
• 1 GTP (equivalent to 1 ATP)
• 2 carbon dioxide molecules (CO₂)
• Therefore, 12 ATPs are made per cycle per acetyl CoA molecule oxidized

Regulation of TCA Cycle

• Some enzymes, such as citrate synthase, and isocitrate dehydrogenase are critically important
• These are all upregulated by low ATP to ADP ratios.

Role of TCA in Anabolism

• TCA intermediates serve as precursors for other biomolecules.
• The cycle provides building blocks for other necessary molecules such as amino acids, porphyrins, and heme