Biochemistry: Pyruvate Oxidation and TCA Cycle

Pyruvate Oxidation

• Pyruvate is oxidized to acetyl-CoA and CO2 inside the mitochondria
• Pyruvate dehydrogenase complex (PDHc) is responsible for this process
• PDHc contains three enzymes: E1, E2, and E3
• Four water-soluble vitamins are crucial for pyruvate oxidation: thiamin, riboflavin, niacin, and pantothenic acid
• Lipoate is also involved in the process
• PDH is regulated by allosteric effectors and covalent modification
• Deficiencies or dysregulation of PDHc can have clinical significance

TCA Cycle

• Acetyl-CoA is oxidized in the TCA cycle to generate CO2, reducing equivalents (NADH & FADH2), and GTP
• The cycle consists of eight enzymatic steps:
  • Citrate synthase catalyzes the formation of citrate from oxaloacetate and acetyl-CoA
  • Aconitase converts citrate to isocitrate
  • Isocitrate dehydrogenase oxidizes isocitrate while forming NADH and CO2
  • Alpha-ketoglutarate dehydrogenase complex (α-KGDH) is similar to PDHc and catalyzes the formation of succinyl-CoA, NADH, and CO2
  • Succinyl-CoA synthetase converts succinyl-CoA to succinate, producing GTP
  • Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate, producing FADH2
  • Fumarase converts fumarate to malate
  • Malate dehydrogenase produces oxaloacetate from malate, generating NADH
• The cycle is highly exergonic and thermodynamically favorable
• The TCA cycle is regulated by substrate availability and allosteric effectors
• Dysregulation of TCA cycle enzymes, such as SDH, can have clinical significance

Oxidative Phosphorylation and ETC

• Four protein complexes (I, II, III, IV) and mobile electron carriers (Q & cytochrome c) are involved in the ETC
• The respiratory chain oxidizes reducing equivalents (NADH and FADH2), acting as a proton pump
• The chemiosmotic theory explains ATP synthesis through proton gradient generation across the inner mitochondrial membrane
• Inhibitors can block specific steps in the ETC or oxidative phosphorylation, including rotenone, cyanide, antimycin A, and oligomycin
• The glycerol phosphate shuttle and malate-aspartate shuttle transfer reducing equivalents from the cytoplasm to the mitochondria
• Superoxide dismutase protects the ETC from oxidative damage

Electron Transport Chain

• The ETC (respiratory chain) involves the sequential transfer of electrons from NADH or FADH2 to oxygen
• Complex I (NADH-Q oxidoreductase) receives electrons from NADH and transfers them to Q
• Complex II (succinate-Q reductase) receives electrons from FADH2 and transfers them to Q
• Complex III (Q-cytochrome c oxidoreductase) accepts electrons from Q and passes them to cytochrome c
• Complex IV (cytochrome c oxidase) receives electrons from cytochrome c and transfers them to oxygen, producing water

Oxidative Phosphorylation

• The proton gradient generated by the ETC drives ATP synthesis
• ATP synthase uses the gradient to phosphorylate ADP to ATP
• The ETC and oxidative phosphorylation generate ATP efficiently from glucose oxidation
• FADH2 produces less ATP than NADH due to its entry point in the ETC

Amphibolic Nature of TCA

• The TCA cycle is amphibolic, meaning it plays roles in both energy production and biosynthetic pathways, including gluconeogenesis, amino acid synthesis, and fatty acid synthesis
• Anaplerotic reactions replenish TCA cycle intermediates, ensuring its continuous operation### ATP Synthase
• Inhibits oxidation and phosphorylation by blocking proton flow through ATP synthase.

Glycerol 3-phosphate Shuttle

• Transfers reducing equivalents from cytosol to mitochondrial matrix
• Utilizes cytosolic and mitochondrial isoforms of Glycerol-3-P DH.

Malate Shuttle

• Transfers reducing equivalents from cytosol to mitochondrial matrix.
• Utilizes cytosolic and mitochondrial isoforms of Malate DH.
• The same enzyme but different locations.

Glutamate-Oxaloacetate Transaminase

• Also known as Aspartate aminotransferase.
• Transports reducing equivalents from cytosol to mitochondrial matrix.

Brown Adipose Tissue

• Rich in mitochondria to generate heat (thermogenesis).
• 2,4-Dinitrophenol (2,4-DNP) increases uncoupling in brown adipose tissue.
• Increases mitochondrial permeability and reduces ATP synthesis, leading to increased heat production.

Superoxide Dismutase

• Neutralizes superoxide radicals (O2-)
• Superoxide radicals are reactive oxygen species.

Explore the intricacies of pyruvate oxidation and the TCA cycle in this quiz. Focus on the key processes, enzymes involved, and the importance of specific vitamins in energy metabolism. Test your understanding of metabolic pathways and their regulation.