Cellular Respiration: Pyruvate Oxidation and Citric Acid Cycle Notes

Overview of Cellular Respiration

• Cellular respiration involves: 1) Glycolysis, 2) Pyruvate Processing, 3) Citric Acid Cycle, and 4) Electron Transport and Chemiosmosis.

• Pyruvate (two for every glucose) is processed into Acetyl CoA.

• The electron transport chain establishes a proton gradient to produce ATP.

Pyruvate Metabolism to Acetyl CoA

• Pyruvate moves from the cytoplasm to inside the mitochondrion.

• Oxidation and decarboxylation of pyruvate produce CO₂ and Acetyl-CoA, creating one NADH.

• The pyruvate dehydrogenase complex contains multiple copies of 3 enzymes (E1, E2, E3) and uses five co-enzymes.

Pyruvate Dehydrogenase Complex (PDH)

• PDH is a multi-enzyme complex with three enzymes associated together.

  • Advantages: Higher reaction rate, minimum side reactions, and coordinated control.

• Cofactors include TPP, lipoate, FAD, CoA-SH, and NAD+ (four are B vitamins).

Enzyme Functions:

• E₁ (pyruvate dehydrogenase): Oxidative decarboxylation of pyruvate.

• E₂ (dihydrolipoyl transacetylase): Transfer of acetyl group to CoA.

• E₃ (dihydrolipoyl dehydrogenase): Regeneration of oxidized lipoamide.

Reaction Steps

1. E1: Pyruvate dehydrogenase decarboxylates pyruvate, yielding a hydroxyethyl-TPP carbanion

2. E2: Transfers hydroxyethyl group from TPP to lipoamide, forming a thioester linkage and oxidizing C-OH to C=O.

3. E2: COASH transfer removes the 2C unit as Acetyl-S-CoA, leaving reduced lipoamide attached to E2.

4. E3: FAD gets reduced to FADH2, regenerating lipoamide to its oxidized form.

5. E3: Hydrogens from FADH2 are transferred to NAD+ to create NADH and H+.

Products per Pyruvate

• 1 CO₂ (waste), 1 NADH (electron carrier), and 1 Acetyl CoA (used in the next reaction pathway).

• Multiply by two for each glucose.

Regulation

• High $[NAD^+]$ promotes the reaction; low $[NAD^+]$ inhibits it.

Inhibition

• Arsenic inhibits the PDH complex by reacting with lipoamide, preventing it from picking up acetyl group.

Citric Acid Cycle

• Acetyl CoA can come from glucose, fatty acids and amino acids.

• GTP is produced, and 8 electrons leave via 4 reduced electron carriers.

Overview

• 8 steps occur in the mitochondrial matrix.

• Each turn produces 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂.

  • Two turns for every glucose molecule.

Individual Reactions

1. Citrate Synthase: Input of Acetyl CoA (2C) to oxaloacetate (4C) forming Citrate (6C).

   • Enzyme: Citrate Synthase, $\Delta G°' = -32.2$ kJ/mol

2. Aconitase: Isomerization to Isocitrate.

   • Enzyme: Aconitase, $\Delta G°' = +6.3$ kJ/mol

3. Isocitrate Dehydrogenase: Oxidation and decarboxylation; Isocitrate (6C) to a-Ketoglutarate (5C).

   • Enzyme: Isocitrate Dehydrogenase, $\Delta G°' = -20.9$ kJ/mol

4. Alpha-ketoglutarate dehydrogenase: Oxidation and decarboxylation; a-Ketoglutarate (5C) to Succinyl CoA (4C).

   • Enzyme: Alpha ketoglutarate dehydrogenase, $\Delta G°' = -33.5$ kJ/mol

5. Succinyl-CoA synthetase: Removal of CoA; Succinyl CoA (4C) to Succinate (4C).

   • Enzyme: Succinyl-CoA synthetase, $\Delta G°' = -2.9$ kJ/mol

6. Succinate dehydrogenase: Oxidation; Succinate (4C) to Fumarate (4C).

   • Enzyme: Succinate dehydrogenase, $\Delta G°' = 0$ kJ/mol. Embedded in the inner mitochondrial membrane

7. Fumerase: Addition of water; Fumarate (4C) to Malate (4C).

   • Enzyme: Fumerase, $\Delta G°' = -3.8$ kJ/mol

8. Malate dehydrogenase: Oxidation; Malate (4C) to Oxaloacetate (4C).

   • Enzyme: Malate dehydrogenase, $\Delta G°' = +29.7$ kJ/mol

Products and Redox

• For every glucose oxidized to 2 Acetyl-CoA we obtain:
  2 ATP, 10 ATP from 2 NADH, 3 ATP from 2 FADH2, and 2 GTP.

Regulation

• The cycle is mainly impacted by availability of $NAD^+$ and $FAD^+$.

• Regulation occurs via: ATP, acetyl CoA, and NADH inhibit, and ADP and pyruvate activate.

Anaplerotic Reactions

• Many cycle intermediates are also intermediates in other pathways.

• Amino acids can be made from or donate to cycle intermediates.