The Citric Acid Cycle (TCA/Krebs Cycle): Structure, Function, and Regulation

The Citric Acid Cycle (TCA/Krebs Cycle)

Overview and Nomenclature

The Citric Acid Cycle, also known as the Tricarboxylic Acid (TCA) Cycle or Krebs Cycle, is a central metabolic pathway in aerobic organisms. It is the second stage of cellular respiration and is responsible for the majority of cellular CO2 production.

• Respiration involves cellular processes that require oxygen and produce CO2.

• The TCA cycle occurs in the mitochondria and is essential for energy production.

Acetyl-CoA: The Starting Point

Acetyl-CoA is the entry molecule for the TCA cycle, formed from pyruvate produced during glycolysis.

• Pyruvate is oxidized to Acetyl-CoA via the Pyruvate Dehydrogenase (PDH) Complex.

• This reaction is an oxidative decarboxylation and occurs in the mitochondria.

• Standard free energy change: $\Delta G^\circ = -33.4\ \text{kJ/mol}$

Formation of Acetyl-CoA: The PDH Complex

The PDH complex is a multi-enzyme, hetero-oligomer system that catalyzes the conversion of pyruvate to Acetyl-CoA.

• Composed of three main enzymes:

  • Pyruvate Dehydrogenase (E1): Catalyzes decarboxylation of pyruvate. (redox)

  • Dihydrolipoyl Transacetylase (E2): Transfers the acetyl group. (moving aceyl-group)

  • Dihydrolipoyl Dehydrogenase (E3): Regenerates oxidized lipoamide. (redox)

• Requires five coenzymes: TPP, FAD, NAD+, CoA, lipoate.

Coenzymes and Their Functions

• CoA: A reactive thiol attached to a modified ADP, acts as an acyl group carrier.

• Lipoate: Contains two sulfhydryl groups, forms internal disulfide bonds, acts as a redox cofactor and acyl carrier. (short chain)

• TPP, FAD, NAD+: Essential for electron transfer and decarboxylation steps.

• Pyruvate is sequentially modified by the three enzymes in the PDH complex

• E1 decarboxylates pyruvate with aid of TPP

• E2 accepts acetyl and 2 protons from TPP on lipollysmes forming disulfide

• Trans-esterification of CoA sulfhydryl to release the acetyl from the enzyme and form AcetylCoA

• Protons from reduce lipollysine transferred to FAD associated with E3

• The FADH2 on E3 transfers the protons to free NAD+

Substrate Channeling in PDH Complex

Substrate channeling refers to the direct transfer of intermediates between enzyme active sites, increasing efficiency and preventing loss of intermediates.

• Multiple enzymes are physically close, allowing intermediates to be passed directly from one to another.

Oxidation of Acetyl-CoA

• AcetylCoA is the starting material of the TCA cycle

• Lehninger showed that the entire set of reactions in the TCA cycle occur in the mitochondria

• 8 step process

Steps of the Citric Acid Cycle

Summary of Intermediates

• Citrate

• Isocitrate

• α-Ketoglutarate

• Succinyl-CoA

• Succinate

• Fumarate

• Malate

• Oxaloacetate

Stepwise Reactions and Enzymes

1. Condensation of Acetyl-CoA and Oxaloacetate to form Citrate

   • Enzyme: Citrate Synthase

   • $\Delta G^\circ = -32.2\ \text{kJ/mol}$ (favorable)

2. Formation of Isocitrate via cis-Aconitate

   • Enzyme: Aconitase (uses Fe-S center to remove water)

   • Intermediate: cis-Aconitate

   • $\Delta G^\circ = 13.3\ \text{kJ/mol}$ (unfavorable)

3. Oxidation of Isocitrate to α-Ketoglutarate and CO2

   • Enzyme: Isocitrate Dehydrogenase

   • Requires Mn2+

   • Produces NADH and CO2

4. Oxidation of α-Ketoglutarate to Succinyl-CoA and CO2

   • Enzyme: α-Ketoglutarate Dehydrogenase Complex

     • reaction and enzyme complex related to PDH

   • Succinyl CoA has a high energy thioester

   • electrons transferred to NAD

   • Produces NADH and CO2

   • $\Delta G^\circ = -33.5\ \text{kJ/mol}$ (favorable)

5. Conversion of Succinyl-CoA to Succinate

   • Enzyme: Succinyl-CoA Synthetase

   • High-energy thioester bond is broken; energy transferred to GDP + Pi to form GTP

   • Regenerates CoA-SH

   • $\Delta G^\circ = -2.9\ \text{kJ/mol}$(favorable)

6. Oxidation of Succinate to Fumarate

   • Enzyme: Succinate Dehydrogenase

   • Coupled to reduction of FAD to FADH2

   • Located at the inner mitochondrial membrane

   • $\Delta G^\circ = 0\ \text{kJ/mol}$

7. Hydration of Fumarate to L-Malate

   • Enzyme: Fumarase

   • Stereospecific hydration via carbanion intermediate

   • importantly discovered carbanion was inferred of this reaction

   • $\Delta G^\circ = -3.8\ \text{kJ/mol}$ (favorable)

8. Oxidation of Malate to Oxaloacetate

   • Enzyme: L-Malate Dehydrogenase

   • Coupled to reduction of NAD+ to NADH

   • Thermodynamically unfavorable under standard conditions

   • $\Delta G^\circ = 29.7\ \text{kJ/mol}$ (unfavorable)

Net Reaction of the TCA Cycle

• Used: 2 H2O, 1 CoA

• Produced: 1 CoA, 1 H2O (waste), 2 CO2, 2 NADH

• Net: -1 H2O, +2 CO2, +2 NADH

Amphibolic Nature of the TCA Cycle

The TCA cycle is amphibolic, meaning it is involved in both catabolic and anabolic processes.

• Catabolism: Breaks down sugars, fatty acids, and amino acids for energy.

• Anabolism: Provides intermediates for synthesis of nucleic acids, amino acids, and other biomolecules.

• Many intermediates can be modified for biosynthetic pathways.

Regulation of the Citric Acid Cycle

General Principles - Regulation

The TCA cycle is tightly regulated to ensure efficient energy production and metabolic balance.

• Regulation occurs via allosteric inhibition by downstream products (feedback inhibition).

• Enzyme activity is modulated by levels of ATP, NADH, and other metabolites.

• Large number of enzymes and cofactors allows for fine control over TCA cycle progression

• Many enzymes in the cycle are negatively regulated in an allosteric manner by downstream products

Enzyme	Inhibitor	ΔG (kJ/mol)
Pyruvate Dehydrogenase	ATP, Acetyl-CoA, NADH	-33.4 (Favorable)
Citrate Synthase	ATP, NADH, citrate, Succinyl-CoA	-32.2 (Favorable)
Isocitrate Dehydrogenase	ATP	---
α-Ketoglutarate Dehydrogenase	NADH, Succinyl-CoA	-33.5 (Favorable)

Pyruvate Carboxylase: Anaplerotic Reaction

Pyruvate Carboxylase replenishes oxaloacetate for the TCA cycle.

• Carboxylates pyruvate to form oxaloacetate.

• Regulated by acetyl-CoA levels.

• Requires biotin as a cofactor, which is linked to an active site lysine.

• ATP reacts with bicarbonate to form carboxyphosphate, which allows carboxylation of biotin.

• Biotin gives up CO2 to form pyruvate enolate, which reacts with liberated CO2 to form oxaloacetate.

Summary Table: TCA Cycle Intermediates and Enzymes

Step	Intermediate	Enzyme	Key Product
1	Citrate	Citrate Synthase	---
2	Isocitrate	Aconitase	---
3	α-Ketoglutarate	Isocitrate Dehydrogenase	NADH, CO2
4	Succinyl-CoA	α-Ketoglutarate Dehydrogenase	NADH, CO2
5	Succinate	Succinyl-CoA Synthetase	GTP
6	Fumarate	Succinate Dehydrogenase	FADH2
7	Malate	Fumarase	---
8	Oxaloacetate	Malate Dehydrogenase	NADH

Example: Energy Yield from One Turn of the TCA Cycle

• 3 NADH (used in electron transport chain)

• 1 FADH2 (used in electron transport chain)

• 1 GTP (can be converted to ATP)

• 2 CO2 (waste product)

Note: The TCA cycle is a central hub for metabolism, connecting carbohydrate, fat, and protein catabolism and anabolism. Its regulation is crucial for cellular energy homeostasis and biosynthetic needs.