Tricarboxylic Acid Cycle (TCA Cycle) and Pyruvate Dehydrogenase Complex: Structure, Function, and Regulation

Tricarboxylic Acid Cycle (TCA Cycle) Overview

Introduction to the TCA Cycle

The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, is a central metabolic pathway that plays several critical roles in cellular metabolism. It is the final common pathway for the oxidative catabolism of carbohydrates, amino acids, and fatty acids, with their carbon skeletons being converted to carbon dioxide (CO2).

• Location: Occurs in the mitochondria of eukaryotic cells.

• Function: Provides energy for ATP production and supplies intermediates for various anabolic reactions.

• Aerobic Pathway: Requires oxygen as the final electron acceptor.

Reactions of the TCA Cycle

General Features

In the TCA cycle, oxaloacetate (OAA) condenses with an acetyl group from acetyl coenzyme A (acetyl CoA) to form citrate, which is then regenerated as the cycle completes.

• Acetyl groups enter the cycle as acetyl CoA and are oxidized to two molecules of CO2.

• One acetyl CoA is metabolized per cycle turn; the cycle does not result in net production or consumption of intermediates.

Acetyl CoA Production

Pyruvate Dehydrogenase Complex (PDHC)

The major source of acetyl CoA for the TCA cycle is the oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex (PDHC), a multienzyme complex.

• Pyruvate is transported from the cytosol into the mitochondrial matrix via a specific carrier protein.

• Within the matrix, PDHC converts pyruvate to acetyl CoA.

Structure and Components of PDHC

• PDHC is a large protein aggregate composed of three enzymes:

  • E1: Pyruvate dehydrogenase (pyruvate decarboxylase)

  • E2: Dihydrolipoyl transacetylase

  • E3: Dihydrolipoyl dehydrogenase

• Contains two regulatory enzymes: pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase.

• Requires five coenzymes:

  • Thiamine pyrophosphate (TPP) (for E1)

  • Lipoic acid and CoA (for E2)

  • FAD and NAD+ (for E3)

Regulation of PDHC

Mechanisms of Regulation

• PDH kinase phosphorylates and inactivates E1.

• PDH phosphatase dephosphorylates and activates E1.

• PDH kinase is allosterically activated by ATP, acetyl CoA, and NADH, and inhibited by pyruvate.

• PDH phosphatase is activated by Ca2+, especially important in muscle during contraction.

Arsenic Poisoning

Mechanism and Effects

• Arsenate can interfere with ATP production at the glyceraldehyde 3-phosphate step of glycolysis.

• Arsenic inhibits enzymes that require lipoic acid as a coenzyme, including PDH, α-ketoglutarate dehydrogenase, and branched-chain α-keto acid dehydrogenase.

• Arsenite (a trivalent form of arsenic) forms a stable complex with the thiol groups of lipoic acid, preventing its function as a coenzyme.

• Inhibition of lipoic acid in PDHC leads to accumulation of pyruvate and lactate, especially affecting the brain and causing neurologic disturbances and death.

Detailed Steps of the TCA Cycle

1. Citrate Synthesis

• Enzyme: Citrate synthase

• Reaction: Acetyl CoA + Oxaloacetate → Citrate

• Highly exergonic; inhibited by citrate (product inhibition).

2. Isomerization of Citrate

• Enzyme: Aconitase (an iron-sulfur protein)

• Reaction: Citrate ⇌ Isocitrate (via cis-aconitate intermediate)

3. Oxidative Decarboxylation of Isocitrate

• Enzyme: Isocitrate dehydrogenase

• Reaction: Isocitrate + NAD+ → α-Ketoglutarate + CO2 + NADH

• First release of CO2; rate-limiting step; activated by ADP and Ca2+, inhibited by ATP and NADH.

4. Oxidative Decarboxylation of α-Ketoglutarate

• Enzyme: α-Ketoglutarate dehydrogenase complex

• Reaction: α-Ketoglutarate + NAD+ + CoA → Succinyl CoA + CO2 + NADH

• Requires TPP, lipoic acid, FAD, NAD+, and CoA; inhibited by NADH and succinyl CoA, activated by Ca2+.

5. Cleavage of Succinyl CoA

• Enzyme: Succinyl CoA synthetase (succinate thiokinase)

• Reaction: Succinyl CoA + GDP + Pi → Succinate + CoA + GTP

• Example of substrate-level phosphorylation.

• GTP and ATP are interconvertible via nucleoside diphosphate kinase:

$\text{GTP} + \text{ADP} \rightarrow \text{GDP} + \text{ATP}$

6. Oxidation of Succinate

• Enzyme: Succinate dehydrogenase (Complex II of the electron transport chain)

• Reaction: Succinate + FAD → Fumarate + FADH2

• Embedded in the inner mitochondrial membrane.

7. Hydration of Fumarate

• Enzyme: Fumarase (fumarate hydratase)

• Reaction: Fumarate + H2O → Malate

8. Oxidation of Malate

• Enzyme: Malate dehydrogenase

• Reaction: Malate + NAD+ → Oxaloacetate + NADH

• Regenerates oxaloacetate to continue the cycle.

Energy Yield of the TCA Cycle

ATP Production per Acetyl CoA

For each acetyl CoA oxidized in one turn of the TCA cycle:

Reaction	Product	ATP Yield (via ETC)
Isocitrate dehydrogenase	1 NADH	3
α-Ketoglutarate dehydrogenase	1 NADH	3
Malate dehydrogenase	1 NADH	3
Succinate dehydrogenase	1 FADH2	2
Substrate-level phosphorylation	1 GTP (→ 1 ATP)	1
Total		12

Additional info: The oxidation of pyruvate to acetyl CoA by PDHC also produces 1 NADH (3 ATP), so the total ATP yield from complete oxidation of one pyruvate is 15 ATP.

Regulation of the TCA Cycle

Key Regulatory Enzymes

• Regulation occurs primarily at enzymes catalyzing reactions with highly negative ΔG:

• Citrate synthase

• Isocitrate dehydrogenase

• α-Ketoglutarate dehydrogenase

These enzymes are regulated by substrate availability, product inhibition, and allosteric effectors (e.g., ATP, NADH, ADP, Ca2+).

Summary Table: Key Features of the TCA Cycle and PDHC

Enzyme/Complex	Key Substrates	Products	Regulation	Coenzymes
Pyruvate Dehydrogenase Complex	Pyruvate, NAD+, CoA	Acetyl CoA, NADH, CO2	Inhibited by ATP, NADH, Acetyl CoA; Activated by ADP, Ca2+	TPP, Lipoic acid, FAD, NAD+, CoA
Citrate Synthase	Acetyl CoA, Oxaloacetate	Citrate	Inhibited by citrate	—
Isocitrate Dehydrogenase	Isocitrate, NAD+	α-Ketoglutarate, NADH, CO2	Inhibited by ATP, NADH; Activated by ADP, Ca2+	NAD+
α-Ketoglutarate Dehydrogenase	α-Ketoglutarate, NAD+, CoA	Succinyl CoA, NADH, CO2	Inhibited by NADH, Succinyl CoA; Activated by Ca2+	TPP, Lipoic acid, FAD, NAD+, CoA

Clinical Relevance

• PDHC deficiency is the most common biochemical cause of congenital lactic acidosis (often X-linked dominant).

• Arsenic poisoning inhibits enzymes requiring lipoic acid, leading to neurologic symptoms and potentially death.