CITRIC ACID CYCLE: INDIVIDUAL REACTIONS AND ENERGY YIELD

Overview of the Citric Acid Cycle

The citric acid cycle (also known as the tricarboxylic acid cycle or Krebs cycle) is a central metabolic pathway in biochemistry, responsible for the oxidation of acetyl-CoA to CO2 and the generation of high-energy electron carriers. The cycle consists of eight enzyme-catalyzed reactions, beginning with the condensation of acetyl-CoA and oxaloacetate to form citrate.

• Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

• Two carbons are released as CO2 per cycle turn.

• Three NAD+ are reduced to NADH, one FAD to FADH2, and one GTP is produced.

• Oxaloacetate is regenerated, allowing the cycle to continue.

Summary of the Eight Reactions

Step 1: Formation of Citrate

The cycle begins with the condensation of acetyl-CoA and oxaloacetate, catalyzed by citrate synthase. This reaction forms a new carbon–carbon bond, producing citrate and releasing CoA-SH.

• Reaction:

• Enzyme: Citrate synthase

• Type: Condensation

Step 2: Isomerization of Citrate to Isocitrate

Citrate is converted to isocitrate by aconitase, requiring Fe2+ as a cofactor. The reaction proceeds via the intermediate cis-aconitate, involving removal and re-addition of water in a stereospecific manner.

• Reaction: Citrate → Isocitrate

• Enzyme: Aconitase

• Type: Isomerization

• Intermediate: cis-Aconitate

Step 3: First Oxidative Decarboxylation

Isocitrate is oxidized and decarboxylated to α-ketoglutarate, producing CO2 and NADH. The reaction occurs via the intermediate oxalosuccinate.

• Reaction:

• Enzyme: Isocitrate dehydrogenase

• Type: Oxidative decarboxylation

• Electron carrier: NAD+ → NADH

Step 4: Second Oxidative Decarboxylation

α-Ketoglutarate is converted to succinyl-CoA, releasing CO2 and producing NADH. The reaction is catalyzed by the α-ketoglutarate dehydrogenase complex, which requires several cofactors.

• Reaction:

• Enzyme: α-Ketoglutarate dehydrogenase complex

• Cofactors: TPP, FAD, lipoic acid, Mg2+

Step 5: Formation of Succinate and GTP

Succinyl-CoA is converted to succinate, releasing CoA-SH and producing GTP via substrate-level phosphorylation. The energy for GTP formation comes from hydrolysis of the thioester bond in succinyl-CoA.

• Reaction:

• Enzyme: Succinyl-CoA synthetase

• Type: Substrate-level phosphorylation

• GTP ↔ ATP: (nucleoside diphosphate kinase)

Step 6: Formation of Fumarate

Succinate is oxidized to fumarate by succinate dehydrogenase, which is unique as an inner mitochondrial membrane enzyme and a flavoprotein. FAD is reduced to FADH2.

• Reaction:

• Enzyme: Succinate dehydrogenase

• Type: Oxidation

• Location: Inner mitochondrial membrane

• Features: Covalently bound FAD, Fe–S clusters, direct connection to electron transport chain

Step 7: Formation of L-Malate

Fumarate is hydrated to L-malate by fumarase. The reaction is stereospecific, producing only the L-isomer.

• Reaction:

• Enzyme: Fumarase

• Type: Hydration

Step 8: Regeneration of Oxaloacetate

L-malate is oxidized to oxaloacetate by malate dehydrogenase, producing NADH. This step is essential for the cycle's continuity.

• Reaction:

• Enzyme: Malate dehydrogenase

• Type: Oxidation

Product Accounting per Acetyl-CoA

Energy Yield and Electron Carriers

• NADH: Each NADH yields approximately 2.5 ATP via oxidative phosphorylation.

• FADH2: Each FADH2 yields approximately 1.5 ATP.

• GTP: Energetically equivalent to ATP.

Distinction from Pyruvate Oxidation

• Pyruvate → Acetyl-CoA produces 1 NADH and 1 CO2 per pyruvate.

• One turn of the TCA cycle (per acetyl-CoA) produces 3 NADH, 1 FADH2, 1 GTP, and 2 CO2.

• For one glucose (2 acetyl-CoA): 6 NADH, 2 FADH2, 2 GTP, 4 CO2 (cycle only).

Functional Stages of the Cycle

1. Entry: Acetyl-CoA + Oxaloacetate → Citrate

2. Rearrangement: Citrate → Isocitrate

3. Carbon loss and NADH production: Isocitrate → α-Ketoglutarate + CO2 + NADH; α-Ketoglutarate → Succinyl-CoA + CO2 + NADH

4. Direct high-energy phosphate production: Succinyl-CoA → Succinate + GTP

5. Regeneration of oxaloacetate: Succinate → Fumarate + FADH2; Fumarate → Malate; Malate → Oxaloacetate + NADH

Enzyme Map of the Cycle

• Citrate synthase: Condensation

• Aconitase: Isomerization

• Isocitrate dehydrogenase: Oxidative decarboxylation

• α-Ketoglutarate dehydrogenase: Oxidative decarboxylation

• Succinyl-CoA synthetase: Substrate-level phosphorylation

• Succinate dehydrogenase: Oxidation

• Fumarase: Hydration

• Malate dehydrogenase: Oxidation

The Big Picture

The citric acid cycle is essential for capturing energy from acetyl-CoA oxidation. Most energy is stored in NADH and FADH2, which fuel ATP production via the electron transport chain. GTP is produced directly, and oxaloacetate is regenerated to sustain the cycle.

Summary Table: Citric Acid Cycle Products

Additional info: The notes have been expanded to include definitions, enzyme functions, and energy accounting for clarity and completeness.