4.2.5 Krebs Cycle

Introduction

The Krebs cycle is the third phase of aerobic respiration. It is also known as the Citric Acid Cycle or the Tri Carboxylic Acid (TCA) cycle. It occurs in the mitochondrial matrix and completes the oxidation of acetyl-CoA to CO₂, producing NADH, FADH₂, and ATP. This cycle was discovered by British scientist Sir Hans Krebs, for which he won the Nobel Prize in 1953.

Definition

Krebs Cycle: The Krebs cycle is a series of enzymatic reactions that oxidize acetyl-CoA to carbon dioxide, producing reduced coenzymes (NADH and FADH₂) and a small amount of ATP. It is also called the Citric Acid Cycle or Tri Carboxylic Acid (TCA) cycle because the first product is citric acid.

In simple words: The Krebs cycle is a series of reactions that break down acetyl-CoA to release energy in the form of NADH, FADH₂, and ATP.

Core Concept

• This cycle was discovered by British scientist Sir Hans Krebs, therefore, called the Krebs cycle.
• It is also called the Citric acid cycle or Tri carboxylic acid (TCA) cycle because the first product is citric acid.
• It occurs in the mitochondrial matrix.
• Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
• The cycle produces:
• NADH
• FADH₂
• ATP (GTP)
• CO₂
• The cycle turns twice per glucose molecule (because 2 acetyl-CoA are produced per glucose).

Detailed Explanation

Overview of the Krebs Cycle

• Acetyl-CoA (2C)
• 3 NAD⁺
• 1 FAD
• 1 GDP + Pi
• 1 H₂O

• 2 CO₂
• 3 NADH
• 1 FADH₂
• 1 GTP (ATP)
• CoA

• 4 CO₂
• 6 NADH
• 2 FADH₂
• 2 ATP
• CoA

Equation (per turn): Acetyl-CoA + 3NAD⁺ + FAD + GDP + Pi + 2H₂O → 2CO₂ + 3NADH + FADH₂ + GTP + CoA

Steps of the Krebs Cycle

Step 1: Synthesis of Citrate

Reaction: Acetyl CoA (2-carbon compound) and a water molecule combine with oxaloacetate (4-carbon compound) to form a 6-carbon compound called citrate (citric acid).

Enzyme: Citrate synthetase

• Acetyl-CoA (2C) combines with oxaloacetate (4C).
• Water is used.
• Citrate (6C) is formed.
• CoA is liberated.

Equation: Acetyl-CoA (2C) + Oxaloacetate (4C) + H₂O → Citrate (6C) + CoA

Step 2: Dehydration

Reaction: Citrate undergoes reorganization by the removal of a water molecule.

Enzyme: Aconitase

• Citrate loses a water molecule.
• cis-Aconitate is formed.
• cis-Aconitate is an intermediate molecule of isomerization of citrate.

Equation: Citrate (6C) → cis-Aconitate (6C) + H₂O

Step 3: Hydration

Reaction: With the addition of a water molecule, cis-aconitate is converted into isocitrate.

Enzyme: Aconitase

• cis-Aconitate gains a water molecule.
• Isocitrate is formed.
• Citrate and isocitrate are isomers of each other.

Equation: cis-Aconitate (6C) + H₂O → Isocitrate (6C)

Step 4: Oxidative Decarboxylation

Reaction: This is a two-step process, which involves first oxidation/dehydrogenation of isocitrate, followed by decarboxylation to form α-ketoglutarate.

Enzyme: Isocitrate dehydrogenase complex

• Isocitrate is oxidized.
• NAD⁺ is reduced to NADH.
• CO₂ is released.
• α-Ketoglutarate (5C) is formed.

Equation: Isocitrate (6C) + NAD⁺ → α-Ketoglutarate (5C) + CO₂ + NADH + H⁺

Step 5: Oxidative Decarboxylation and Addition of CoA

Reaction: α-Ketoglutarate again undergoes oxidative decarboxylation. The hydrogen and electrons which are released from α-ketoglutarate are taken up by NAD⁺ to form NADH while the carboxyl group is released in the form of CO₂. Then, it combines with coenzyme A to form succinyl CoA.

Enzyme: α-Ketoglutarate dehydrogenase complex

• α-Ketoglutarate is oxidized.
• NAD⁺ is reduced to NADH.
• CO₂ is released.
• Succinyl-CoA (4C) is formed.

Equation: α-Ketoglutarate (5C) + NAD⁺ + CoA → Succinyl-CoA (4C) + CO₂ + NADH + H⁺

Step 6: Synthesis of Substrate Level ATP

Reaction: Coenzyme A is removed from Succinyl CoA by the help of enzyme succinyl thiokinase to form succinate. The reaction releases sufficient energy which is used to combine GDP and Pi forming GTP. GTP reacts with ADP to form ATP while GTP is again converted into GDP. In this way a molecule of ATP is generated in this reaction.

Enzyme: Succinyl thiokinase

• Succinyl-CoA is converted to succinate.
• GDP + Pi → GTP
• GTP + ADP → ATP + GDP
• ATP is produced by substrate-level phosphorylation.

Equation: Succinyl-CoA (4C) + GDP + Pi → Succinate (4C) + GTP + CoA GTP + ADP → ATP + GDP

Step 7: Dehydrogenation/Oxidation

Reaction: Succinate undergoes dehydrogenation/oxidation with the help of enzyme succinate dehydrogenase to form fumarate. The hydrogen and electrons which are released from succinate are taken up by FAD to form FADH₂.

Enzyme: Succinate dehydrogenase

• Succinate is oxidized.
• FAD is reduced to FADH₂.
• Fumarate (4C) is formed.

Equation: Succinate (4C) + FAD → Fumarate (4C) + FADH₂

Step 8: Hydration

Reaction: A molecule of water gets added to fumarate to form malate.

Enzyme: Fumarase

• Fumarate gains a water molecule.
• Malate (4C) is formed.

Equation: Fumarate (4C) + H₂O → Malate (4C)

Step 9: Dehydrogenation/Oxidation

Reaction: Malate undergoes dehydrogenation/oxidation in the presence of enzyme malate dehydrogenase to produce oxaloacetate. The hydrogen and electrons which are released from malate are taken up by NAD⁺ to form NADH. Oxaloacetate picks up another molecule of acetyl CoA to repeat the cycle.

Enzyme: Malate dehydrogenase

• Malate is oxidized.
• NAD⁺ is reduced to NADH.
• Oxaloacetate (4C) is formed.
• Oxaloacetate is regenerated to start the cycle again.

Equation: Malate (4C) + NAD⁺ → Oxaloacetate (4C) + NADH + H⁺

Summary of the Krebs Cycle

Outputs of Krebs Cycle

Energetics of the Krebs Cycle

• 3 NADH → 9 ATP
• 1 FADH₂ → 2 ATP
• 1 GTP → 1 ATP
• Total: 12 ATP

• 6 NADH → 18 ATP
• 2 FADH₂ → 4 ATP
• 2 GTP → 2 ATP
• Total: 24 ATP

Importance of the Krebs Cycle

• Produces NADH and FADH₂, which carry electrons to the ETC.
• Produces ATP (GTP) by substrate-level phosphorylation.

• Oxidizes acetyl-CoA to CO₂.
• Releases energy stored in acetyl-CoA.

• Intermediates are used to synthesize:
• Amino acids
• Fatty acids
• Heme
• Other molecules

• Links carbohydrate, fat, and protein metabolism.
• Intermediates can be used for various metabolic pathways.

5. Amphibolic Nature: - The Krebs cycle is both catabolic (breaks down molecules) and anabolic (provides precursors).

Intermediate Level

Steps of the Krebs Cycle in Detail

Outputs of the Krebs Cycle

Energetics of the Krebs Cycle

• 6 NADH → 18 ATP
• 2 FADH₂ → 4 ATP
• 2 GTP → 2 ATP
• Total: 24 ATP

Importance of the Krebs Cycle

Important Points

• 1. This cycle was discovered by British scientist Sir Hans Krebs, therefore, called Krebs cycle.
• 2. It is also called Citric acid cycle or Tri carboxylic acid (TCA) cycle because the first product is citric acid.
• 3. Steps of Krebs cycle:
• - 1. Synthesis: Acetyl CoA (2-carbon compound) and a water molecule combine with oxaloacetate (4-carbon compound) to form a 6-carbon compound called citrate (citric acid) with the help of enzyme citrate synthetase. It is the first product of Krebs cycle. CoA is liberated.
• - 2. Dehydration: Citrate undergoes reorganization by the removal of a water molecule with the help of enzyme aconitase. The resulting compound cis-aconitate is an intermediate molecule of isomerization of citrate.
• - 3. Hydration: With the addition of water molecule cis-aconitate is converted into isocitrate with the help of same enzyme aconitase. Actually, citrate and isocitrate are isomers of each other.
• - 4. Oxidative decarboxylation: This is a two-step process, which involves first oxidation/dehydrogenation of isocitrate, followed by the decarboxylation to form alpha-ketoglutarate. Both interlinked steps are catalyzed by isocitrate dehydrogenase complex. The hydrogen and electrons which are released from isocitrate are taken up by NAD⁺ to form NADH while the carboxyl group is released in the form of CO₂.
• - 5. Oxidative decarboxylation and addition of CoA: α-Ketoglutarate again undergoes oxidative decarboxylation. The hydrogen and electrons which are released from α-ketoglutarate are taken up by NAD⁺ to form NADH while the carboxyl group is released in the form of CO₂. Then, it combines with coenzyme A to form succinyl CoA. This reaction is controlled by enzyme complex α-ketoglutarate dehydrogenase.
• - 6. Synthesis of substrate level ATP: Coenzyme A is removed from Succinyl CoA by the help of enzyme succinyl thiokinase to form succinate. The reaction releases sufficient energy which is used to combine GDP and Pi forming GTP. GTP reacts with ADP to form ATP while GTP is again converted into GDP. In this way a molecule of ATP is generated in this reaction.
• - 7. Dehydrogenation/oxidation: Succinate undergoes dehydrogenation/oxidation with the help of enzyme succinate dehydrogenase to form fumarate. The hydrogen and electrons which are released from succinate are taken up by FAD to form FADH₂.
• - 8. Hydration: A molecule of water gets added to fumarate to form malate with the help of enzyme fumarase.
• - 9. Dehydrogenation/oxidation: Malate undergoes dehydrogenation/oxidation in the presence of enzyme malate dehydrogenase to produce oxaloacetate. The hydrogen and electrons which are released from malate are taken up by NAD⁺ to form NADH. Oxaloacetate picks up another molecule of acetyl CoA to repeat the cycle.
• 4. Outputs of Krebs Cycle:
• - Per Acetyl-CoA (one turn): 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP (GTP)
• - Per Glucose (two turns): 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP

Remember

• - Krebs cycle = citric acid cycle = TCA cycle
• - Location = mitochondrial matrix
• - First product = citric acid (6C)
• - Acetyl-CoA = 2C; Oxaloacetate = 4C; Citrate = 6C
• - Steps: 9 enzymatic reactions
• - Outputs per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP
• - Outputs per glucose: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP
• - ATP yield per glucose: 24 ATP (from NADH, FADH₂, and ATP)
• - Amphibolic = both catabolic and anabolic

Common Mistakes

• 1. Confusing the Krebs cycle with the Calvin cycle – Krebs cycle = respiration; Calvin cycle = photosynthesis.
• 2. Thinking the Krebs cycle occurs in the cytoplasm – The Krebs cycle occurs in the mitochondrial matrix.
• 3. Thinking the Krebs cycle produces glucose – The Krebs cycle breaks down acetyl-CoA to CO₂.
• 4. Confusing the number of ATP produced – Per glucose, the Krebs cycle produces 2 ATP directly (via GTP).
• 5. Thinking the Krebs cycle produces only ATP – It also produces NADH, FADH₂, and CO₂.
• 6. Forgetting that the cycle turns twice per glucose – The cycle turns twice per glucose because 2 acetyl-CoA are produced.
• 7. Confusing the enzymes – Each step has a specific enzyme.
• 8. Thinking the Krebs cycle is reversible – Some steps are irreversible (citrate synthetase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase).
• 9. Forgetting the role of oxaloacetate – Oxaloacetate is regenerated at the end of the cycle.
• 10. Thinking the Krebs cycle only produces energy – It also provides precursors for biosynthesis.
• 11. Confusing substrate-level phosphorylation with oxidative phosphorylation – Substrate-level = direct transfer of phosphate; Oxidative = via ETC.
• 12. Thinking the Krebs cycle occurs in prokaryotes – Prokaryotes do not have mitochondria; the Krebs cycle occurs in the cytoplasm.

Quick Revision

• Synthesis: Acetyl CoA (2-carbon compound) and a water molecule combine with oxaloacetate (4-carbon compound) to form a 6-carbon compound called citrate (citric acid) with the help of enzyme citrate synthetase. It is the first product of Krebs cycle. CoA is liberated.
• Dehydration: Citrate undergoes reorganization by the removal of a water molecule with the help of enzyme aconitase. The resulting compound cis-aconitate is an intermediate molecule of isomerization of citrate.
• Hydration: With the addition of water molecule cis-aconitate is converted into isocitrate with the help of same enzyme aconitase. Actually, citrate and isocitrate are isomers of each other.
• Oxidative decarboxylation: This is a two-step process, which involves first oxidation/dehydrogenation of isocitrate, followed by the decarboxylation to form alpha-ketoglutarate. Both interlinked steps are catalyzed by isocitrate dehydrogenase complex. The hydrogen and electrons which are released from isocitrate are taken up by NAD⁺ to form NADH while the carboxyl group is released in the form of CO₂.
• Oxidative decarboxylation and addition of CoA: α-Ketoglutarate again undergoes oxidative decarboxylation. The hydrogen and electrons which are released from α-ketoglutarate are taken up by NAD⁺ to form NADH while the carboxyl group is released in the form of CO₂. Then, it combines with coenzyme A to form succinyl CoA. This reaction is controlled by enzyme complex α-ketoglutarate dehydrogenase.
• Synthesis of substrate level ATP: Coenzyme A is removed from Succinyl CoA by the help of enzyme succinyl thiokinase to form succinate. The reaction releases sufficient energy which is used to combine GDP and Pi forming GTP. GTP reacts with ADP to form ATP while GTP is again converted into GDP. In this way a molecule of ATP is generated in this reaction.
• Dehydrogenation/oxidation: Succinate undergoes dehydrogenation/oxidation with the help of enzyme succinate dehydrogenase to form fumarate. The hydrogen and electrons which are released from succinate are taken up by FAD to form FADH₂.
• Hydration: A molecule of water gets added to fumarate to form malate with the help of enzyme fumarase.
• Dehydrogenation/oxidation: Malate undergoes dehydrogenation/oxidation in the presence of enzyme malate dehydrogenase to produce oxaloacetate. The hydrogen and electrons which are released from malate are taken up by NAD⁺ to form NADH. Oxaloacetate picks up another molecule of acetyl CoA to repeat the cycle.

60-Second Revision

• Krebs cycle = citric acid cycle = TCA cycle
• Location = mitochondrial matrix
• First product = citric acid (6C)
• Acetyl-CoA = 2C; Oxaloacetate = 4C; Citrate = 6C
• Steps: 9 enzymatic reactions
• Outputs per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP
• Outputs per glucose: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP
• ATP yield per glucose: 24 ATP (from NADH, FADH₂, and ATP)
• Amphibolic = both catabolic and anabolic
• Warning: The Krebs cycle occurs in the mitochondrial matrix. It turns twice per glucose. It produces NADH, FADH₂, ATP, and CO₂. It is amphibolic. Some steps are irreversible. It provides precursors for biosynthesis. In prokaryotes, it occurs in the cytoplasm.