Krebs Cycle or Citric Acid Cycle: Steps, Products & Significance

Table of Contents
1. Starting Material and Location
2. Sequential Steps of TCA Cycle
3. Net Products Per Acetyl-CoA (One Turn of Cycle)
4. Total Yield from Complete Glucose Oxidation (Till TCA Cycle)
5. Essential Requirements for TCA Cycle Continuity
6. Role of Oxygen (O₂) in Respiration
7. ATP Synthesis and Energy Yield
8. Key Points for NEET
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The Tricarboxylic Acid Cycle (TCA cycle), also called Citric Acid Cycle or Krebs Cycle, is the second major stage of aerobic respiration. It occurs in the mitochondrial matrix. This cycle completely oxidizes acetyl groups derived from pyruvate to CO₂, producing reduced coenzymes (NADH, FADH₂) and ATP. Understanding each step, enzyme, substrate, and product is crucial for NEET preparation.

1. Starting Material and Location

• Substrate: Acetyl-CoA (2-carbon acetyl group attached to Coenzyme A)
• Location: Mitochondrial matrix (inside the inner mitochondrial membrane)
• Source of Acetyl-CoA: Produced during oxidative decarboxylation of pyruvic acid (link reaction)
• Initial Acceptor: Oxaloacetic Acid (OAA) - a 4-carbon compound that must be continuously regenerated

2. Sequential Steps of TCA Cycle

2.1 Step 1: Formation of Citric Acid

• Reaction: Acetyl-CoA (2C) + Oxaloacetic acid (4C) + H₂O → Citric acid (6C) + CoA
• Enzyme: Citrate synthase
• Type: Condensation reaction (joining of two molecules)
• Product: Citric acid (tricarboxylic acid with 6 carbons)
• Key Point: CoA is released and can be reused in link reaction

2.2 Step 2: Isomerization to Isocitrate

• Reaction: Citrate (6C) → Isocitrate (6C)
• Enzyme: Aconitase
• Type: Isomerization (rearrangement of atoms)
• Product: Isocitric acid (6-carbon compound)
• Purpose: Prepares molecule for oxidative decarboxylation

2.3 Step 3: First Decarboxylation

• Reaction: Isocitrate (6C) + NAD⁺ → α-ketoglutaric acid (5C) + CO₂ + NADH + H⁺
• Enzyme: Isocitrate dehydrogenase
• Type: Oxidative decarboxylation (removal of CO₂ with oxidation)
• Products: α-ketoglutaric acid, CO₂, NADH + H⁺
• First CO₂ release: One carbon is lost as carbon dioxide
• First NADH formation: NAD⁺ is reduced to NADH + H⁺

2.4 Step 4: Second Decarboxylation

• Reaction: α-ketoglutarate (5C) + NAD⁺ + CoA → Succinyl-CoA (4C) + CO₂ + NADH + H⁺
• Enzyme: α-ketoglutarate dehydrogenase complex
• Type: Oxidative decarboxylation
• Products: Succinyl-CoA, CO₂, NADH + H⁺
• Second CO₂ release: Another carbon is lost as carbon dioxide
• Second NADH formation: Another NAD⁺ is reduced

2.5 Step 5: Substrate-Level Phosphorylation

• Reaction: Succinyl-CoA (4C) + GDP + Pᵢ → Succinic acid (4C) + GTP + CoA
• Enzyme: Succinyl-CoA synthetase (also called succinate thiokinase)
• Type: Substrate-level phosphorylation (direct ATP synthesis)
• Products: Succinic acid, GTP, CoA
• Energy capture: GTP (Guanosine Triphosphate) is synthesized
• Coupled reaction: GTP + ADP → GDP + ATP (GTP energy is transferred to ATP)
• Important: This is the only step where direct ATP equivalent is produced in TCA cycle

2.6 Step 6: Oxidation to Fumarate

• Reaction: Succinate (4C) + FAD → Fumaric acid (4C) + FADH₂
• Enzyme: Succinate dehydrogenase
• Type: Dehydrogenation (removal of hydrogen)
• Products: Fumaric acid, FADH₂
• Coenzyme: FAD (Flavin Adenine Dinucleotide) is reduced to FADH₂
• Special feature: Only enzyme of TCA cycle located in inner mitochondrial membrane

2.7 Step 7: Hydration to Malate

• Reaction: Fumarate (4C) + H₂O → Malic acid (4C)
• Enzyme: Fumarase
• Type: Hydration (addition of water)
• Product: Malic acid (4-carbon compound)

2.8 Step 8: Regeneration of OAA

• Reaction: Malate (4C) + NAD⁺ → Oxaloacetic acid (4C) + NADH + H⁺
• Enzyme: Malate dehydrogenase
• Type: Dehydrogenation (oxidation)
• Products: Oxaloacetic acid, NADH + H⁺
• Third NADH formation: Third NAD⁺ is reduced in the cycle
• Cycle completion: OAA is regenerated to accept another Acetyl-CoA

3. Net Products Per Acetyl-CoA (One Turn of Cycle)

• CO₂ released: 2 molecules (Step 3 and Step 4)
• NADH + H⁺ produced: 3 molecules (Steps 3, 4, 8)
• FADH₂ produced: 1 molecule (Step 6)
• GTP (ATP equivalent) produced: 1 molecule (Step 5)
• CoA molecules released: 2 (Steps 1 and 5)

3.1 Summary Equation for TCA Cycle

Pyruvic acid + 4NAD⁺ + FAD + H₂O + ADP + Pᵢ → 3CO₂ + 4(NADH + H⁺) + FADH₂ + ATP

• This equation represents complete oxidation starting from pyruvate through TCA cycle
• One glucose produces 2 pyruvate, so TCA cycle runs twice per glucose molecule

4. Total Yield from Complete Glucose Oxidation (Till TCA Cycle)

4.1 From Glycolysis (Cytoplasm)

• ATP: 2 molecules (net gain via substrate-level phosphorylation)
• NADH + H⁺: 2 molecules (during oxidation of glyceraldehyde-3-phosphate)

4.2 From Link Reaction (2 Pyruvate → 2 Acetyl-CoA)

• NADH + H⁺: 2 molecules (one per pyruvate oxidation)
• CO₂: 2 molecules released

4.3 From TCA Cycle (2 Turns for 2 Acetyl-CoA)

• NADH + H⁺: 6 molecules (3 per turn × 2 turns)
• FADH₂: 2 molecules (1 per turn × 2 turns)
• GTP/ATP: 2 molecules (1 per turn × 2 turns)
• CO₂: 4 molecules released (2 per turn × 2 turns)

4.4 Cumulative Products (Before Electron Transport Chain)

• Total ATP by substrate-level phosphorylation: 4 molecules (2 from glycolysis + 2 from TCA)
• Total NADH + H⁺: 10 molecules (2 glycolysis + 2 link + 6 TCA)
• Total FADH₂: 2 molecules (from TCA cycle only)
• Total CO₂ released: 6 molecules (2 link + 4 TCA)

5. Essential Requirements for TCA Cycle Continuity

5.1 Continuous Replenishment of OAA

• OAA is regenerated: At the end of each cycle (Step 8)
• Necessity: Without OAA, Acetyl-CoA cannot enter the cycle
• Cycle nature: OAA acts as a catalyst - used and regenerated repeatedly

5.2 Regeneration of Oxidized Coenzymes

• NAD⁺ regeneration: NADH + H⁺ must be oxidized back to NAD⁺
• FAD regeneration: FADH₂ must be oxidized back to FAD
• Where regeneration occurs: Electron Transport Chain (ETC) in inner mitochondrial membrane
• Why necessary: Without oxidized coenzymes (NAD⁺, FAD), dehydrogenase enzymes cannot function
• Consequence of no regeneration: TCA cycle stops due to unavailability of electron acceptors

6. Role of Oxygen (O₂) in Respiration

• Trap Alert: O₂ is NOT directly involved in TCA cycle reactions
• Indirect role: O₂ acts as final electron acceptor in Electron Transport Chain
• Connection: ETC oxidizes NADH and FADH₂ back to NAD⁺ and FAD
• Why called aerobic: Without O₂, ETC stops → coenzymes remain reduced → TCA cycle cannot continue
• O₂ consumption site: Inner mitochondrial membrane (Complex IV of ETC)

7. ATP Synthesis and Energy Yield

7.1 Limited ATP from TCA Cycle Directly

• Direct ATP: Only 2 ATP per glucose (via GTP in substrate-level phosphorylation)
• Why so low: Most energy is stored in reduced coenzymes (NADH, FADH₂)
• Actual energy carriers: 10 NADH + 2 FADH₂ hold majority of released energy

7.2 Role of NADH and FADH₂

• Function: Carry high-energy electrons to Electron Transport Chain
• Energy conversion: Electrons from NADH and FADH₂ drive ATP synthesis in ETC
• NADH yield: Each NADH produces approximately 3 ATP (via oxidative phosphorylation)
• FADH₂ yield: Each FADH₂ produces approximately 2 ATP (via oxidative phosphorylation)
• Maximum theoretical ATP per glucose: 38 ATP (including glycolysis, link, TCA, and ETC)

7.3 Why Respiration Discussion Matters

• Energy transformation: Chemical energy in glucose is gradually released in controlled steps
• Prevents heat loss: Stepwise oxidation captures energy efficiently as ATP
• Complete oxidation: Glucose (C₆H₁₂O₆) is fully converted to 6CO₂ + 6H₂O
• Next phase: Oxidative phosphorylation in ETC produces bulk of ATP from NADH and FADH₂

8. Key Points for NEET

• Amphibolic pathway: TCA cycle is both catabolic (breaks down Acetyl-CoA) and anabolic (provides intermediates for biosynthesis)
• Cycle intermediates: α-ketoglutarate, OAA, Succinyl-CoA serve as precursors for amino acid synthesis
• Decarboxylation points: Two CO₂ molecules released at Steps 3 and 4 (Isocitrate → α-ketoglutarate → Succinyl-CoA)
• Dehydrogenation points: Four oxidation steps (Steps 3, 4, 6, 8) produce 3 NADH + 1 FADH₂
• Rate-limiting enzymes: Citrate synthase, Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase regulate cycle speed
• Inhibitors: Malonate competitively inhibits succinate dehydrogenase (Step 6)
• Common mistake: Students often forget that 1 glucose requires 2 turns of TCA cycle (because 2 pyruvate → 2 Acetyl-CoA)

The TCA cycle is central to aerobic respiration and energy metabolism. It completely oxidizes acetyl groups to CO₂ while capturing energy in NADH and FADH₂. These reduced coenzymes then feed into the Electron Transport Chain for massive ATP production via oxidative phosphorylation. Mastering each step, enzyme, substrate, and product count is essential for NEET success in respiration questions.