Krebs Cycle

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Krebs Cycle (Citric Acid Cycle / TCA Cycle) occurs in the mitochondrial matrix and is the central hub of aerobic metabolism. One acetyl-CoA entering produces 3 NADH, 1 FADH₂, and 1 GTP (or ATP). The cycle runs twice per glucose (2 acetyl-CoA). Key enzymes to memorize: citrate synthase (step 1, fastest), isocitrate dehydrogenase (rate-limiting, regulated by ADP/ATP and Ca²⁺), and α-ketoglutarate dehydrogenase (complex similar to pyruvate dehydrogenase).

⚡ Exam tip: Questions often ask which step is rate-limiting (isocitrate dehydrogenase) or which cofactors are needed (TPP, lipoic acid, CoA, FAD, NAD⁺ — remember “TLCONF” for pyruvate dehydrogenase; same for α-ketoglutarate dehydrogenase).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Krebs Cycle — FMGE Study Guide

What it is: The Krebs Cycle (also called Citric Acid Cycle or Tricarboxylic Acid Cycle) is a series of enzyme-catalyzed reactions in the mitochondrial matrix that complete the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. It does NOT directly use oxygen, but it requires aerobic conditions because NAD⁺ and FAD are regenerated in the electron transport chain (which needs O₂).

Why it matters for FMGE: This is one of the most frequently tested topics in FMGE Biochemistry. Questions commonly ask about the sequence of reactions, enzymes involved, ATP yield, regulation, and clinical correlations (e.g., huntington’s disease, thiamine deficiency).

The 8 Steps:

Step	Reaction	Enzyme	Products
1	Acetyl-CoA + Oxaloacetate → Citrate	Citrate synthase	Citrate + CoA
2	Citrate → Isocitrate	Aconitase	Isocitrate
3	Isocitrate → α-Ketoglutarate + CO₂	Isocitrate dehydrogenase	NADH, CO₂
4	α-Ketoglutarate → Succinyl-CoA + CO₂	α-Ketoglutarate dehydrogenase	NADH, CO₂, Succinyl-CoA
5	Succinyl-CoA → Succinate	Succinyl-CoA synthetase	GTP (or ATP), Succinate
6	Succinate → Fumarate	Succinate dehydrogenase	FADH₂, Fumarate
7	Fumarate → Malate	Fumarase	Malate
8	Malate → Oxaloacetate	Malate dehydrogenase	NADH, Oxaloacetate

Energy yield per acetyl-CoA: 3 NADH (→ 7.5 ATP), 1 FADH₂ (→ 1.5 ATP), 1 GTP (→ 1 ATP) = 10 ATP per acetyl-CoA. Per glucose (2 turns): ~30-32 ATP total from Krebs + glycolysis + oxidative phosphorylation.

⚡ Exam tip: Succinate dehydrogenase is the only membrane-bound enzyme of the Krebs cycle — it’s embedded in the inner mitochondrial membrane (Complex II of ETC). This is a high-yield fact!

Regulation:

• Citrate synthase: Inhibited by ATP, NADH, succinyl-CoA; activated by ADP
• Isocitrate dehydrogenase: Rate-limiting step; activated by ADP and Ca²⁺; inhibited by ATP and NADH
• α-Ketoglutarate dehydrogenase: Inhibited by NADH and succinyl-CoA; activated by Ca²⁺

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Krebs Cycle — Comprehensive FMGE Notes

---

📍 Location & Overview

The Krebs Cycle (Citric Acid Cycle / Tricarboxylic Acid Cycle / TCA Cycle) operates in the mitochondrial matrix of eukaryotic cells. It was elucidated by Hans Krebs in 1937, who received the Nobel Prize in Physiology or Medicine in 1953.

The cycle serves three critical purposes:

1. Complete oxidation of acetyl-CoA to CO₂, generating high-energy electrons (NADH, FADH₂)
2. Intermediate biosynthesis — cycle intermediates are siphoned off for biosynthetic pathways (anaplerosis)
3. Energy production — direct substrate-level phosphorylation (GTP)

Key point: The Krebs Cycle does NOT directly require oxygen, but it cannot function anaerobically because NAD⁺ and FAD must be regenerated via the electron transport chain, which requires oxygen as the final electron acceptor. Under anaerobic conditions, the cycle stops.

---

🔬 The 8 Steps (Detailed)

Step 1: Citrate Synthase

• Reaction: Acetyl-CoA + Oxaloacetate + H₂O → Citrate + CoA + H⁺
• Enzyme: Citrate synthase (most rapid/fastest enzyme of the cycle)
• Note: This is a condensation reaction, not an oxidation. CoA is released.
• Regulation: Inhibited by ATP, NADH, citrate, succinyl-CoA; activated by ADP

Step 2: Aconitase

• Reaction: Citrate ⇌ Isocitrate (via cis-aconitate)
• Enzyme: Aconitase (iron-sulfur protein containing [4Fe-4S] cluster)
• Note: Citrate is isomerized to isocitrate (no carbon loss)
• Clinical: Fluoroacetate (rat poison) inhibits aconitase after being converted to fluorocitrate

Step 3: Isocitrate Dehydrogenase (Rate-Limiting Step)

• Reaction: Isocitrate + NAD⁺(or NADP⁺ in some tissues) → α-Ketoglutarate + CO₂ + NADH
• Enzyme: Isocitrate dehydrogenase (decarboxylation)
• Regulation: This is the major rate-limiting step of the cycle
  • Activated by ADP, Ca²⁺, and NAD⁺
  • Inhibited by ATP and NADH
• Note: First CO₂ release; first NADH produced

Step 4: α-Ketoglutarate Dehydrogenase Complex

• Reaction: α-Ketoglutarate + NAD⁺ + CoA → Succinyl-CoA + CO₂ + NADH
• Enzyme: α-Ketoglutarate dehydrogenase complex (similar 5-cofactor complex to pyruvate dehydrogenase)
• Cofactors required (remember “TLCONF”):
  • Thiamine pyrophosphate (TPP) — from vitamin B1
  • Lipoic acid
  • CoA — from pantothenic acid (B5)
  • Oxidized FAD — from riboflavin (B2)
  • NAD⁺ — from niacin (B3)
• Note: Second CO₂ release; second NADH produced
• Regulation: Inhibited by NADH and succinyl-CoA; activated by Ca²⁺
• Clinical correlation: Thiamine (B1) deficiency → impaired α-ketoglutarate dehydrogenase → accumulate α-ketoglutarate → neurological dysfunction (Wernicke-Korsakoff syndrome, beriberi)

Step 5: Succinyl-CoA Synthetase (Substrate-Level Phosphorylation)

• Reaction: Succinyl-CoA + GDP (or ADP) + Pi → Succinate + GTP (or ATP) + CoA
• Enzyme: Succinyl-CoA synthetase
• Note: Only step in Krebs cycle that directly generates high-energy phosphate (substrate-level phosphorylation)
• GTP/ATP: In most tissues, uses GDP + Pi → GTP. In brain, uses ADP → ATP.

Step 6: Succinate Dehydrogenase (Membrane-Bound!)

• Reaction: Succinate → Fumarate + FADH₂
• Enzyme: Succinate dehydrogenase (embedded in inner mitochondrial membrane as Complex II of ETC)
• Note: ONLY membrane-bound enzyme of the Krebs cycle; also part of the electron transport chain
• Clinical correlation: Malonate (competitive inhibitor) blocks this step → succinate accumulates
• Inhibitors: Malonate, oxaloacetate, meso-tartrate

Step 7: Fumarase

• Reaction: Fumarate + H₂O → Malate
• Enzyme: Fumarase (fumarate hydratase)
• Note: Addition of water across the double bond

Step 8: Malate Dehydrogenase

• Reaction: Malate + NAD⁺ → Oxaloacetate + NADH + H⁺
• Enzyme: Malate dehydrogenase
• Note: Highly unfavorable equilibrium (Keq ~ 0.03); driven forward by rapid consumption of oxaloacetate by citrate synthase
• Clinical correlation: Fumarase deficiency → accumulation of fumarate → developmental delay, seizures

---

⚡ Energy Yield

Per acetyl-CoA:

• 3 NADH × 2.5 ATP/NADH = 7.5 ATP
• 1 FADH₂ × 1.5 ATP/FADH₂ = 1.5 ATP
• 1 GTP = 1 ATP
• Total: ~10 ATP per acetyl-CoA

Per glucose (2 acetyl-CoA from 2 pyruvate):

• Glycolysis: 2 ATP + 2 NADH (cytosolic, worth ~3-5 ATP depending on shuttle)
• Pyruvate dehydrogenase: 2 NADH
• Krebs Cycle (×2): 6 NADH + 2 FADH₂ + 2 GTP
• Total: ~30-32 ATP per glucose

⚡ Exam tip: The old calculation (before the modern P/O ratios were established) gave 36-38 ATP/glucose. Current estimates are 30-32 ATP. FMGE questions may use either depending on the source — check which system your reference uses.

---

🔄 Anaplerotic Reactions (“Filling Up” the Cycle)

Anaplerosis = reactions that replenish/intermediate cycle intermediates that have been drawn off for biosynthesis.

Major anaplerotic reactions:

1. Pyruvate carboxylase (acetyl-CoA activates): Pyruvate + CO₂ + ATP → Oxaloacetate
2. Glutamate dehydrogenase: Glutamate + NAD⁺ → α-Ketoglutarate + NH₃ + NADH
3. Branched-chain amino acid transaminases: Various amino acids → various Krebs intermediates

Clinical correlation: Biotin deficiency → impaired pyruvate carboxylase → reduced oxaloacetate → reduced gluconeogenesis → hypoglycemia and neurological symptoms.

---

🧬 Regulation of the Krebs Cycle

The Krebs cycle is regulated at three key steps:

Enzyme	Regulatory Molecules
Citrate synthase	Inhibitors: ATP, NADH, citrate, succinyl-CoA; Activator: ADP
Isocitrate dehydrogenase	Inhibitors: ATP, NADH; Activators: ADP, Ca²⁺, NAD⁺
α-Ketoglutarate dehydrogenase	Inhibitors: NADH, succinyl-CoA; Activators: Ca²⁺

Calcium (Ca²⁺) is a key regulator — during muscle contraction, Ca²⁺ released into the matrix activates both isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, increasing energy production.

Energy charge concept: High ATP/ADP ratio = cycle slows down. Low energy charge = cycle speeds up. This is classic feedback regulation.

---

🏥 Clinical Correlations for FMGE

1. Thiamine (B1) deficiency → α-ketoglutarate dehydrogenase and pyruvate dehydrogenase both require TPP → accumulate α-ketoglutarate and pyruvate → Wernicke-Korsakoff syndrome, lactic acidosis, peripheral neuropathy

2. Huntington’s Disease — mutant huntingtin protein inhibits pyruvate dehydrogenase and α-ketoglutarate dehydrogenase → impaired energy metabolism → neuronal death in striatum

3. Fluoroacetate poisoning — fluoroacetate is converted to fluorocitrate, which inhibits aconitase → citrate accumulates → toxicity (used as rat poison)

4. Fumarase deficiency — rare metabolic disorder; accumulation of fumarate; symptoms: developmental delay, hypotonia, seizures, facial dysmorphism

5. Succinate dehydrogenase deficiency — leads to mitochondrial Complex II deficiency; can cause neurodegeneration, cardiomyopathy

6. Cancer metabolism (Warburg effect) — cancer cells favor glycolysis even in presence of oxygen; this may involve altered Krebs cycle function

7. Isocitrate dehydrogenase mutations — found in gliomas and AML; mutant enzyme produces 2-hydroxyglutarate (oncometabolite) instead of α-ketoglutarate

---

🎯 High-Yield Facts for FMGE

1. Location: Mitochondrial matrix
2. Rate-limiting enzyme: Isocitrate dehydrogenase
3. Only membrane-bound enzyme: Succinate dehydrogenase (Complex II of ETC)
4. Only substrate-level phosphorylation step: Succinyl-CoA synthetase (step 5)
5. Two CO₂ release steps: Steps 3 and 4
6. Co-factors for α-ketoglutarate dehydrogenase: TPP, lipoic acid, CoA, FAD, NAD⁺ (“TLCONF” or remember same as pyruvate dehydrogenase)
7. Anaplerotic reaction enzyme: Pyruvate carboxylase (biotin-dependent)
8. GTP produced: Step 5 (succinyl-CoA synthetase)
9. Key alloster activators: Ca²⁺, ADP; Inhibitors: ATP, NADH
10. Enzyme requiring B1 (thiamine): α-Ketoglutarate dehydrogenase complex

⚡ Exam tip: If asked “which step requires biotin?” — the answer is NOT in the Krebs cycle directly; it’s pyruvate carboxylase (anaplerotic reaction) that requires biotin. Don’t get confused!

---

⚠️ Common Pitfalls

• Confusing glycolysis regulation with Krebs regulation: Isocitrate dehydrogenase is the rate-limiting enzyme for Krebs cycle, NOT phosphofructokinase (that’s glycolysis)
• Forgetting membrane-bound nature of succinate dehydrogenase: It’s the only membrane-bound enzyme AND part of ETC Complex II
• Not knowing co-factors: α-Ketoglutarate dehydrogenase needs 5 cofactors — questions about B1/B2/B3/B5 deficiency tie directly to this
• Miscounting NADH per glucose: 2 from glycolysis (if malate-aspartate shuttle), 2 from pyruvate dehydrogenase, 6 from Krebs (3 per acetyl-CoA × 2) = 10 total NADH per glucose
• Not remembering that acetyl-CoA cannot be converted back to glucose (fatty acids cannot make glucose via acetyl-CoA)

---

📝 Quick Reference Table

Feature	Details
Location	Mitochondrial matrix
Turns per glucose	2 (one per acetyl-CoA)
Rate-limiting enzyme	Isocitrate dehydrogenase
Steps producing CO₂	Steps 3 and 4
Steps producing NADH	Steps 3, 4, and 8
Step producing FADH₂	Step 6 (succinate dehydrogenase)
Step producing GTP	Step 5
Membrane-bound enzyme	Succinate dehydrogenase
Anaplerotic enzyme	Pyruvate carboxylase
Key alloster regulator	Ca²⁺ (activator), ATP/NADH (inhibitors)

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.