The Krebs Cycle: Cellular Respiration Simplified

The Krebs cycle—also known as the citric acid cycle or TCA (tricarboxylic acid) cycle—is the central metabolic hub of aerobic respiration. Occurring in the mitochondrial matrix of eukaryotic cells (and the cytosol of prokaryotes), it oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins to CO₂, while harvesting high-energy electrons for ATP synthesis. Discovered by Sir Hans Krebs in 1937, this eight-step enzymatic pathway not only powers life by fueling the electron transport chain, but also supplies key precursors for biosynthesis. In this detailed yet accessible guide, we’ll simplify each reaction, explain enzyme roles, energy yields, regulation mechanisms, and underline why the Krebs cycle is essential to every living organism.

1. Overview of Aerobic Respiration

Cellular respiration is the process by which cells convert nutrients into usable energy (ATP). It occurs in three main stages:

Glycolysis – Glucose (6C) → 2 pyruvate (3C) + net 2 ATP + 2 NADH (cytosol)
Link (Pyruvate) Reaction – Pyruvate → Acetyl-CoA + CO₂ + NADH (mitochondrial matrix)
Krebs Cycle (Citric Acid Cycle) – Acetyl-CoA oxidation to 2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP/ATP
Electron Transport Chain (ETC) & Oxidative Phosphorylation – NADH/FADH₂ electrons drive proton pumping → ATP synthase → ~26–28 ATP

Overall yield per glucose under ideal conditions: ~30–32 ATP plus H₂O and CO₂. The Krebs cycle lies at the heart of this process, bridging carbohydrate breakdown to oxidative phosphorylation.

2. From Glycolysis to Acetyl-CoA

Glycolysis generates two pyruvate molecules per glucose. Before entering the Krebs cycle, each pyruvate undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex (PDC). This multienzyme assembly:

Decarboxylates pyruvate, releasing CO₂
Oxidizes the remaining two-carbon fragment, transferring electrons to NAD⁺ → NADH
Links the two-carbon acetyl unit to coenzyme A → acetyl-CoA

For each pyruvate: 1 CO₂ + 1 NADH + 1 acetyl-CoA. Thus per glucose: 2 CO₂ + 2 NADH + 2 acetyl-CoA feed into the Krebs cycle.

3. The Eight Steps of the Krebs Cycle

The Krebs cycle consists of eight enzyme-catalyzed reactions. Each turn of the cycle handles one acetyl-CoA and yields key high-energy molecules.

Step 1: Citrate Synthesis

Enzyme: Citrate synthase
Acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C) + CoA-SH
This irreversible condensation “primes” the cycle.

Step 2: Citrate Isomerization

Enzyme: Aconitase
Citrate ↔ cis-aconitate ↔ isocitrate (both 6C)
A two-stage dehydration–rehydration reorients the hydroxyl group for subsequent oxidation.

Step 3: First Oxidative Decarboxylation

Enzyme: Isocitrate dehydrogenase
Isocitrate → α-ketoglutarate (5C) + CO₂ + NADH
This rate-limiting step also consumes NAD⁺.

Step 4: Second Oxidative Decarboxylation

Enzyme: α-Ketoglutarate dehydrogenase complex
α-Ketoglutarate + CoA-SH + NAD⁺ → succinyl-CoA (4C) + CO₂ + NADH
Mechanistically similar to pyruvate dehydrogenase.

Step 5: Substrate-Level Phosphorylation

Enzyme: Succinyl-CoA synthetase
Succinyl-CoA + GDP (or ADP) + Pi ↔ succinate + GTP (or ATP) + CoA-SH
One high-energy phosphate is captured directly.

Step 6: Succinate Oxidation

Enzyme: Succinate dehydrogenase (Complex II of ETC)
Succinate → fumarate + FADH₂
FAD (tightly bound) accepts two electrons, feeding them directly into the electron transport chain.

Step 7: Hydration of Fumarate

Enzyme: Fumarase
Fumarate + H₂O → malate
A simple addition of water across the double bond.

Step 8: Oxidation to Oxaloacetate

Enzyme: Malate dehydrogenase
Malate + NAD⁺ → oxaloacetate + NADH
Completes the cycle; oxaloacetate is ready to condense with a new acetyl-CoA.

4. Net Energy Yield per Turn

For each acetyl-CoA oxidized, the Krebs cycle produces:

3 NADH → ~7.5 ATP via oxidative phosphorylation
1 FADH₂ → ~1.5 ATP
1 GTP (or ATP) directly → 1 ATP
2 CO₂ expelled as waste

Total ≈10 ATP equivalents per turn. Since one glucose yields two acetyl-CoA, the cycle contributes ≈20 ATP per glucose (in addition to glycolysis and the link reaction).

5. Regulation of the Krebs Cycle

Flux through the cycle is tightly controlled by energy demand and substrate availability:

Allosteric activation of key enzymes by ADP, AMP, or Ca²⁺ (signals of low energy/high workload in muscle cells).
Feedback inhibition by high levels of NADH, ATP, or succinyl-CoA, which signal sufficient energy supply.
Substrate availability – acetyl-CoA and oxaloacetate concentrations determine cycle entry.

For example, isocitrate dehydrogenase is activated by ADP and inhibited by ATP/NADH; α-ketoglutarate dehydrogenase is inhibited by its product succinyl-CoA and by NADH.

6. Anaplerotic and Cataplerotic Reactions

Beyond energy production, the Krebs cycle supplies carbon skeletons for biosynthesis:

Anaplerotic reactions replenish intermediates. Example: pyruvate carboxylase converts pyruvate → oxaloacetate.
Cataplerotic reactions withdraw intermediates for amino acid, nucleotide, and heme synthesis. Example: α-ketoglutarate → glutamate (amino acid precursor).

A dynamic balance between these pathways ensures both energy generation and biosynthetic needs are met.

7. Biological and Clinical Significance

The Krebs cycle is universally conserved—from bacteria to humans—underscoring its pivotal role:

Metabolic diseases: Deficiencies in enzymes (e.g., fumarase deficiency) cause severe neurological symptoms.
Cancer metabolism: Certain tumors exhibit “Warburg effect,” favoring glycolysis; some harbor mutations in isocitrate dehydrogenase leading to oncometabolite production.
Biotechnology: Engineering microorganisms to overproduce cycle intermediates (citric acid, succinic acid) for industrial applications.

8. Variations and Evolutionary Adaptations

Some organisms display modified or reverse TCA cycles:

Archaea and anaerobic bacteria can run a reductive (reverse) TCA cycle to fix CO₂ into biomass.
Plant peroxisomes host portions of the glyoxylate cycle—bypassing decarboxylations to allow net carbohydrate synthesis from fats during seed germination.

These adaptations highlight the plasticity of central metabolism across life forms.

9. Visual Summary of the Cycle

(A simplified diagram is often found in biochemistry texts, illustrating the cyclical flow from acetyl-CoA and oxaloacetate → citrate, → isocitrate, → α-ketoglutarate, → succinyl-CoA, → succinate, → fumarate, → malate, → oxaloacetate again. Key cofactors—NAD⁺, FAD, GDP/ADP—are reduced or phosphorylated along the way, forging the link to ATP production in the mitochondrion’s inner membrane.)

10. Conclusion

The Krebs cycle stands as a biochemical cornerstone: a finely tuned sequence of eight reactions that extracts hydrogen and electrons from nutrient-derived acetyl groups, converts them into CO₂, and captures free energy in NADH, FADH₂, and GTP/ATP. Its integration with glycolysis, the pyruvate dehydrogenase complex, and oxidative phosphorylation ensures efficient energy transduction. Moreover, its intermediates feed anabolic pathways, illustrating metabolism’s unified design. Understanding the Krebs cycle clarifies how cells power life, adapt to varied nutritional states, and carry out essential biosynthetic functions.

Why is it called the Krebs cycle?

It’s named after Sir Hans Krebs, who first elucidated the sequence of reactions in 1937, earning the Nobel Prize in 1953.

How many ATP molecules are produced per glucose from the Krebs cycle?

Each turn yields ~10 ATP equivalents from NADH, FADH₂, and GTP. Since glucose produces two acetyl-CoA, the cycle yields ~20 ATP per glucose.

Where in the cell does the Krebs cycle occur?

In eukaryotes, it takes place in the mitochondrial matrix; in prokaryotes, the cytosol.

How is the Krebs cycle regulated?

Key enzymes—citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase—are allosterically activated by ADP and Ca²⁺, and inhibited by ATP, NADH, and succinyl-CoA.