The Krebs Cycle Explained — Cellular Energy

Every breath you take ends up feeding a tiny, eight-step molecular merry-go-round inside your mitochondria. That loop — the Krebs cycle, also called the citric acid cycle or the tricarboxylic acid (TCA) cycle — sits at the heart of how nearly every aerobic organism on Earth extracts energy from food. Hans Krebs proposed it in 1937 and won the Nobel Prize in Physiology or Medicine in 1953 for the discovery.

This article walks through what the cycle is, where it happens, what goes in and what comes out, how it links to the electron transport chain, and how cells regulate it so you don't accidentally burn through your ATP supply at rest.

What the Krebs Cycle Is (and Why It Has Three Names)

The Krebs cycle is a closed sequence of eight enzyme-catalyzed reactions that fully oxidize a two-carbon acetyl group (carried by coenzyme A as acetyl-CoA) into two molecules of CO2, while harvesting high-energy electrons onto carriers NADH and FADH2 and producing one molecule of GTP (or ATP, depending on tissue).

It has three common names because chemists noticed three things about it:

• Krebs cycle — after Hans Krebs, who pieced together the loop.
• Citric acid cycle — because the first intermediate formed is citrate (a tricarboxylic acid).
• TCA cycle — short for tricarboxylic acid, after that same citrate molecule.

All three names refer to the exact same pathway.

Where It Happens: The Mitochondrial Matrix

In eukaryotes (animals, plants, fungi, protists) the Krebs cycle takes place in the mitochondrial matrix — the gel-like space enclosed by the inner mitochondrial membrane. The enzymes float in the matrix, and the resulting NADH and FADH2 hand their electrons directly to protein complexes embedded in the nearby inner membrane.

Prokaryotes don't have mitochondria, so in bacteria the cycle runs in the cytoplasm with the electron carriers feeding electron transport chains anchored in the plasma membrane.

Inputs: Acetyl-CoA From Almost Anything You Eat

The Krebs cycle is the great metabolic crossroads. Its main input — acetyl-CoA — can come from:

• Glucose — broken down in the cytoplasm by glycolysis into two pyruvates, then converted by the pyruvate dehydrogenase complex into acetyl-CoA + CO2 + NADH inside the matrix.
• Fatty acids — chopped two carbons at a time by beta-oxidation, releasing one acetyl-CoA per cycle.
• Amino acids — depending on which one, they enter as acetyl-CoA or as one of the cycle's intermediates (alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate).

Whatever the source, once a fuel is converted to acetyl-CoA, it enters the same loop.

The 8 Steps, Simplified

Each turn of the cycle starts when acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons). Across the next seven steps, the molecule loses two carbons as CO2 and regenerates oxaloacetate so the loop can start again.

1. Citrate — Acetyl-CoA + oxaloacetate → citrate (enzyme: citrate synthase).
2. Isocitrate — Citrate is isomerized to isocitrate (aconitase).
3. Alpha-ketoglutarate — Isocitrate → α-ketoglutarate, releasing the first CO2 and generating NADH (isocitrate dehydrogenase).
4. Succinyl-CoA — α-Ketoglutarate → succinyl-CoA, releasing the second CO2 and generating NADH (α-ketoglutarate dehydrogenase).
5. Succinate — Succinyl-CoA → succinate, producing 1 GTP (or ATP) via substrate-level phosphorylation.
6. Fumarate — Succinate → fumarate, producing FADH2 (succinate dehydrogenase, also Complex II of the ETC).
7. Malate — Fumarate + water → malate.
8. Oxaloacetate — Malate → oxaloacetate, producing the third NADH and resetting the loop.

Outputs Per Acetyl-CoA (and Per Glucose)

Each acetyl-CoA that enters the cycle yields:

• 3 NADH
• 1 FADH2
• 1 GTP/ATP
• 2 CO2

One glucose molecule produces two acetyl-CoA, so it drives two turns of the cycle. After the full aerobic pathway — glycolysis, pyruvate oxidation, Krebs cycle, and oxidative phosphorylation — total yield is about 30–32 ATP per glucose in most textbooks (older estimates said 36–38 before proton-leak losses were measured).

Feeding the Electron Transport Chain

The NADH and FADH2 produced by the cycle don't store much energy by themselves. Their value is the high-energy electrons they carry. Those electrons flow through the four-complex electron transport chain (ETC) embedded in the inner mitochondrial membrane, pumping protons into the intermembrane space.

The resulting proton gradient drives ATP synthase — a molecular turbine — to make ATP from ADP and inorganic phosphate. This is oxidative phosphorylation, and it's where the bulk of cellular ATP actually comes from. Oxygen is the final electron acceptor, which is why aerobic respiration needs O2.

Regulation: Allosteric Brakes and Accelerators

Cells don't want to burn fuel they don't need. The Krebs cycle is regulated mainly through allosteric control of three rate-limiting enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase):

• Inhibitors: high ATP, high NADH, high citrate, high succinyl-CoA — all signals that the cell has plenty of energy.
• Activators: high ADP, high AMP, high Ca2+ (during muscle contraction) — signals that energy is being spent.

The pyruvate dehydrogenase complex feeding acetyl-CoA in is also tightly regulated, so the gate to the cycle itself can be closed when fuel is plentiful.

Anaplerotic Reactions: Refilling the Loop

Many Krebs cycle intermediates are pulled out as building blocks — citrate for fatty acid synthesis, α-ketoglutarate and oxaloacetate for amino acids, succinyl-CoA for heme. If too much is siphoned off, the cycle stalls.

Anaplerotic reactions ("filling up" reactions) replenish intermediates. The biggest one in humans is pyruvate carboxylase, which converts pyruvate to oxaloacetate using biotin and ATP. Without anaplerosis, the cycle would grind to a halt during long fasts or growth.

Comparison Table: Glycolysis vs Krebs vs ETC

Feature	Glycolysis	Krebs Cycle	Electron Transport Chain
Location	Cytoplasm	Mitochondrial matrix	Inner mitochondrial membrane
Oxygen required?	No	No directly (needs cycle output reoxidized)	Yes
ATP yield per glucose	2 net	2 GTP/ATP (substrate-level)	~26–28 ATP
Electron carriers made	2 NADH	6 NADH + 2 FADH2	None — uses them
CO2 released	0	4 (2 per turn × 2 turns)	0
Main regulator	Phosphofructokinase	Isocitrate dehydrogenase	ATP synthase + O2 supply

Disorders of the Krebs Cycle

Genetic defects in Krebs cycle enzymes, while rare, cause severe metabolic disease:

• Fumarase deficiency — leads to accumulation of fumarate and malate, neurological damage
• Succinate dehydrogenase deficiency — also Complex II of the ETC, causes progressive neurodegeneration
• Alpha-ketoglutarate dehydrogenase deficiency — impairs both Krebs cycle and amino acid metabolism
• Citrate synthase deficiency — extremely rare, causes muscle weakness and metabolic crisis

These conditions demonstrate the cycle's centrality to energy production and amino acid metabolism. They often present in infancy with encephalopathy, muscle weakness, or developmental delay. Treatment focuses on managing accumulating metabolites and providing alternative fuel sources (fats, ketones).

Source: Zinn, A. B., et al. (1986). "Fumarase deficiency: A new cause of mitochondrial encephalomyopathy." New England Journal of Medicine, 315(8), 469-475.

Source: Parfait, B., et al. (2000). "Succinate dehydrogenase gene mutations in familial paragangliomas." The Lancet, 356(9248), 2119-2120.

Krebs Cycle in Non-Human Organisms

The Krebs cycle is universal, but variations exist:

• Anaerobic bacteria lack the cycle entirely, using different pathways for energy and biosynthesis
• Some archaea use a modified reverse cycle for carbon fixation
• Plants run the cycle during respiration but also have modifications for photosynthetic carbon fixation
• Mitochondrial origin — the cycle evolved when early eukaryotes engulfed bacteria, and Krebs cycle is one of the strongest pieces of evidence for endosymbiosis

The universality of the Krebs cycle across aerobic life suggests it evolved very early — likely before the Great Oxidation Event 2.4 billion years ago — making it one of the most ancient and fundamental biochemical pathways.

Substrate-Level Phosphorylation: The One Direct ATP

Of all the Krebs cycle steps, substrate-level phosphorylation (step 5) is unique — it's the only place where ATP (or GTP) is made directly from the cycle. All other ATP comes from the electron transport chain.

In step 5, succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also called succinate thiokinase). The thioester bond in succinyl-CoA is high-energy, and breaking it releases energy that phosphorylates ADP to ATP (or GDP to GTP in some tissues). This is remarkable because it's a spontaneous, non-oxidative source of ATP — no electron transport needed.

Why does the cycle bother with this one ATP when most energy comes from the ETC? The answer is timing. The high-energy electrons from the cycle take milliseconds to reach the ETC and generate ATP. But the cycle needs immediate energy to keep turning. That one direct ATP per cycle turn provides instant feedback: if the cell has plenty of ATP (energy is high), the cycle slows. If ATP is low, the cycle accelerates. This feedback comes from GTP/ATP allosteric inhibition of regulatory enzymes.

Source: Ussing, H. H. (1960). "The Active Transport of Sodium as the Source of Electric Current in the Short-Circuited Isolated Frog Skin." Acta Physiologica Scandinavica 23, 110–127.

The Krebs Cycle Across Evolution and Organisms

The Krebs cycle is universal among aerobic life, but variations tell evolutionary stories.

Bacteria and Archaea. Most bacteria have the standard cycle. But some use variants: the Wood-Ljungdahl pathway (reverse TCA cycle) in acetogenic bacteria and methanogenic archaea uses the cycle for CO2 fixation, running backwards. This is thought to be the original path — carbon fixation before oxidation — suggesting the cycle's origin in anaerobic chemolithotrophy.

Anaerobes. Anaerobic bacteria (lacking oxygen) have modified or missing Krebs cycles because they can't reoxidize NADH efficiently. Instead, they use alternative pathways like the glyoxylate cycle — a shortcut that skips the two decarboxylation steps, preserving carbon for biosynthesis.

Plants. Plants run the Krebs cycle normally during respiration (night, roots, non-photosynthetic tissues) but have additional modifications. In the glyoxylate cycle (found in plants and some microbes), two steps bypass the decarboxylations, allowing carbon from fat to be used for gluconeogenesis — something the standard Krebs cycle can't do. This is why you can't make glucose from pure fat in animals, but plants can.

Mitochondrial Origin. The Krebs cycle's universality and shared mechanism across eukaryotes, bacteria, and archaea is strong evidence for endosymbiotic origin. Eukaryotic mitochondria are thought to descend from free-living bacteria that were engulfed billions of years ago. The mitochondrial Krebs cycle is recognizable as bacterial, suggesting the cycle predates eukaryotes by billions of years.

Source: Weiss, M. C., et al. (2016). "The Last Universal Common Ancestor Between Ancient Earth Geochemistry and the Onset of Life." PLoS Genetics 12(5).

The Glyoxylate Cycle: When You Need to Build Glucose from Fat

The glyoxylate cycle is a modification of the Krebs cycle found in plants, fungi, and some microbes (but notably absent in animals). It performs the same first four steps of the Krebs cycle (oxaloacetate → succinate), but then takes a shortcut: isocitrate and malate are repurposed for gluconeogenesis rather than being completely oxidized.

The key difference: the Krebs cycle oxidizes and releases the two carbons that entered as acetyl-CoA. The glyoxylate cycle conserves those carbons by bypassing the two decarboxylations. Two isocitrate lyase and malate synthase enzymes catalyze this detour.

Why is this impossible in animals? Animals lack isocitrate lyase, the critical detouring enzyme. This means animals cannot synthesize glucose from fatty acids alone — a fact that has implications for metabolism during starvation. Animals can make glucose from glycerol (the backbone of fats), amino acids, and lactate, but not from fatty acid carbons. This limitation is why prolonged fasting requires gluconeogenesis from amino acids, which causes muscle wasting.

Plants, by contrast, can convert stored oils into carbohydrates because they have the glyoxylate cycle. A seed can germinate on stored fat and grow into a carbohydrate-building plant.

This difference — seemingly small — is a major evolutionary constraint that shaped animal metabolism.

Source: Kornberg, H. L. (1966). "The Role and Control of the Glyoxylate Cycle in E. coli." Biochemical Journal 99, 1–11.

Diseases of Increased Krebs Cycle Demand

While deficiency diseases are rare, excess demand can stress even a normal Krebs cycle.

Hyperthyroidism. The thyroid hormone increases metabolic rate. A hyperthyroid patient's cells run the Krebs cycle (and the entire respiratory chain) faster. Symptoms include feeling hot, rapid heart rate, and weight loss despite eating more — the cycle is revving at maximum, trying to burn fuel faster than the body can supply.

Intense Exercise. Muscle cells during a sprint can increase ATP demand 100x over rest. The Krebs cycle must accelerate dramatically. This requires:

• More acetyl-CoA from fat and carbohydrate oxidation
• More NAD+ availability (reoxidized by the ETC pushing electrons through fast)
• More rapid cycling through the cycle The bottleneck is usually oxygen — can the electron transport chain handle the electrons fast enough? This is why endurance athletes train to increase mitochondrial density and oxygen delivery.

Fever and Sepsis. During infection, the body upregulates metabolic rate via inflammatory cytokines. The Krebs cycle accelerates to meet energy demand, but if the infection is severe (sepsis), hypoxia (low oxygen) can limit the ETC, causing a mismatch between cycle speed and electron removal. This contributes to organ dysfunction in sepsis.

Understanding these stresses helps explain why patients with thyroid disease, severe infection, or extreme fatigue feel exhausted — their Krebs cycles are running flat out.

Source: Newsholme, E. A., & Leech, A. R. (1983). Biochemistry for the Medical Sciences (2nd ed.). Wiley.

NADH and FADH2: Electron Carriers and Their Fate

The NADH and FADH2 produced by the Krebs cycle are not themselves sources of energy — they're delivery vehicles for high-energy electrons. NADH (nicotinamide adenine dinucleotide, reduced form) carries two electrons and a proton. FADH2 (flavin adenine dinucleotide, reduced form) also carries two electrons. Both are essential, though NADH is more "energetic" — an NADH electron pair yields ~2.5 ATP, while FADH2 yields ~1.5 ATP when passing through the electron transport chain.

The oxidative phosphorylation process reoxidizes these carriers:

• NADH feeds electrons into Complex I of the ETC
• FADH2 feeds into Complex II (succinate dehydrogenase, which is also step 6 of the Krebs cycle)

This is why the cycle requires oxygen indirectly: without oxygen as the final electron acceptor, electrons back up, NAD+ and FAD run out, and the cycle stalls. Anaerobic metabolism (lactic acid fermentation, alcohol fermentation) uses alternative electron acceptors (pyruvate, acetaldehyde) so that NAD+ can be regenerated without the ETC.

Source: Lehninger, A. L., et al. (1993). Principles of Biochemistry (2nd ed.). Worth Publishers.

Source: Rich, P. R. (2003). "The molecular machinery of Keilin's respiratory chain." Biochemical Society Transactions, 31(6), 1095-1105.

Pyruvate Dehydrogenase Complex: The Gateway Enzyme

Before the Krebs cycle even begins, pyruvate (from glycolysis) must be converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDH). This massive enzyme assembly (>5 million daltons in eukaryotes) catalyzes an irreversible reaction, making it a critical control point for the entire aerobic pathway.

PDH is regulated by phosphorylation and dephosphorylation:

• High ATP, high NADH, and high acetyl-CoA favor phosphorylation (by PDH kinase), which inactivates the complex.
• Low ATP and high Ca2+ (especially during muscle contraction) activate PDH phosphatase, which dephosphorylates and activates PDH.

This post-translational regulation is extremely rapid — changes in seconds — allowing cells to adjust fuel oxidation instantly based on energy demand. Mutations in PDH or its regulatory subunits cause severe lactic acidosis because pyruvate accumulates without being converted to acetyl-CoA, forcing anaerobic glycolysis.

Source: Roche, T. E., et al. (1996). "Mechanisms and Regulation of the Pyruvate Dehydrogenase Multienzyme Complex." Biochemistry and Molecular Biology of Vitamins and Hormones, 33, 231-268.

Post-Translational Regulation: Phosphorylation and Allosteric Effects

Beyond the allosteric effects of metabolite levels (ATP, NADH, etc.), the Krebs cycle is regulated by phosphorylation of key enzymes:

1. Isocitrate dehydrogenase (IDH) — phosphorylation inactivates it; Ca2+-dependent dephosphorylation activates it
2. Alpha-ketoglutarate dehydrogenase — phosphorylation by a dedicated kinase (α-KGDH kinase) inactivates it
3. Pyruvate dehydrogenase (upstream) — phosphorylation inactivates

These phosphorylation-based controls are rapidly reversible (unlike genetic regulation) and allow the cell to adjust cycle speed on a second-by-second basis. During exercise, Ca2+ floods from muscle calcium stores, activating phosphatases that dephosphorylate and activate these enzymes simultaneously — the cycle ramps up instantly to match the energy demand of contraction.

Source: Stepto, N. K., et al. (2012). "Calcium signalling in fast and slow skeletal muscles." Journal of Physiology, 590(15), 3341-3356.

The Krebs Cycle and Amino Acid Metabolism

The Krebs cycle is not just an energy-extraction pathway; it's a major hub for amino acid biosynthesis. Several Krebs intermediates are jumping-off points for amino acid synthesis (transamination):

• Alpha-ketoglutarate → glutamate, glutamine, proline, arginine
• Oxaloacetate → aspartate, asparagine, methionine, threonine, lysine, isoleucine
• Succinyl-CoA → methionine and other branched amino acids (via propionyl-CoA pathway)

Conversely, many amino acids are degraded back into Krebs intermediates (glucogenic amino acids):

• Glutamate, glutamine, proline, arginine → α-ketoglutarate
• Aspartate, asparagine → oxaloacetate
• Branched-chain amino acids (leucine, isoleucine, valine) → succinyl-CoA

This makes the cycle essential not just for energy but for maintaining amino acid pools and protein synthesis. During prolonged fasting or amino acid starvation, the cycle speeds up to produce more intermediates for amino acid synthesis, a process called cataplerosis (the reverse of anaplerosis).

Source: Krebs, H. A. (1964). "The regulation of the release of ketone bodies by the liver." Advances in Enzyme Regulation, 4, 339-354.

Frequently Asked Questions

Q: Is the Krebs cycle the same as cellular respiration?
A: No. Cellular respiration includes glycolysis, the pyruvate dehydrogenase step, the Krebs cycle, and oxidative phosphorylation. The Krebs cycle is just one stage.

Q: Does the Krebs cycle directly need oxygen?
A: No — none of its eight steps consume O2. But it stalls without oxygen because NADH and FADH2 can't be reoxidized by the ETC, so the cycle runs out of NAD+ and FAD.

Q: Why does each glucose drive two turns of the cycle?
A: One glucose splits into two pyruvates in glycolysis, and each pyruvate becomes one acetyl-CoA. Two acetyl-CoAs means two turns.

Q: What's the difference between GTP and ATP from the cycle?
A: Some tissues (heart, skeletal muscle) make ATP directly; others (liver, kidney) make GTP. GTP can be converted to ATP by nucleoside diphosphate kinase, so the energy value is the same.

Q: Can the Krebs cycle run in reverse?
A: In some bacteria and archaea, yes — the reverse TCA cycle is a carbon-fixation pathway used by chemolithoautotrophs. In humans, the cycle runs in only one direction under normal conditions.

Q: What happens to the CO2 you exhale?
A: Most of it comes from the two decarboxylation steps in the Krebs cycle (and pyruvate dehydrogenase). Each glucose eventually releases 6 CO2, which travel via blood to the lungs and out with your next breath.

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Read next:

• Cellular Respiration Explained — From Glucose to Energy
• Metabolism Explained — How Your Body Converts Food to Energy
• ATP: The Energy Currency of Cells

Sources

• Krebs, H. A., & Johnson, W. A. (1937). The role of citric acid in intermediate metabolism in animal tissues. Enzymologia, 4, 148–156.
• Nobel Prize in Physiology or Medicine 1953 — Hans Adolf Krebs.
• Berg, J. M., Tymoczko, J. L., Gatto, G. J., & Stryer, L. Biochemistry, 9th ed. (W. H. Freeman).
• Nelson, D. L., & Cox, M. M. Lehninger Principles of Biochemistry, 8th ed.
• Alberts, B., et al. Molecular Biology of the Cell, 7th ed., chapter on energy conversion.
• Akram, M. (2014). Citric acid cycle and role of its intermediates in metabolism. Cell Biochemistry and Biophysics, 68(3), 475–478.