The Krebs Cycle: How Cells Extract Energy from Food

Hans Krebs was working at the University of Sheffield in 1937 when he published a paper proposing a cyclic series of reactions by which cells burn pyruvate from glucose to carbon dioxide and water. The journal Nature initially rejected the paper as too long — he published it elsewhere, and it earned him the Nobel Prize in Physiology or Medicine in 1953. The cycle Krebs described — variously called the citric acid cycle, the tricarboxylic acid (TCA) cycle, or simply the Krebs cycle — is the central hub of cellular energy metabolism, operating in virtually every aerobic organism on Earth.

Where the Cycle Fits: The Context of Cellular Respiration

Glucose breakdown proceeds in three stages. Glycolysis splits glucose (6 carbons) into two pyruvate molecules (3 carbons each) in the cytoplasm, yielding 2 ATP and 2 NADH per glucose. Pyruvate then enters the mitochondrial matrix, where the pyruvate dehydrogenase complex converts each pyruvate into acetyl-CoA (2 carbons) plus CO₂ and NADH. Two acetyl-CoA molecules then enter the Krebs cycle per original glucose molecule. The cycle does not directly produce large amounts of ATP — it produces the electron carriers NADH and FADH₂ that power oxidative phosphorylation in the electron transport chain, where the bulk of ATP is made.

The Eight Reactions of the Krebs Cycle

The cycle begins when a 2-carbon acetyl group enters and is attached to the 4-carbon oxaloacetate (OAA), forming 6-carbon citrate. The cycle then progressively oxidizes this molecule, releasing two CO₂ molecules and regenerating OAA to accept the next acetyl group.

Step	Substrate → Product	Enzyme	Energy Output
1	OAA + Acetyl-CoA → Citrate	Citrate synthase	—
2	Citrate → Isocitrate	Aconitase	—
3	Isocitrate → α-Ketoglutarate + CO2	Isocitrate dehydrogenase	1 NADH
4	α-Ketoglutarate → Succinyl-CoA + CO2	α-Ketoglutarate dehydrogenase	1 NADH
5	Succinyl-CoA → Succinate	Succinyl-CoA synthetase	1 GTP (≈ ATP)
6	Succinate → Fumarate	Succinate dehydrogenase (Complex II)	1 FADH2
7	Fumarate → Malate	Fumarase	—
8	Malate → Oxaloacetate	Malate dehydrogenase	1 NADH

Per turn of the cycle (one acetyl-CoA): 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂. For a full glucose molecule (two turns): 6 NADH, 2 FADH₂, 2 GTP, and 4 CO₂. These electron carriers then donate their electrons to the mitochondrial electron transport chain, which uses the energy to pump protons across the inner mitochondrial membrane and drive ATP synthase — yielding approximately 34 additional ATP molecules per glucose.

Key Regulatory Points

The Krebs cycle is tightly regulated to match the cell's energy demand. Three enzymes function as the main control points:

• Citrate synthase (Step 1): Inhibited by ATP, NADH, and citrate itself. High energy charge slows the cycle's entry point. Stimulated by ADP and oxaloacetate.
• Isocitrate dehydrogenase (Step 3): The primary regulatory enzyme. Inhibited by NADH and ATP; activated by ADP and Ca²⁺. During muscle contraction, rising Ca²⁺ accelerates the cycle to meet increased ATP demand.
• α-Ketoglutarate dehydrogenase (Step 4): Inhibited by succinyl-CoA and NADH. The complex is structurally analogous to pyruvate dehydrogenase and requires the same five cofactors: CoA, NAD⁺, FAD, thiamine pyrophosphate (vitamin B1), and lipoic acid.

The Cycle as a Metabolic Hub

The Krebs cycle does more than generate energy. Its intermediates serve as biosynthetic precursors for dozens of essential molecules. This dual role — catabolism and anabolism — makes the cycle central to metabolism in a way that goes beyond ATP production.

• α-Ketoglutarate: Precursor for glutamate synthesis and for transamination reactions that produce other amino acids. Also produced during amino acid degradation, linking protein catabolism to the cycle.
• Oxaloacetate: Precursor for aspartate and asparagine synthesis. Removed from the cycle by phosphoenolpyruvate carboxykinase (PEPCK) to initiate gluconeogenesis during fasting.
• Succinyl-CoA: Required for heme synthesis. Porphyria — a family of diseases involving defective heme synthesis — disrupts succinyl-CoA utilization.
• Citrate: Exported from mitochondria to the cytoplasm, where it is cleaved back to acetyl-CoA for fatty acid and cholesterol synthesis.

Intermediate	Biosynthetic Pathway Fed	End Products
Citrate	Fatty acid synthesis; cholesterol synthesis	Fatty acids, steroids
α-Ketoglutarate	Amino acid transamination	Glutamate, glutamine, proline, arginine
Succinyl-CoA	Tetrapyrrole synthesis	Heme, chlorophyll, vitamin B12
Oxaloacetate	Gluconeogenesis; amino acid synthesis	Glucose, aspartate, asparagine
Fumarate	Urea cycle (connects to amino acid metabolism)	Argininosuccinate → arginine → urea

Anaplerotic Reactions: Replenishing the Cycle

When intermediates are drawn off for biosynthesis, the cycle would run out of oxaloacetate and stop if not replenished. Anaplerotic reactions feed intermediates back in. The most important in animals is the reaction of pyruvate with CO₂, catalyzed by pyruvate carboxylase, to produce oxaloacetate directly. This enzyme requires biotin (vitamin B7) as a cofactor and is activated by acetyl-CoA — a signal that the cycle needs more OAA to keep running.

In plants and bacteria, the glyoxylate cycle — a modified Krebs cycle with two extra enzymes, isocitrate lyase and malate synthase — allows net synthesis of oxaloacetate from acetyl-CoA. Animals lack these enzymes and cannot convert fats into glucose, which is why a high-fat, zero-carbohydrate diet still requires gluconeogenesis from amino acids rather than from fat directly.

The eight reactions Krebs assembled from scattered biochemical observations in the 1930s now stand as one of the most thoroughly understood pathways in all of biochemistry. Every aerobic cell alive today runs this same cycle — in mitochondria in eukaryotes, in the cytoplasm of aerobic bacteria — a molecular process conserved across 1.5 billion years of evolution.