- The Krebs cycle extracts energy from acetyl-CoA through eight reactions in mitochondria.
- Each turn produces 3 NADH, 1 FADH2, 1 GTP, and 2 CO2 molecules.
- Its intermediates supply raw materials for amino acid and nucleotide synthesis.
The Krebs cycle is a sequence of eight enzyme-driven reactions inside mitochondria that extracts energy from acetyl-CoA, the common breakdown product of carbohydrates, fats, and proteins, and channels it toward ATP production. Also called the citric acid cycle or the tricarboxylic acid (TCA) cycle, it operates in nearly every organism that uses oxygen.
Key figure
8
enzyme-driven reactions per turn
Why It Matters
The Krebs cycle sits at the center of cellular metabolism. Every major fuel source (glucose, fatty acids, amino acids) feeds into it, and virtually every biosynthetic pathway draws raw materials from it. Without this single loop of chemistry, aerobic respiration could not function.
That dual role makes the cycle far more than an energy generator. Its intermediates supply the carbon skeletons for amino acid synthesis, the precursors for heme and nucleotide construction, and the signaling molecules that regulate gene expression. Research over the past decade has established that TCA intermediates, particularly succinate and fumarate, act as immune signaling molecules. A 2021 review in Life Sciences documented how succinate stabilizes HIF-1alpha in macrophages, directly shaping inflammatory responses.
Mutations in Krebs cycle enzymes have clinical consequences. Isocitrate dehydrogenase (IDH) mutations appear in roughly 80% of grade II and III gliomas and in approximately 20% of acute myeloid leukemias when IDH1 and IDH2 mutations are combined, according to clinical studies. These mutations produce an abnormal metabolite, 2-hydroxyglutarate, that disrupts normal cell differentiation.
How the Krebs Cycle Works
The cycle begins when acetyl-CoA (a two-carbon unit attached to coenzyme A) combines with oxaloacetate (four carbons) to form citrate (six carbons). The enzyme citrate synthase catalyzes this step. Over the next seven reactions, two carbon atoms leave as carbon dioxide, four pairs of electrons transfer to carrier molecules, and oxaloacetate regenerates, ready for the next turn.
Each complete turn yields three molecules of NADH, one FADH2, one GTP (equivalent to one ATP), and two molecules of CO2. The NADH and FADH2 carry their electrons to the electron transport chain on the inner mitochondrial membrane, where oxidative phosphorylation generates the bulk of cellular ATP. In total, each acetyl-CoA molecule entering the cycle contributes to the production of 10 to 12 ATP molecules, depending on the efficiency of the electron transport chain.
Key figure
10-12 ATP
produced per acetyl-CoA molecule
Three enzymes regulate the cycle's speed: citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. All three respond to the cell's energy status. When ATP levels are high and NADH accumulates, these enzymes slow. When ADP rises and NAD+ is plentiful, they accelerate. This feedback system matches energy production to demand with notable precision.
Key Context
Hans Adolf Krebs and William Arthur Johnson identified the cycle in 1937 at the University of Sheffield. Krebs submitted his manuscript to Nature on June 10 of that year. The journal rejected it on June 14, citing a backlog of material. The paper appeared instead in the Dutch journal Enzymologia. In 1953, Krebs received the Nobel Prize in Physiology or Medicine for this work, shared with Fritz Lipmann, who had discovered coenzyme A in 1945.
The cycle operates in the mitochondrial matrix of eukaryotic cells and in the cytoplasm of prokaryotes. In humans, all eight enzymes reside in the matrix except succinate dehydrogenase (Complex II), which is embedded in the inner mitochondrial membrane and participates directly in the electron transport chain.
FAQ
What is the difference between the Krebs cycle and glycolysis?
Glycolysis splits one glucose molecule into two pyruvate molecules in the cytoplasm, producing a net gain of two ATP. The Krebs cycle picks up where glycolysis leaves off: it oxidizes the acetyl-CoA derived from pyruvate inside the mitochondria, generating electron carriers (NADH and FADH2) that drive far greater ATP production through oxidative phosphorylation.
Does the Krebs cycle require oxygen?
The cycle itself does not use oxygen directly. However, it depends on the electron transport chain to recycle NAD+ and FAD from their reduced forms. Because the electron transport chain requires oxygen as its final electron acceptor, the Krebs cycle stalls without it.
Can the Krebs cycle run in reverse?
Certain bacteria and archaea use a reversed TCA cycle (the reductive citric acid cycle) to fix carbon dioxide into organic molecules. This pathway, identified in green sulfur bacteria, functions as a carbon-fixation mechanism rather than an energy-releasing one.
Why do Krebs cycle mutations cause cancer?
Mutations in succinate dehydrogenase, fumarase, and isocitrate dehydrogenase allow abnormal metabolites to accumulate. These metabolites inhibit enzymes that regulate DNA methylation and the cellular oxygen-sensing pathway (HIF), promoting uncontrolled cell growth. IDH mutations are found in roughly 80% of low-grade gliomas.
Related Reading
Sources
- Primary Research: Physiology, Krebs Cycle (StatPearls, National Library of Medicine)
- Additional Context:
- Tricarboxylic acid cycle (Encyclopaedia Britannica)
- Hans Krebs: Facts (Nobel Prize)
- Regulation and function of the mammalian tricarboxylic acid cycle (Arnold & Bhatt, 2023)
Fact Check: Claim-by-Claim Verification Verified
All 10 claims verified as Supported or Mostly Supported. Corrections applied for AML mutation frequency clarification, ATP yield range, and Nature rejection date. External fact-check via Perplexity confirmed all claims after reconciliation.
Sources used for verification
- Physiology, Krebs Cycle - ncbi.nlm.nih.gov
- Tricarboxylic acid cycle - britannica.com
- Hans Krebs: Facts - nobelprize.org
- Regulation and function of the mammalian TCA cycle - pmc.ncbi.nlm.nih.gov
