The Krebs Cycle (Citric Acid Cycle)

The Krebs cycle (citric acid cycle) is a cycle of eight enzyme-catalyzed reactions in the mitochondrial matrix that completely oxidizes the acetyl group of acetyl-CoA to CO₂, producing for each acetyl-CoA 3 NADH, 1 FADH₂ and 1 ATP (or GTP). In brief: acetyl-CoA (2 carbons) joins oxaloacetate (4 carbons) to form citrate (6 carbons). Two carbons are lost as CO₂ in the next steps, and oxaloacetate is regenerated. Since one glucose gives two acetyl-CoA, the cycle runs twice per glucose, giving 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂.

The cycle releases the stored energy of food as reduced coenzymes, which then drive ATP synthesis. This guide describes the stages of the cycle, the products per turn and per glucose, the regulation, the role of the cycle as a metabolic hub, compares the yields in a table, works through the carbon balance and corrects common mistakes.

What is the Krebs cycle?

The Krebs cycle takes place in the matrix of the mitochondrion (in the cytosol of bacteria). It follows the link reaction. Its main role is not to make ATP directly, but to transfer electrons from carbon compounds to the coenzymes NAD⁺ and FAD, forming NADH and FADH₂, which carry them to the electron transport chain. The cycle is aerobic in the sense that it depends on a supply of NAD⁺ and FAD regenerated by the electron transport chain, which needs oxygen. With no oxygen, the cycle stops.

What are the steps of the cycle?

  1. Citrate formation: acetyl-CoA (2C) combines with oxaloacetate (4C) to make citrate (6C). Coenzyme A is released and is reused.
  2. Isomerization: citrate is rearranged into isocitrate (6C).
  3. First oxidation and decarboxylation: isocitrate loses CO₂ and is oxidized, producing alpha-ketoglutarate (5C) and the first NADH.
  4. Second oxidation and decarboxylation: alpha-ketoglutarate loses CO₂ and is oxidized, forming succinyl-CoA (4C) and the second NADH.
  5. Substrate-level phosphorylation: succinyl-CoA is converted to succinate (4C), and the energy released makes one GTP (or ATP).
  6. Third oxidation: succinate is oxidized to fumarate (4C), and FAD is reduced to FADH₂.
  7. Hydration: fumarate takes up water and becomes malate (4C).
  8. Fourth oxidation: malate is oxidized to oxaloacetate (4C), producing the third NADH. The oxaloacetate is ready to accept another acetyl group.

Summing up one turn: the two carbons added from acetyl-CoA are lost as two molecules of CO₂, in steps 3 and 4. These carbon atoms are not the same ones that were added in this turn, but they are balanced overall. There are four oxidation steps: three give NADH and one gives FADH₂.

Diagram of the Krebs cycle showing carbon numbers and products 6C 6C 5C 4C 4C 4C 4C 4C Citrate Isocitrate α-KG Succinyl-CoA Succinate Fumarate Malate Oxaloacetate CO₂ NADH CO₂ NADH ATP FADH₂ NADH Acetyl-CoA (2C)
Schematic of one turn of the Krebs cycle (clockwise, starting at the top). Each circle shows the number of carbons in the intermediate; alpha-KG is alpha-ketoglutarate. Acetyl-CoA (2C) joins oxaloacetate (4C) to make citrate (6C), two carbons leave as CO₂ as the 6C compound becomes 4C, and each turn gives 3 NADH, 1 FADH₂ and 1 ATP.

What are the products?

Per one acetyl-CoA (one turn): 3 NADH, 1 FADH₂, 1 ATP (or GTP) and 2 CO₂. Per glucose, two turns are needed, and the total products from the Krebs cycle alone are: 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂. Adding the link reaction (2 NADH, 2 CO₂) and glycolysis (2 NADH, 2 ATP, none of the CO₂), the whole of glucose oxidation up to this point gives 10 NADH, 2 FADH₂, 4 ATP and 6 CO₂. All six carbons of glucose have been released as CO₂.

Products per turn and per glucose

Yields of the stages of glucose oxidation before the electron transport chain
Stage NADH FADH₂ ATP (direct) CO₂
Glycolysis2020
Link reaction2002
Krebs cycle (2 turns)6224
Total per glucose10246

How is the cycle regulated?

The cycle responds to the cell’s energy needs by feedback. High levels of ATP and NADH inhibit key enzymes, such as isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, slowing the cycle when the cell has enough energy, whereas ADP activates it. Citrate, a product of the first step, also helps to inhibit glycolysis. Availability of oxaloacetate and NAD⁺ also limits the rate.

Why is the cycle a metabolic hub?

The cycle is the common final pathway for the oxidation of carbohydrates, fats and proteins. Fatty acids are broken down to acetyl-CoA by beta-oxidation, and many amino acids can be converted into cycle intermediates or acetyl-CoA. In reverse, intermediates leave the cycle to build other molecules: citrate to make fatty acids, alpha-ketoglutarate and oxaloacetate to make amino acids and oxaloacetate to make glucose. This shows how closely catabolism and anabolism are linked. Because the intermediates are removed, they must be replenished by other reactions, for example by the conversion of pyruvate to oxaloacetate.

Worked example: balancing carbon and counting coenzymes

Count what happens to the carbon in one turn, and the number of coenzymes for the oxidation of 3 molecules of glucose.

  1. Carbon in one turn: acetyl-CoA contributes 2 carbons, and oxaloacetate has 4, so citrate has 6. Two carbons leave as CO₂ in steps 3 and 4, leaving 4 carbons as oxaloacetate again. The cycle is balanced: 2 carbons in, 2 carbons out.
  2. For 3 glucose molecules: 3 × 2 = 6 acetyl-CoA, so 6 turns of the cycle.
  3. Krebs cycle products: NADH = 6 × 3 = 18; FADH₂ = 6 × 1 = 6; ATP = 6 × 1 = 6; CO₂ = 6 × 2 = 12.
  4. Potential ATP from the coenzymes of the Krebs cycle: using about 2.5 ATP per NADH and 1.5 ATP per FADH₂ (current estimates), 18 × 2.5 = 45 and 6 × 1.5 = 9, so 54 ATP, plus 6 ATP made directly, gives 60 ATP for the 3 glucose molecules, or 20 ATP per glucose from the Krebs cycle.

Adding the 2 ATP from glycolysis and 2 NADH from glycolysis and 2 from the link reaction (4 × 2.5 = 10 ATP equivalents), we have 20 + 2 + 10 = 32 ATP per glucose, which is the maximum theoretical yield; the real number is lower, because energy is needed to transport substances across the mitochondrial membrane and some protons leak. Using 3 and 2 ATP per NADH and FADH₂ (older values) gives 38.

20 2 10 32 20 + 2 + 10 = 32

How was the cycle discovered?

Hans Krebs proposed the cycle in 1937 while working in Sheffield, building on earlier work by Albert Szent-Györgyi on the role of small organic acids such as succinate and fumarate in respiration. Krebs received the Nobel Prize in Physiology or Medicine in 1953, shared with Fritz Lipmann, who discovered coenzyme A. The cycle is also called the tricarboxylic acid (TCA) cycle or the citric acid cycle. Krebs and William Johnson first sent a paper to the journal Nature in 1937, which was rejected, and it was then published in another journal.

Which mistakes are common?

Mistake 1: Saying that the cycle makes most of the ATP

It makes only 1 ATP per turn directly. Most ATP comes from the oxidation of NADH and FADH₂ in the electron transport chain.

Mistake 2: Forgetting that the cycle turns twice per glucose

One glucose gives two acetyl-CoA. Double the yields per turn.

Mistake 3: Thinking that oxygen is used in the cycle

Oxygen is not used directly, but the cycle depends on oxygen in the electron transport chain to regenerate NAD⁺ and FAD.

Mistake 4: Mixing up the locations

The Krebs cycle takes place in the mitochondrial matrix. Glycolysis is in the cytosol.