How many ATP are produced in the Krebs cycle?
Per turn, per glucose, and what its NADH and FADH₂ are worth
Matic Broz, PhD7 min
One turn of the Krebs cycle produces 1 ATP directly, or its close relative GTP, along with 3 NADH, 1 FADH₂ and 2 carbon dioxide. Each glucose molecule supplies two acetyl-CoA, so the cycle turns twice per glucose and makes 2 ATP, 6 NADH and 2 FADH₂.
The NADH and FADH₂ carry most of the energy. Once oxidative phosphorylation spends them, at about 2.5 ATP per NADH and 1.5 per FADH₂, one turn is worth 10 ATP and one glucose about 20 ATP. Older textbooks say 24, because they credited each NADH with 3 ATP and each FADH₂ with 2. The Krebs cycle, the citric acid cycle and the tricarboxylic acid (TCA) cycle are three names for the same pathway.
How many ATP does one turn of the Krebs cycle produce?
Each turn makes one ATP or GTP, at a single step. Succinyl-CoA synthetase breaks the high-energy thioester bond of succinyl-CoA and couples the released energy to phosphorylating a nucleoside diphosphate, the same kind of substrate-level phosphorylation that makes ATP in glycolysis.[1] None of the cycle's other seven steps makes ATP.
A turn starts when a two-carbon acetyl group from acetyl-CoA joins four-carbon oxaloacetate to form citrate, and ends when oxaloacetate is regenerated. Along the way two carbons leave as carbon dioxide, and four oxidation steps pass electrons to carriers: three to NAD⁺ and one to FAD.[2][1] Table 1 shows where each product forms.
| Reaction | Enzyme | Product |
|---|---|---|
| Isocitrate to α-ketoglutarate | Isocitrate dehydrogenase | 1 NADH, 1 carbon dioxide |
| α-Ketoglutarate to succinyl-CoA | α-Ketoglutarate dehydrogenase complex | 1 NADH, 1 carbon dioxide |
| Succinyl-CoA to succinate | Succinyl-CoA synthetase | 1 GTP or ATP |
| Succinate to fumarate | Succinate dehydrogenase | 1 FADH₂ |
| Malate to oxaloacetate | Malate dehydrogenase | 1 NADH |
Whether the product is ATP or GTP depends on the tissue. For nearly 50 years the animal enzyme was thought to make only GTP, until Lambeth and colleagues showed that mammals express both an ATP-forming and a GTP-forming form, in proportions that vary by tissue.[3] The ATP form is found where demand for ATP is high, as in heart and skeletal muscle, and the GTP form in biosynthetic tissues such as liver.[4]
The count is the same either way. GTP is energetically equivalent to ATP, and transferring its terminal phosphate to ADP produces one ATP, so one GTP counts as one ATP equivalent.[2][4] Sources that write 1 ATP, 1 GTP or 1 ATP equivalent per turn all mean the same thing.
One enzyme of the cycle also differs in where it works. Seven of the cycle's steps run in the mitochondrial matrix, but succinate dehydrogenase sits in the inner membrane of mitochondria, where it doubles as complex II of the electron transport chain.[1] Its FADH₂ hands electrons straight to the chain, and that entry point decides what the FADH₂ is worth.
How many ATP does the Krebs cycle produce per glucose?
Per glucose, the Krebs cycle produces 2 ATP or GTP, 6 NADH, 2 FADH₂ and 4 carbon dioxide. Glycolysis splits glucose into two pyruvate, the pyruvate dehydrogenase complex converts each pyruvate into one acetyl-CoA, and each acetyl-CoA drives one turn, so every per-turn number doubles.[2]
The step before the cycle is a common source of confusion. Converting the two pyruvates releases 2 more carbon dioxide and makes 2 more NADH, but no ATP.[2] Those belong to pyruvate oxidation, so counts of 8 NADH per glucose, or 4 per pyruvate, include that step. Together the two stages make 8 of the 10 NADH that one glucose yields and release all six of its carbons as carbon dioxide.
The direct yield of 2 ATP equals the net 2 ATP of glycolysis, although the two pathways make it at different reactions. For the Krebs cycle, though, the direct ATP is a small part of the output.
How many ATP do the NADH and FADH₂ from the Krebs cycle produce?
Hinkle's review of mitochondrial P/O ratios, the ATP made per pair of electrons passed to oxygen, found that about 2.5 for NADH and 1.5 for succinate, which donates electrons at the FADH₂ level, fit most measurements.[5] At those values the products of the Krebs cycle support about 20 ATP per glucose: 15 from 6 NADH, 3 from 2 FADH₂ and 2 made directly. Per turn, or per acetyl-CoA, that is 10 ATP.
| Product | Per turn | Per glucose | ATP per glucose, modern | ATP per glucose, older |
|---|---|---|---|---|
| GTP or ATP | 1 | 2 | 2 | 2 |
| NADH | 3 | 6 | 15 | 18 |
| FADH₂ | 1 | 2 | 3 | 4 |
| Carbon dioxide | 2 | 4 | 0 | 0 |
| Total ATP | 20 | 24 |
The fractions come from counting protons. Electrons from NADH enter the respiratory chain at complex I and drive about 10 protons across the inner membrane on their way to oxygen. Electrons from FADH₂ enter at ubiquinone through complex II, skip complex I and move about 6.[7] Textbooks round the cost of making and exporting one ATP to 4 protons, which gives 10 ÷ 4 = 2.5 and 6 ÷ 4 = 1.5.
Structural work puts the cost a little lower. The rotor ring of bovine ATP synthase has eight c subunits, so one full turn uses 8 protons to make 3 ATP, or about 3.7 protons per ATP once transport is added.[8] On that basis Mookerjee and colleagues calculated ceilings of 2.73 ATP per NADH and 1.64 per FADH₂.[7] Living mitochondria recover less, because some protons leak back across the membrane without passing through ATP synthase.[7]
Only the 2 ATP from succinyl-CoA synthetase are made inside the cycle. The other 18 are made later by ATP synthase during oxidative phosphorylation, the only step in the breakdown of food that uses oxygen directly.[2] The 20 is a theoretical yield for complete, tightly coupled oxidation, not a count of ATP measured from every turn.
Why do some sources say the Krebs cycle produces 24 ATP?
The 24 comes from older whole-number ratios of 3 ATP per NADH and 2 per FADH₂. Six NADH at 3 and two FADH₂ at 2 give 22 ATP, and the 2 made directly bring the total to 24 per glucose, or 12 per turn. The same integers give the 38 ATP per glucose still quoted for cellular respiration as a whole.[6]
Those integers were long read from early experiments as exact. Hinkle traced the apparent disagreements between measurements made since 1937 to systematic errors, and concluded that the fractional values follow from how many protons each step moves.[5] A 24 in a recent source signals the older accounting rather than different biology, and both accountings agree on the direct yield of 2 ATP per glucose.
How the Krebs cycle fits into cellular respiration
The cycle makes few ATP itself, yet it supplies most of the ATP that glucose yields. Cellular respiration produces about 30 to 32 ATP per glucose under modern accounting,[9] and about 20 of them, roughly two-thirds, trace back to the Krebs cycle. Glycolysis adds 2 ATP plus 2 NADH worth 3 or 5 ATP, depending on the shuttle that carries their electrons into the mitochondrion, and pyruvate oxidation adds 2 NADH worth 5.
Glucose is not the only fuel the cycle burns. Fatty acids are broken down to acetyl-CoA in mitochondria as well,[2] so every acetyl-CoA released by beta oxidation is worth the same 10 ATP per turn. Eight of them account for most of the 106 ATP that one palmitate yields.
