Bbiochemtools
Daily question About All tools →

Citric Acid Cycle Explorer

Walk through all 8 reactions of one turn of the citric acid (Krebs/TCA) cycle. The NADH, FADH₂, GTP, and CO₂ tally updates as you go, every enzyme is named, and the three regulated steps are flagged.

Step
0 / 8
NADH
0
FADH₂
0
GTP
0
CO₂
0

One turn of the cycle, and why it runs twice per glucose

One acetyl-CoA entering the cycle (one "turn") yields 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂, while regenerating the oxaloacetate the cycle started with so it can accept another acetyl-CoA. Glycolysis produces 2 pyruvate per glucose, and pyruvate dehydrogenase converts each pyruvate into one acetyl-CoA — so the cycle runs twice per original glucose, doubling everything to 6 NADH, 2 FADH₂, 2 GTP, and 4 CO₂. Those are exactly the citric acid cycle numbers used in the ATP yield calculator.

Model note: standard textbook citric acid cycle (Lehninger). GTP is counted as ATP-equivalent (nucleoside diphosphate kinase interconverts them freely), matching the convention used across this site.

Related tools: ATP yield calculator · Glycolysis pathway explorer · all biochem tools.

Worked example: the running tally, step by step

Tracking cumulative NADH, FADH₂, GTP, and CO₂ after each step — exactly what the tally bar above computes as you click through:

Step 1 Citrate synthase (no change) Step 2 Aconitase (no change) Step 3 Isocitrate dehydrogenase NADH +1, CO2 +1 → cumulative NADH: 1, CO2: 1 Step 4 α-Ketoglutarate dehydrogenase NADH +1, CO2 +1 → cumulative NADH: 2, CO2: 2 Step 5 Succinyl-CoA synthetase GTP +1 → cumulative GTP: 1 Step 6 Succinate dehydrogenase FADH2 +1 → cumulative FADH2: 1 Step 7 Fumarase (no change) Step 8 Malate dehydrogenase NADH +1 → cumulative NADH: 3 Final (one turn): NADH = 3, FADH2 = 1, GTP = 1, CO2 = 2

Both carbons lost as CO₂ (steps 3 and 4) come from the oxaloacetate/citrate carbon skeleton, not directly from the acetyl-CoA that entered this turn — but the cycle still nets zero carbons gained, since oxaloacetate is fully regenerated by step 8.

FAQ

What is the yield of one turn of the cycle?
Per acetyl-CoA: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂. Twice per glucose (2 acetyl-CoA from 2 pyruvate): 6 NADH, 2 FADH₂, 2 GTP, 4 CO₂.

What are the three regulated steps?
Citrate synthase (step 1), isocitrate dehydrogenase (step 3, rate-limiting), and α-ketoglutarate dehydrogenase (step 4) — all irreversible under cellular conditions, inhibited by high ATP/NADH, activated by ADP/calcium.

Why is succinate dehydrogenase different from the rest?
It's the only cycle enzyme embedded in the inner mitochondrial membrane, since it doubles as Complex II of the electron transport chain — its FADH₂ electrons feed the ETC directly, unlike the NADH from the other three oxidative steps.

Is the GTP the same as ATP?
Functionally yes for energy accounting — nucleoside diphosphate kinase readily interconverts GTP and ATP, so most textbooks (and this site's ATP yield calculator) count them as equivalent.

Practice problems

1. A cell processes 10 acetyl-CoA through the citric acid cycle (10 turns). What's the total NADH, FADH₂, GTP, and CO₂?

Show answer
Per turn: 3 NADH, 1 FADH2, 1 GTP, 2 CO2 — scales linearly since each acetyl-CoA is processed independently. 10 × 3 NADH = 30 NADH 10 × 1 FADH2 = 10 FADH2 10 × 1 GTP = 10 GTP 10 × 2 CO2 = 20 CO2

2. Using the modern P/O ratios from the ATP yield calculator (2.5 ATP/NADH, 1.5 ATP/FADH2), how much total ATP does one full turn of the citric acid cycle contribute, counting both oxidative phosphorylation and the substrate-level GTP?

Show answer
Oxidative phosphorylation: 3 NADH × 2.5 + 1 FADH2 × 1.5 = 7.5 + 1.5 = 9 ATP Substrate-level: 1 GTP = 1 ATP-equivalent Total per turn = 9 + 1 = 10 ATP Per glucose (×2 turns) = 20 ATP — which is exactly the citric acid cycle's share of the 32 ATP total shown by default in the ATP yield calculator.